Diagnosis - A/t GMC Typhoon I

THM 3T40 APPLICATION

THM 3T40 APPLICATION

THM 4L60/700-R4 APPLICATION (1991-92)

THM 4L60/700-R4 APPLICATION (1991-92)

THM 4T60 & 4T60-E APPLICATION (1991-92)

THM 4T60 & 4T60-E APPLICATION (1991-92)

THM 4L80-E APPLICATION

THM 4L80-E APPLICATION

THM 4L80-E SERIES CODE DESIGNATION

THM 4L80-E SERIES CODE DESIGNATION

MP6 & MP7 APPLICATIONS (1991-92)

MP6 & MP7 APPLICATIONS

DIAGNOSTIC FLOW

TCC diagnosis should be performed in the following order

1. See «TROUBLE SHOOTING»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__trouble-shooting) .
2. See «RETRIEVING TROUBLE CODES»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__retrieving-trouble-codes) or DISPLAYING TROUBLE CODES under appropriate TESTING & DIAGNOSIS.
3. Verify TCC system fault.
4. See «TORQUE CONVERTER CLUTCH CHARTS (TRUCKS)»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at) .
5. See «DIAGNOSTIC FLOW CHARTS (CARS)»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at) or «DIAGNOSTIC CODE CHARTS (TRUCKS)»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at) .

Note. This article contains test charts that are part of General Motors Computerized Engine Controls. Only those charts required to test Torque Converter Clutch (TCC) system and related circuits are included. Other diagnostic codes may appear while performing TCC electrical diagnosis. See appropriate article in the ENGINE PERFORMANCE section for complete information on General Motors Computerized Engine Control systems.

DESCRIPTION

Note. ECM is often referred to as PCM in diagnostic charts.

The Torque Converter Clutch (TCC) assembly consists of a 3-element torque converter with the addition of a converter clutch. The converter clutch is an internal mechanism with friction material attached to front face. It is splined to the turbine assembly in converter.

The purpose of the torque converter clutch feature is to eliminate power loss due to slippage when vehicle is cruising. This allows the convenience of an automatic transmission and the fuel economy of a manual transmission.

When the TCC solenoid ground circuit is completed by the Electronic Control Module (ECM), the torque converter clutch is applied, resulting in a direct mechanical coupling between engine and transmission. When TCC solenoid is deactivated, the torque converter clutch is released, allowing torque converter to operate in a conventional manner.

TCC CONTROL COMPONENTS

The following components are used in TCC system. Not all components will be present on all vehicles.

Brake Switch

Power from ignition switch passes through brake switch to TCC solenoid. When brake pedal is depressed with TCC engaged, power to TCC solenoid is interrupted, releasing converter clutch and preventing engine from stalling.

Scheme 54

Coolant Temperature Sensor

This sensor provides ECM with engine coolant temperature information. ECM will not allow TCC operation until signal from this sensor indicates coolant temperature greater than 130-150°F (55-65°C).

Electronic Control Module (ECM)

To determine application of torque converter clutch, ECM receives and processes information from various input devices. These devices may include the vehicle speed sensor, coolant temperature sensor, throttle position sensor, 3rd and 4th gear switches and brake switch. The ECM controls application of torque converter clutch by providing a ground circuit for the TCC solenoid circuit.

TCC Solenoid Assembly

Solenoid is energized by ECM to redirect transmission fluid to the converter clutch apply valve in the auxiliary control valve assembly.

Throttle Position Sensor (TPS)

Provides ECM with throttle position information. TCC operation is prevented when throttle position signal is less than a specified value.

Vacuum Sensor

Sends engine vacuum (load) information to ECM.

Vehicle Speed Sensor (VSS)

This sensor sends vehicle speed information to ECM. Vehicle speed must be greater than a certain value before TCC can be applied. Two types of speed sensor are used. A light emitting diode type is used in the instrument cluster on some models. Other models use a Permanent Magnet (PM) generator mounted in the transmission.

3rd & 4th Gear Switches

When open, 3rd and 4th gear switches prevent TCC operation. Switch status may be monitored by ECM, or switch may be an integral portion (series circuit) of TCC solenoid power supply.

TROUBLE SHOOTING

Note. Every diagnosis of automatic transmission problems should begin with a check of the transmission fluid and linkage. Most of the conditions listed in TROUBLE SHOOTING can be caused by one or more of the following factors

1. Incorrect fluid level
2. Contaminated fluid
3. Improperly adjusted linkage
4. Damaged or worn linkage

When diagnosing converter clutch problems, ensure engine and vacuum systems are operating properly.

Problem In Electronic Control Module

1. Verify Electronic Control Module (ECM) operation. See appropriate CHART C-8.

Electrical Problems

1. Voltage not reaching transmission. Ensure 12 volts reaches transmission to engage solenoid.
2. Ground inside transmission. Ensure solenoid is not grounded inside transmission case.
3. Defective connector, wiring harness, or solenoid. Check and repair or replace if necessary. Defective pressure switch (if equipped). Check and replace pressure switch if necessary.
4. 3rd and 4th gear switches inoperative. Check and replace switches if necessary.

Valve Body Assembly

1. Sticking converter clutch shift and/or apply valve. Clean and/or replace valve body if necessary.
2. Solenoid "O" ring leaking or solenoid screen blocked. Check and repair or replace if necessary.
3. Inspect valve body and service as necessary.

Oil Pump Assembly

1. Orifice cup plug restricted or damaged. Clean and inspect orifice for blockage.
2. Solenoid "O" ring damaged or missing. Check and replace "O" ring if necessary.
3. Oil pump wear plate or gasket mispositioned or damaged. Check and replace wear plate or gasket.
4. Improper torque on oil pump-to-converter housing bolts. Tighten bolts to specifications.
5. Turbine shaft seals damaged. Check and replace seals.
6. Converter clutch valve stuck or assembled backwards or retaining ring mispositioned. Check and repair as necessary.

CONVERTER CLUTCH APPLY SLIPS, ROUGH OR SHUDDERS

1. Torque converter assembly has internal damage. Replace as necessary.
2. Damaged or missing check ball in end of turbine shaft. Check and replace turbine shaft if necessary.
3. Converter clutch valve stuck or restricted oil passage. Clean and/or repair if necessary.
4. Crack in oil filter body, restricted filter neck or "O" ring seal cut or damaged. Check and repair or replace if necessary.

CONVERTER CLUTCH DOES NOT RELEASE

1. External ground or clogged exhaust orifice. Check and repair if necessary.
2. ECM has external ground. Verify ECM operation. See appropriate CHART C-8.
3. Converter clutch apply valve stuck. Clean and/or replace if necessary.
4. Check for damaged torque converter. Replace torque converter if necessary.

DIAGNOSTIC TOOLS

Note. On all models, diagnosis of the TCC system may be performed by following the appropriate CHART C-8 for the transmission/engine application in question.

When utilizing C-8 charts, specific tools which may be required are a tachometer, test light, digital volt/ohmmeter (DVOM) with 10-megohm input impedance (minimum), and jumper wires. A test light, rather than a voltmeter, must be used when indicated by diagnostic chart.

Note. In the diagnostic and trouble code charts, special scan testers are referred to (and recommended) by the manufacturer to read data parameters and check voltages in the system.

RETRIEVING TROUBLE CODES

The ECM stores component failure information under a related trouble code, which can be recalled later for diagnosis and repair. When recalled, these codes can be displayed on a scan tester, or by entering diagnostic mode and observing flashes of the SERVICE ENGINE SOON light. It is NOT necessary to enter diagnostic mode to use TCC test charts; however, it will be necessary to enter ECM diagnostic mode to verify the presence of any codes.

Note. Even when using a scan tester, it is recommended that the diagnostic mode be accessed in advance to verify the computer's ability to self-diagnose.

Scheme 55

1. To enter diagnostic mode, turn ignition switch on, but do not start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Diagnostic Link (ALDL) connector attached to ECM wiring harness under instrument panel. Insert jumper wire across diagnostic mode "test" terminal "B" and ground terminal "A". (Scheme 55) CAUTION: Inserting jumper wire in terminals of ALDL connector completes test circuit. DO NOT ground ALDL connector until after ignition switch is on. Inserting jumper wire in terminals of ALDL connector with engine running will cause fuel injected vehicles to enter field service mode. Flashes of the SERVICE ENGINE SOON light will not indicate codes if this is done.
2. SERVICE ENGINE SOON light should flash Code 12 (FLASH, pause, FLASH, FLASH) followed by a longer pause. Trouble Code 12 will be repeated 3 more times, then if any trouble codes are stored in the ECM memory, they will be displayed in the same manner.
3. Trouble codes will be displayed from lowest to highest numbered codes (3 times each) and be repeated as long as the test terminal of the ALDL connector is grounded.
4. To exit diagnostic mode, remove jumper wire from ALDL connector and turn ignition switch off.

Note. If Code 12 is not flashed when the ALDL diagnostic test terminal is grounded, information received by a scan tester on the serial data line may not be accurate. For complete testing of the ECM diagnostic circuit, refer to appropriate article in the ENGINE PERFORMANCE section.

CLEARING TROUBLE CODES (EXCEPT SATURN)

Trouble codes are cleared by removing battery voltage from ECM for at least 30 seconds. Ensure test terminal is not grounded. Turn ignition switch off and remove ECM fuse from fuse block for 30 seconds, and then reinsert fuse. Another way of removing battery voltage is to disconnect the battery positive cable (or pigtail) from the battery for 30 seconds, and then reconnect it. In either case, ensure the ignition switch is in the OFF position before removing battery voltage. Otherwise, a voltage spike could occur and damage the ECM. When power to ECM is removed, degraded driveability may be exhibited until control module relearns optimum operational parameters. See COMPUTER RELEARN PROCEDURES article in the GENERAL INFORMATION section. See menu below.

CLEARING TROUBLE CODES (SATURN)

To clear general information codes, turn ignition switch on and jumper diagnostic test terminal "B" to ground terminal "A" at ALDL connector 3 times within 5 seconds. (Scheme 55) General information codes will also clear automatically if code does not reoccur within 50 ignition "ON" cycles. Malfunction history codes and information flags can only be cleared using a scan tester.

ENTERING SELF-DIAGNOSTICS

Note. It will be necessary to enter service mode to use Reatta, Riviera, Toronado and Trofeo TCC test charts. Enter appropriate test condition (i.e., ECM INPUTS, BCM CODE, etc.) as indicated in charts.

1. On Reatta and Riviera, turn ignition switch on. Simultaneously depress OFF and TEMP buttons on the Electronic Climate Control Panel (ECCP). (Scheme 56) Continue to depress OFF and TEMP buttons until all segments and bulbs of the Instrument Panel Cluster (IPC), the Driver Information Center (DIC) and ECCP illuminate. When all segments are lit, system has entered self-diagnostic mode. Release OFF and TEMP buttons. Go to step 4).
2. On Toronado and Trofeo, turn ignition switch on. On models without Cathode Ray Tube (CRT), simultaneously depress OFF and WARM buttons on the Electronic Climate Control Panel (ECCP). On CRT equipped vehicles, simultaneously push OFF button and touch WARM key on CRT monitor control panel. (Scheme 57)
3. On both non-CRT and CRT monitor equipped vehicles, continue to depress OFF and WARM button/key until all segments and bulbs of the IPC, the Driver Information Center (DIC) and ECCP illuminate. (Scheme 58) When all segments are lit, system has entered self-diagnostic mode. Release OFF and WARM button/key.
4. On all models, illuminating the segments of the IPC, DIC and ECCP ensures all display segments are working properly. If all segments illuminate, proceed to «DISPLAYING TROUBLE CODES»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__displaying-trouble-codes). Failure of any segment to illuminate may result in inaccurate test results. All inoperative segments of the display must be made to operate before proceeding with self-diagnostic procedures.
5. Partial segment check is possible by depressing TEST button on IPC when vehicle is in Park or Neutral. Holding button depressed will light all vacuum fluorescent displays and all telltales on IPC in order to check for faulty bulbs or panels.

EXITING SELF-DIAGNOSTICS

During self-diagnostics procedure, it is possible to exit service mode without erasing trouble codes. On non-CRT equipped vehicles, depress BI-LEV button on ECCP or turn ignition switch to OFF position. On CRT monitor equipped panel vehicles, touch RTN key on CRT monitor panel or turn ignition switch to OFF position.

DISPLAYING TROUBLE CODES

After diagnostic service mode is entered, any trouble codes stored in the computer memory will be displayed. Codes will also be followed by a "C" or "H". Letter "H" stands for history and indicates that the failure was not present when last tested. Letter "C" stands for current and indicates the fault still exists. ECM codes will be displayed first. If no ECM codes are stored, a NO E CODE message is displayed.

After ECM codes have been displayed, the Body Control Module (BCM) codes will be displayed. BCM codes mainly deal with body control functions, with climate control being one of its main features. If no BCM codes are present, a NO B CODE message will be displayed. Trouble codes for both the ECM and BCM will be displayed in numerical succession of lowest to highest numbered code. Trouble code display can be bypassed at any time by depressing FAN DOWN button on ECCP or touching RTN key on CRT monitor control panel.

ECM & BCM TROUBLE CODES

Scheme 56

Scheme 57

Scheme 58

CLEARING TROUBLE CODES

See CLEAR CODES under TEST TYPE SELECTION .

SYSTEM SELECTION

1. After all trouble codes have been displayed, the SERVICE MODE system can be directed to perform specific system diagnostic tests. See «DISPLAYING TROUBLE CODES»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__displaying-trouble-codes). Following the trouble code display, the first system available for testing will automatically be displayed. For example, ECM? may now be present on the display.
2. To select the desired system, (ECM or BCM), advance and stop the DIC display as follows: On non-CRT equipped vehicles, depressing the FAN DOWN button on the ECCP will cycle the system selection list. When the desired diagnostic system is displayed, depress FAN UP button on ECCP and the displayed system will be selected for testing.
3. On CRT equipped vehicles, touching NO key on CRT monitor panel will cycle system selection list. When desired diagnostic system is displayed on DIC display panel, touch YES key on CRT monitor panel and displayed system will be selected for testing. (Scheme 57)
4. On non-CRT vehicles, depressing BI-LEVEL button on ECCP will exit diagnostics and return to normal IPC and ECCP operation.

Note. To cancel a system selection and repeat the system selection process, depress the OFF button on the ECCP for non-CRT equipped vehicles or touch LEVL key on CRT monitor panel for CRT equipped vehicles.

TEST TYPE SELECTION

1. After the diagnostic system has been selected, 6 test types are now made available for selection. The menu choices are: CLEAR CODES?, DATA?, INPUTS?, OUTPUTS?, OVERRIDE? and SNAPSHOT?. The Season Odometer/DIC display may be showing any one of these 6 test type choices. The Trip Odometer or DIC will display selected value as seen by computer. (Scheme 59)- (Scheme 66).
2. To advance the display, depress FAN DOWN button on the ECCP. When the desired test type is displayed, depress the FAN UP button on the ECCP.

Note. To cancel a system selection and repeat the system selection process, depress the OFF button on the ECCP for non-CRT equipped vehicles or touch LEVL key on CRT monitor panel for CRT equipped vehicles. To exit diagnostics without clearing codes, depress the BI-LEV button on ECCP or touch RTN key on CRT monitor panel.

The 6 test types are as follows

CLEAR CODES

Selection of CLEAR CODES? will result in the message CODES CLEAR or CODES NOT CLEAR being displayed, indicating whether the codes were successfully cleared. This message appears for 3 seconds, indicating all stored trouble codes have been erased from that system's memory. After 3 seconds, the display will automatically return to the next test type available for testing. (Scheme 59)- (Scheme 66).

Note. After trouble code has been cleared, test drive vehicle and recheck to ensure code did not reset.

DATA

Data test displays component/system present specifications, which can be compared with specifications of a properly functioning system. Trip Odometer/DIC displays data value.

INPUTS

1. Input test displays the voltage level of the circuit being tested as HI of LO. Input test display is shown on Trip Odometer/DIC panel.
2. HI or LO refers to input terminal voltage for the particular circuit. The display also indicates if an input reading has changed since the test was selected. This feature permits technician to activate or deactivate any list device/circuit and then return to the display to see if voltage reading has changed.
3. If a voltage reading change occurs, an X will appear next to the HI/LO indicator; otherwise, a "O" will remain displayed indicating no change occurred. The X will only appear once per selected input. The HI/LO indicator will continue to change as the input changes.

OUTPUTS

Output test displays the voltage level of selected device/circuit as HI or LO. After 3 seconds, this level will cycle between HI and LO voltage. Output test display is shown in Trip Odometer/DIC panel.

OVERRIDE

Override test is represented as a percentage of the tested functions full range during its current operation. This percentage value will be displayed in the ECCP temperature display area. The display will alternate between the override percentage and ("-") and the normal operation value. This alternating display is a reminder that the tested function IS NOT currently being overridden.

Pressing the TEMP UP and TEMP DOWN buttons simultaneously on the ECCP begins the override function. Alternating display will stop, and show override value.

Pressing the TEMP UP button increases the value while pressing the TEMP DOWN button decreases the value. Normal program control can be resumed in 1 of following 3 ways

1. Selection of another override will cancel current override.
2. Selection of another system (i.e. ECM, BCM) will cancel current override.
3. Overriding the value beyond either extreme (O or 99) will display"--" momentarily before resuming value. If the button is released while "--" is being displayed, normal program control will resume and the display will again alternate. The override test type is unique in that any other test type within the selected system may be active at the same time. After selecting an override test, pressing the OFF button will allow selection of another test type. The ECCP will continue to display the selected override. By pressing OFF and TEMP UP or TEMP DOWN buttons at the same time, it is possible to monitor effect of override on different parameter. Pressing the BI-LEV button will exit diagnostics without clearing codes.

SNAPSHOT

Snapshot is a test type that will record all BCM or ECM data and inputs at one instant for review at a later time. Snapshot also permits a technician-triggered recording of specific current BCM/ECM data and input parameters for review at a later time. BCM and ECM have slightly different types of snapshot.

Note. Proceed to SNAPSHOT after selecting snapshot test type.

EXPLANATION OF SPECIFIC TEST SELECTION

Following test type selection, first of many specific tests will be made available for selection; 4 characters of this display represent a test code. First 2 characters of test code are letters which identify system and test type selected. Study the following examples

1. If BCM system and DATA test type were selected, BD will be the first characters of display. BD stands for BCM DATA. The last 2 characters numerically identify specific test selection. For example: BD51 designates BCM DATA, test 51.
2. If ECM system and Data test type were selected, ED will be the first 2 characters of display. ED stands for ECM DATA. The numerals following these 2 characters will represent the particular specific test selection.
3. When BI appears as the first 2 characters of test code, this signifies BCM system and INPUT test type were selected. The last 2 characters numerically identify the specific test selection. For example: BI51 designates BCM INPUT, test 51.

SPECIFIC TEST SELECTION

1. On non-CRT equipped vehicles, scrolling to a lower specific test number is done by depressing the FAN DOWN button. Scrolling to a higher specific test number is done by depressing the FAN UP button. The system will automatically display the values for whatever specific test number is displayed.
2. On CRT monitor equipped vehicles, scrolling to a lower specific test number is done by touching NO key on CRT monitor control panel. Scrolling to a higher specific test number is done by touching YES key. The system will automatically display the values for whatever specific test number is displayed.

Note. To cancel a system selection and repeat the system selection process, depress the OFF button on the ECCP for non-CRT equipped vehicles or touch LEVL key on CRT monitor panel for CRT equipped vehicles.

Note. If directed here from TEST TYPE SELECTION, go to step 2) of either BCM SNAPSHOT or ECM SNAPSHOT.

BCM Snapshot

1. SNAPSHOT allows recall of system operating specifications present at exact time a BCM malfunction code was set. Up to 3 snapshots may be recalled. In addition, one snapshot may be triggered on demand by depressing FAN UP button (ECCP equipped) or touching YES key (CRT equipped) when TAKE BCM SNAPSHOT? is displayed. To enter snapshot: Enter self-diagnostics. See «ENTERING SELF-DIAGNOSTICS»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__entering-self-diagnostics). Display trouble codes stored in memory. See «DISPLAYING TROUBLE CODES»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__displaying-trouble-codes). Select BCM system for testing. See «SYSTEM SELECTION»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__system-selection). Select SNAPSHOT test type. See «TEST TYPE SELECTION»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__test-type-selection). NOTE: To cancel a system selection and repeat the system selection process, depress the OFF button on the ECCP for non-CRT equipped vehicles or touch LEVL key on CRT monitor panel for CRT equipped vehicles.
2. Immediately following selection of SNAPSHOT, system will display BXXX SNAPSHOT. B stands for BCM. XXX represents 3-digit trouble code stored in BCM snapshot. (Scheme 61), (Scheme 62), (Scheme 65) and (Scheme 66).
3. With BXXX displayed, depress FAN DOWN button (ECCP equipped) or touch NO key (CRT equipped) to scroll through list of BCM diagnostic codes for which BCM has stored a snapshot. See «DISPLAYING TROUBLE CODES»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__displaying-trouble-codes) for list of BCM trouble codes which may be present in snapshot. NOTE: A trouble code displayed during trouble code display cycle may not be present as a snapshot trouble code. In this case, exit SNAPSHOT by depressing OFF button (ECCP equipped) or touching LEVL key (CRT equipped). This will return display to next available system selection.
4. After display of last BCM trouble code for which a snapshot exists, pressing FAN DOWN button (ECCP equipped) or touching NO key (CRT equipped) will result in TAKE BCM SNAPSHOT? display. Responding to this display by depressing FAN DOWN button (ECCP equipped) or touching NO key (CRT equipped) will return display to first BXXX SNAP? display.
5. Responding to TAKE BCM SNAPSHOT? display by depressing FAN UP button (ECCP equipped) or touching YES key (CRT equipped) with SNAP DATA? or SNAP INPUTS? displayed will select that test type for display.
6. Display is now controlled as it would be for non-snapshot data and inputs display; however, all values and status information represents memorized vehicle conditions. See «SPECIFIC TEST SELECTION»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__specific-test-selection).
7. Depressing FAN UP key (ECCP equipped) or touching YES key (CRT equipped) again with TAKE BCM SNAPSHOT? displayed will change DIC to display SNAP TAKEN message to indicate new information has been stored in memory.

ECM Snapshot

1. ECM snapshot is slightly different from BCM snapshot because snapshot must be manually taken and then data and inputs can be reviewed. No previously stored information is recalled as in BCM snapshot. To enter ECM snapshot: Enter self-diagnostics. See «ENTERING SELF-DIAGNOSTICS»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__entering-self-diagnostics). Display trouble codes stored in memory. See «DISPLAYING TROUBLE CODES»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__displaying-trouble-codes). Select ECM system for testing. See «SYSTEM SELECTION»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__system-selection). Select SNAPSHOT test type. See «TEST TYPE SELECTION»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__test-type-selection). NOTE: To cancel a system selection and repeat the system selection process, depress the OFF button on the ECCP for non-CRT equipped vehicles or touch LEVL key on CRT monitor panel for CRT equipped vehicles.
2. Immediately following selection of SNAPSHOT, system will display message SNAPSHOT TAKEN. (Scheme 60)and (Scheme 64). Message appears for 3 seconds to indicate all ECM data and inputs have been stored in memory. Following this 3-second information storage verification period, display will automatically proceed to first available snapshot test type (i.e., E SNAP DATA or E SNAP INPUTS).
3. At this point, depressing FAN DOWN button (ECCP equipped) or touching NO key (CRT equipped) will display next available snapshot test type. Display may be toggled between E SNAP DATA and E SNAP INPUTS.
4. Depressing FAN UP button (ECCP equipped) or touching YES key (CRT equipped) with E SNAP DATA or E SNAP INPUTS displayed will select that particular test type. Display is now controlled as it would be for non-snapshot data and inputs displays; however, all values and status information represents memorized vehicle conditions. See «SPECIFIC TEST SELECTION»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__specific-test-selection).
5. Depressing FAN UP button (ECCP equipped) or touching YES key (CRT equipped) with ECM SNAPSHOT? displayed will again display SNAPSHOT TAKEN message to indicate new information has been stored in memory. This information can be accessed as previously described.

STATUS LIGHT INDICATORS

During self-diagnostics, the status light display on ECCP is used to indicate switchable parameter status. Each different mode of operation is indicated by its status light being turned on or off. Status light indicators are relative to system level being tested (ECM or BCM). (Scheme 60)and (Scheme 62).

ECM

The following are brief summaries of ECM status light indicators. (Scheme 60)

HIGH

Indicator light will be on when ECM is in closed loop fuel control. Light comes on only after coolant and O2 sensors reach operating temperature.

MED

Indicator light only indicates whether Torque Converter Clutch (TCC) is enabled (on) or disabled (off) by ECM. Actual TCC engagement depends on TCC system being in working order.

LOW

Indicator light stays on when oxygen sensor signals rich exhaust condition to ECM. Light will flash on and off during warm engine, steady throttle, and proper fuel mix condition.

OFF or VENT

Indicator light will be on when ECM commands 4th gear. Light should only be on in 4th gear.

Left-Side AUTO

Indicator light will be on when ECM commands 3rd gear. Light should only be on in 3rd or 4th gear.

°C

Indicator light will be on when ECM commands 2nd gear. Light should only be on in 2nd, 3rd or 4th gear.

Upper Arrow (Icon)

A/C compressor command upper arrow indicator light will be on only when ECM allows A/C clutch engagement through A/C relay. Light will also be on if BCM requests ECM to turn clutch on during test sequence. Light should be off if ECM commands disengagement. (Scheme 60)

BCM

The following are brief summaries of BCM system status light indicators. (Scheme 62)

Lower Arrow (Icon)

Indicator light will be on if BCM senses low refrigerant pressure switch is open. Light will remain off if A/C system is fully charged and operational; however, light will come on when outside vehicle ambient temperature is less than -5°F (-21°C). This is due to the temperature-pressure relationship of R-12 refrigerant. (Scheme 62)

Indicator light will be on when BCM commands air recirculation.

Indicator light will be on when BCM commands airflow out the A/C vents. Light should also be on in A/C and BI-LEV modes.

Indicator light will be on when BCM commands airflow out defroster outlets or when in defrost and A/C modes.

Indicator light will be on when BCM commands airflow out floor outlets. Light should be on in heater mode.

Indicator light will be on when BCM requests ECM to engage A/C compressor clutch.

Scheme 59

Scheme 60

Scheme 61

Scheme 62

Scheme 63

Scheme 64

Scheme 65

Scheme 66

TCC LOCK-UP PROBLEMS

Note. The following test procedures (part of ATRA TSB 046) apply to all GM lock-up transmissions. Tests address the problems of no lock-up and no lock-up release.

NO LOCK-UP CONDITION

1. Does TCC release with brake switch? If yes, go to step 3). If no, check brake switch. See «BRAKE SWITCH TEST»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__brake-switch-test). If brake switch is okay, check TCC apply solenoid. See «TCC APPLY SOLENOID TEST»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__tcc-apply-solenoid-test). If solenoid is okay, check for stuck lock-up apply valve or lock-up apply valve installed backward. Repair as necessary and retest.
2. If apply valve is okay, block lock-up apply valve in unlocked position. If TCC still does not release, problem is located in torque converter or oil pump. If lock-up does not occur, check appropriate signal oil circuit.
3. If TCC released in step 1), check for short to ground at terminal "D" wire of ALDL connector. (Scheme 55) If short is found, check apply solenoid. See «TCC APPLY SOLENOID TEST»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__tcc-apply-solenoid-test). If short is not found, check for computer system problem. See CHECKING SELF-DIAGNOSTIC SYSTEM.

BRAKE SWITCH TEST

1. Ensure fuse marked "gauges" is okay (most models). Check for 12 volts at both sides of brake switch(es). Some GM vehicles have 2 electric switches at brake pedal. One switch will have 4 wires and the other switch will have 2 wires and a vacuum hose. NOTE: Either of these switches can be used for lock-up. To avoid misdiagnosis, check both switches. If upper switch with vacuum hose is used, check 2 wires at that switch. On 4-wire lower switch, check 2 wires farthest from plunger.
2. Ground test light. DO NOT depress brake pedal. Turn ignition on. Check wires for voltage. Voltage should be present at both wires. Depress brake pedal and retest. Only one wire should have voltage.
3. If brake switch did not pass previous test, remove brake switch and repeat step 2), pushing and releasing brake switch plunger. If brake switch fails test, replace switch. If brake switch is okay, adjust switch as necessary.

CHECKING SELF-DIAGNOSTIC SYSTEM

Checking self-diagnostic system for trouble codes can help in diagnosing lock-up and/or no lock-up conditions. For testing procedures, see appropriate TESTING & DIAGNOSIS section.

TCC APPLY SOLENOID TEST

1. An analog ohmmeter and a 12 volt source will be needed to perform this test. Connect black lead of ohmmeter to Red wire on solenoid. Connect Red lead of ohmmeter to Black wire on solenoid. If solenoid is a one-wire solenoid, connect Red lead of ohmmeter to solenoid body.
2. With ohmmeter set at ohms times one, reading should be greater than 20 ohms but not infinite. Connect Red lead of ohmmeter to Red wire of solenoid, and Black lead to black wire or body of solenoid. Ohmmeter reading should be less than previous reading.
3. Connect a 12 volt source to solenoid. With very low pressure, try to blow through solenoid. Solenoid should be sealed. Disconnect 12 volt source. You should now be able to blow through solenoid. If solenoid does not test as described, replace solenoid.

NO TCC RELEASE CONDITION

1. Check for 12 volts to terminal "A" at transmission. See CHECKING VOLTAGE TO TERMINAL "A" AT TRANSMISSION. If voltage is present, go to step 3). If voltage is not present, ensure fuse marked "gauges" is okay (most models). If fuse is okay, go to next step. If only one side of fuse has voltage, replace fuse and check for shorts. If voltage is not found at fuse, check wire from fuse to ignition switch. Repair as necessary and retest.
2. If fuse was okay in previous step, check brake switch. See «BRAKE SWITCH TEST»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__brake-switch-test) . If brake switch is okay, check for open circuit in wire from terminal "A" to brake switch.
3. If voltage was present to terminal "A" of transmission, check for ground at terminal "D" at transmission. See CHECKING FOR GROUND AT TERMINAL "D" AT TRANSMISSION. Ground terminal "D" at transmission. See «GROUNDING "D" WIRE AT TRANSMISSION USING ALDL»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__grounding-d-wire-at-transmission-using) . If lock-up occurs, computer system problem exists. See CHECKING SELF-DIAGNOSTIC SYSTEM. If lock-up does not occur, check cooler line pressure. See «COOLER LINE PRESSURE TEST»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__cooler-line-pressure-test) .
4. If cooler line pressure is okay, internal problem exists in transmission. If cooler line pressure is not okay, check apply solenoid and electrical switches. See «TCC APPLY SOLENOID TEST»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__tcc-apply-solenoid-test) and «ELECTRICAL SWITCHES TEST»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__electrical-switches-test) . Repair as necessary. Check for stuck lock-up apply valve or apply valve installed backward. If all components are okay, check appropriate signal oil circuit and repair as necessary. Retest for lock-up.

CHECKING VOLTAGE TO TERMINAL "A" AT TRANSMISSION USING ALDL

Connect one end of test light to terminal "A" at ALDL. (Scheme 55) Connect other end to terminal "F" at ALDL. Turn ignition on. Test light should light. Some transmissions must be in 3rd gear before test light will light. If test light does not light, see CHECKING VOLTAGE TO TERMINAL "A" AT TRANSMISSION. If test light lights, 12 volts are present at terminal "A" at transmission. See CHECKING FOR GROUND AT TERMINAL "D" AT TRANSMISSION USING ALDL.

CHECKING VOLTAGE TO TERMINAL "A" AT TRANSMISSION

1. Check for 12 volts to terminal "A" at transmission. See «WIRING DIAGRAMS»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at) . Raise and support vehicle. Ground test light. Unplug connector at transmission case and install tip of test light into terminal "A". DO NOT depress brake pedal. On computer-controlled vehicles, turn ignition on. Test light should light.
2. On all other vehicles, start engine and run to normal operating temperature. Raise engine speed to 1500 RPM. Test light should light. If test light lights, see CHECKING FOR GROUND AT TERMINAL "D" AT TRANSMISSION. If test light does not light, ensure fuse marked "gauges" is okay (most models).

CHECKING FOR GROUND AT TERMINAL "D" AT TRANS. USING ALDL

1. Before performing this test, CHECKING VOLTAGE TO TERMINAL "A" OF TRANSMISSION USING ALDL test must pass. Otherwise, see CHECKING FOR GROUND AT TERMINAL "D" AT TRANSMISSION. Test light should be connected between terminals "A" and "F" at ALDL connector. (Scheme 55)With engine at normal operating temperature, road test vehicle. When road test is started, test light should light.
2. If brake pedal is applied, test light will not light. Watch test light to see if test light goes out during road test. If test light goes out, terminal "D" is grounded at transmission. See «GROUNDING "D" WIRE AT TRANSMISSION USING ALDL»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at__grounding-d-wire-at-transmission-using). If test light does not go out, a computer system problem exists. See CHECKING SELF-DIAGNOSTIC SYSTEM.

CHECKING FOR GROUND AT TERMINAL "D" AT TRANSMISSION

1. Check for ground at terminal "D" at transmission. See «WIRING DIAGRAMS»(/gmc/typhoon/i-1991-1993/remont/automatic-trans/#diagnosis-at) . On non-computer controlled vehicles, see COOLER LINE PRESSURE TEST. On all other vehicles, raise and support vehicle (if necessary). Disconnect transmission electrical connector and connect test light ground wire to terminal "A" of transmission connector. Install tip of test light into terminal "D" of transmission connector.
2. Start engine and run to normal operating temperature. Place gear selector in Drive (O.D. on 4 speed transmissions). Accelerate slowly to 60 MPH. Test light should light. If test light does not light, computer system problem exists. Shave insulation off of wire "D", or pierce wire near transmission connector.
3. Connect one end of jumper wire to wire "D" at shaved or pierced area. Ground other end of jumper wire. With vehicle on lift, road test for lock-up. If unsure lock-up occurred, hold steady speed of 60 MPH and slightly touch and release brake pedal. Disengagement and re-engagement should be felt. Seal bare wire connection with silicon after test is complete (if necessary).

COOLER LINE PRESSURE TEST

Check cooler line pressure or surge. Disconnect a transmission cooler line from radiator. Attach a hose to disconnected line. Insert other end of hose in fill tube of transmission. With drive wheels off ground, start engine. Grip hose firmly. Have a helper place gear selector in Drive and slowly accelerate to 60 MPH. When lock-up valve moves, hose should jump slightly.

ELECTRICAL SWITCHES TEST

Note. If CHECKING VOLTAGE TO TERMINAL "A" AT TRANSMISSION USING ALDL test passed, electrical switches are not causing a no lock-up condition. Check lock-up apply valve.

Single Terminal Switch (8642473) & 2 Terminal Switch

(8643710) - (Normally Open)

On single-terminal switch, connect one ohmmeter lead to switch terminal and other lead to body of switch. On 2-terminal switch, connect one lead to one terminal and other lead to other terminal. Ohmmeter should read infinite. Apply 60 psi of air to switch. Ohmmeter should read zero.

Single Terminal Switch (8642569 & 8634475) & 2 Terminal

Switch (8642346) (Normally Closed)

On single-terminal switch, connect one ohmmeter lead to switch terminal and other lead to body of switch. On 2 terminal switch, connect one lead to one terminal and other lead to other terminal. Ohmmeter should read zero. Apply 60 psi of air to switch. Ohmmeter should read infinite.

GROUNDING "D" WIRE AT TRANSMISSION USING ALDL

Before performing this test, CHECKING VOLTAGE TO TERMINAL "A" AT TRANSMISSION USING ALDL test must pass. Connect one end of test light or jumper wire to terminal "A" at ALDL connector. (Scheme 55) Road test vehicle. Vehicle can be tested on lift. At approximately 35 MPH, connect other end of test light or jumper wire to terminal "F" at ALDL connector. Torque converter should lock up. Proceed to COOLER LINE PRESSURE TEST.

SELF-DIAGNOSTIC SYSTEM CHECK REATTA & RIVIERA

The self-diagnostic system check is an organized approach for identifying a problem caused by on-vehicle computer controlled electronics. Understanding this chart and using it correctly will reduce diagnostic time, prevent unnecessary replacement of parts, and reduce comebacks. To review the basic information on how to use the computer self-diagnostics, go to beginning of this article.

Always Start Electrical Or Electronic Diagnosis Here

1. SERVICE ENGINE SOON telltale light at key ON confirms battery, ignition, and ground integrity to ECM and also verifies Ignition 1 power feed to IPC.
2. IPC odometer is primary display device for diagnosis and must be functional in order to use self-diagnostic system.
3. ECCP serves as access for onboard diagnosis and must be functional in order to use self-diagnostic system. A display other than standard temperature indicates fault that must be corrected before diagnostics can be entered.
4. After entering diagnostics, record all displayed trouble codes and note if they display as CURRENT or HISTORY. Codes displayed as current may be diagnosed using the procedures outlined in this article, but those codes displayed as HISTORY are not presently falling and may require visual inspection of circuitry to isolate.
5. Failure of SERVICE ENGINE SOON light to go out in the expected time period indicates ECM problem.
6. If no fault is found and driveability problems exist, see appropriate TESTS W/O CODES article in the ENGINE PERFORMANCE section for diagnosis by symptom.

Scheme 67

Scheme 68

The SERVICE ENGINE SOON light is attached to the IPC. It is powered by CPS circuit No. 439. PCM completes the ground to turn the light on. The light will be on while engine is not running. With the engine running, a steady light indicates a PCM code is set.

Note. Test numbers refer to numbers on diagnostic chart.

1. SERVICE ENGINE SOON light should be on.
2. Using a test light connected to battery voltage, probe each of the system ground circuits to be sure a good ground is present.

Diagnostic Aids

If engine runs okay, check

1. Faulty light bulb
2. PCM drive circuit open
3. 10-amp ignition fuse open

If engine cranks, but will not run

1. Continuous battery-fusible link open
2. Ignition feed to PCM open
3. Open fuse No. 5 in console relay center
4. Poor connection to PCM

Scheme 69

CHART A-2, SERVICE ENGINE SOON LIGHT WON'T GO OUT OR FLASH (REATTA & RIVIERA)

The SERVICE ENGINE SOON light is attached to the IPC. It is powered by CPS circuit No. 439. PCM completes the ground to turn the light on. The light will be on while engine is not running. With the engine running, a steady light indicates a PCM code is set.

Note. Test numbers refer to numbers on diagnostic chart.

1. If the light goes off when PCM connector is unplugged, PCM drive circuit is not shorted to ground.
2. If there is a problem with PCM that causes a "Scan" tester to not read serial data, PCM should not flash Code 12. IF Code 12 does flash, be sure that the tester is working properly by trying it on another vehicle. If tester is working properly and circuit No. 800 is okay, MEM-CAL or PCM or another component which uses the serial data line may be at fault for the NO ALDL problem.
3. This step will check for an open diagnostic circuit No. 451 (terminal "B" of ALDL).
4. At this point, SERVICE ENGINE SOON light wiring is okay. The problem is a faulty PCM or MEM-CAL. If Code 12 does not flash. PCM should be replaced using original MEM-CAL. Replace MEM-CAL only after trying a PCM, as a defective MEM-CAL is an unlikely cause of the problem.

Scheme 70

Scheme 71

Loss of IPC display and messages can result from the following

1. Loss of 12 volt power to IPC (circuit No. 812)
2. Loss of ground to IPC (circuit No. 803)
3. Internal fault in CPS or IPC. NOTE: Test numbers refer to numbers on diagnostic chart. This test checks the IPC power and grounds. Power, Ignition 3 and ground circuits to IPC are functioning properly; therefore, fault is either in terminal contacts or internal to the IPC. The 12-volt feed to the IPC is not functioning. Check for open in circuit No. 640.

Scheme 72

Scheme 73

IPC indicates loss of serial data communication by displaying ERROR in odometer and illuminating ELECTRICAL PROBLEM telltale light. Loss of serial data communications can occur for the following reasons

1. Short to ground somewhere in circuit No. 800
2. Short to voltage somewhere in circuit No. 800
3. Two opens in circuit No. 800
4. Internal fault in BCM or IPC
5. Open in ECCP ground circuit NOTE: Test numbers refer to numbers on diagnostic chart. This steps checks if BCM is awake and functioning. If it is not, use chart B-1 to isolate fault. This step checks if ECC head has lost data communication along with IPC. Code B556 indicates IPC EEPROM error. Odometer will display ERROR, but diagnostics will be accessible. Since IPC lost data communications, creating an open in data line at ALDL connector should remove data communications from ECCP as well. If not, IPC is receiving data but is unable to communicate due to poor connection or internal problem. This test indicates data line shorted to ground. This test indicates data line functioning properly, but with multiple opens isolating IPC and ECCP from BCM. This test indicates data line shorted to voltage.

Scheme 74

CHART B-1, BCM POWER & GROUNDS - REATTA & RIVIERA

The loss of BCM functions can result from the following

1. Loss of 12-volt power on circuit No. 640 to BCM
2. Loss of ground on circuit No. 803 to BCM
3. Internal BCM fault NOTE: Test numbers refer to numbers on diagnostic chart. BCM is not receiving 12-volt input. Check power and ground supply to BCM. This step checks for short to ground in any part of circuit No. 640. Checks to see if the short to ground is inside a component or in the circuit itself.

Scheme 75

CHART C, ECCP DISPLAY PROBLEMS (LOSS OF ECCP COMMUNICATIONS) (REATTA & RIVIERA)

When ECC communications are lost, service diagnostics usually cannot be entered, but a quick scan of panel will usually help isolate source of fault. Loss of serial data will result in 3 dashes being shown on the panel VF display. This indicates power and ground are okay. If panel displays normal exterior or interior set temperature, power, ground and serial data communications are being received and panel replacement is indicated.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks for power and ground to panel. Voltage should be HI for both power to ground checks.
2. This step checks for serial data voltage. Normal voltage should be more than one volt and varying. Steady voltage indicates second open exists, or ALDL cover is off.

Scheme 76

Scheme 77

With grounded serial data circuit, BCM will not be able to communicate with any other devices in system. Voltage measured anywhere in circuit will zero. IPC will display ELECTRICAL PROBLEM; ECCP will display dashes ("---").

Note. Test numbers refers to test number on diagnostic chart.

1. This step splits the system in half to help isolate source of grounded serial data line. If taking IPC out of the system allows both halves of serial data line to rise more than one volt, source of short has been located. The remaining steps follow same pattern in that components are removed from serial data line until normal voltage range is again measured. When section of circuit is isolated, a simple check of wire for short to ground will differentiate between grounded wire or ground in a device remote.

Scheme 78

CHART D-2, SERIAL DATA CIRCUIT SHORTED TO VOLTAGE - REATTA & RIVIERA

With a shorted serial data circuit, BCM will not be able to communicate with other devices in system. Voltage measured any where in circuit will be more than 5 volts. IPC display showing only ELECTRICAL PROBLEM. (Scheme 77)

Note. Test number refers to test number on diagnostic chart.

1. This step splits system in half to help isolate the source of short in the serial data line. If taking ECM out of system allows both halves of serial data line to fall below 5 volts, source of short was in IPC. The remaining steps follow the same pattern in that components are removed from serial data line until normal voltage range is again measured.

Scheme 79

Scheme 80

Code B556 indicates that EEPROM, which records elapsed odometer mileage, is not being read by BCM. Usually along with Code 556, ERROR will be displayed in the odometer display. The EEPROM contains specific vehicle information such as vehicle ID number, season odometer mileage and certain vehicle options. Since this information is programmed for the specific vehicle in which it is installed, and EEPROM cannot be transferred from one vehicle in which it is installed, an EEPROM cannot be transferred from one vehicle to another. Check for proper EEPROM installation. If the EEPROM is properly installed (no bent pins) but Code B556 persists, replace EEPROM. Obtain replacement EEPROMs through an authorized Delco Service Center.

Scheme 81

The SERVICE ENGINE SOON light is located in IPC. Light is powered by Ignition 1, circuit No. 39 and PCM completes system ground to turn light on. With ignition on and engine not running, light should be on. If with engine running, light is on steady, this indicates a fault code is set.

Note. Test numbers refer to numbers on diagnostic chart.

1. SERVICE ENGINE SOON light should be on.
2. Using a test light connected to battery voltage, probe each of the system ground circuits to be sure a good ground is present.

If engine runs okay, check

1. Faulty light bulb
2. PCM drive circuit open
3. 15-amp BODY IGN fuse open.

If engine cranks, but will not run

1. Continuous battery-fusible link open
2. Ignition feed to PCM open
3. Open fuse No. 7 bad
4. Poor connection to PCM

Scheme 82

CHART A-2, SERVICE ENGINE SOON LIGHT WILL NOT GO OUT OR FLASH - TORONADO & TROFEO

The SERVICE ENGINE SOON light is attached to the IPC. It is powered by circuit No. 39. PCM completes the ground to turn the light on. The light will be on while engine is not running. With the engine running, a steady light indicates a fault code is set. (Scheme 81)

Note. Test numbers refer to numbers on diagnostic chart.

1. If the light goes off when PCM connector is unplugged, PCM drive circuit is not shorted to ground.
2. If there is a problem with PCM that causes a "Scan" tester to not read serial data, PCM should not flash Code 12. If Code 12 does flash, be sure that the tester is working properly by trying it on another vehicle. If tester is working properly and circuit No. 800 is okay, MEM-CAL or PCM or another component which uses the serial data line may be at fault for the NO ALDL problem.
3. This step will check for an open diagnostic circuit No. 451 (terminal "B" of ALDL).
4. At this point, SERVICE ENGINE SOON light wiring is okay. The problem is a faulty PCM or MEM-CAL. If Code 12 does not flash, PCM should be replaced using original MEM-CAL. Replace MEM-CAL only after trying a PCM, as a defective MEM-CAL is an unlikely cause of the problem.

Scheme 83

SELF-DIAGNOSTIC SYSTEM CHECK (NON-CRT TORONADO & TROFEO)

The Self-Diagnostic System Check is an organized approach for identifying problems caused by on-vehicle computer controlled electronics. Understanding this chart and using it correctly will reduce diagnostic time and prevent unnecessary replacement of components. If you have not reviewed the basic information on how to use computer self-diagnostics, go to beginning of this article.

Scheme 84

Scheme 85

Loss of IPC display and messages can result from following

1. Loss of 12 volts to IPC
2. Loss of ground to IPC
3. Loss of wake-up signal to BCM
4. Internal fault in BCM or IPC NOTE: Test numbers refer to numbers on diagnostic chart. This step checks to see if BCM is awake. If BCM is not awake, cause of blank display is power and/or ground related. This step checks BCM power and ground sources. This step checks to see if IPC has lost power or ground. Normally with ignition on, battery voltage should be present at listed terminals. This step checks for short to ground in any of the 640 circuits. This step checks if short to ground is inside component or in circuit. This step checks if IPC is receiving 7 volts from BCM.

Scheme 86

Scheme 87

IPC indicates loss of serial data communication by displaying the message ELECTRICAL PROBLEM. All other segments are blank. If display is fully illuminated and ELECTRICAL PROBLEM message is displayed, enter diagnostics and proceed to appropriate code chart. DO NOT use CHART A. Loss of serial data communications can occur for the following reasons

1. Short to ground anywhere in circuit No. 800
2. Short to voltage anywhere in circuit No. 800
3. Two opens in circuit No. 800
4. Internal BCM or IPC fault
5. Open in ECCP ground circuit or IGN 3 circuit NOTE: Test numbers refer to numbers on diagnostic chart. This step checks to see if ECCP has lost data communication along with IPC. Since IPC has lost data communications, shorting terminal "M" at ALDL connector should NOT remove data communications from ECCP. If this does happen, IPC is receiving data but is unable to communicate due to poor connection or an internal problem.

Scheme 88

CHART C, LOSS OF ECCP COMMUNICATIONS (NON-CRT EQUIPPED TORONADO & TROFEO)

When ECCP communications are lost, service diagnostics usually cannot be entered, but a quick scan of panel will usually help isolate source of fault. ECCP is powered by ignition. If power source is lost, IPC will usually display ELECTRICAL PROBLEM. Loss of serial data will result in 3 dashes being shown on panel VF display. This indicates power and ground are okay. If the panel displays normal outside or interior set temperature, power, ground and serial communications are being received, indicating need to replace panel.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks for power and ground to panel. Test light should be on for both power to ground checks.
2. This step checks for serial data voltage. Normal voltage should be above one volt and varying.

Scheme 89

CHART D, LOSS OF COMMUNICATIONS (NON-CRT EQUIPPED TORONADO & TROFEO)

Note. Test numbers refer to numbers on diagnostic chart.

1. Zero volts indicate an open or grounded data line.
2. This step checks for short to ground on data line
3. Since ECCP displays were lost, an open must exist between BCM and ECCP. This step determines which side of PCM open is on.

Scheme 90

Scheme 91

With grounded serial data circuit, BCM will not be able to communicate with any other devices in system. Voltage measured anywhere along circuit will be zero. Display on IPC will be ELECTRICAL PROBLEM and ECCP will display dashes "---".

Note. Test number refers to test number no diagnostic chart.

1. This step splits system in half to help isolate the source of grounded serial data line. If taking PCM out of system allows both halves of serial data line to rise above one volt, the source of short has been located. The remaining steps of chart follow same pattern in that the components are removed from serial data line until normal voltage range is again measured. When section of circuit is isolated, a simple check of wire for short to ground differentiates between a grounded wire or a ground in the device.

Scheme 92

CHART D-2, SERIAL DATA CIRCUIT SHORTED TO VOLTAGE (NON-CRT EQUIPPED TORONADO & TROFEO)

With shorted serial data circuit BCM will not be able to communicate with other devices in system. Voltage measured anywhere along circuit will be above 5 volts. IPC will display ELECTRICAL PROBLEM.

Note. Test numbers refers to test number on diagnostic chart.

1. This step splits system in half to help isolate source of short in serial data line. If taking PCM out of system allows both halves of serial data line to fall below 5 volts, source of short was in PCM. The remaining steps follow the same pattern in that components are removed from serial data line until normal voltage range is again measured.

Scheme 93

Scheme 94

The Self-Diagnostic System Check is organized approach for identifying problems caused by on-vehicle computer controlled electronics. Understanding this chart and using it correctly will reduce diagnostic time and prevent unnecessary replacement of components. If you have not reviewed the basic information on how to use computer self-diagnostics, go to beginning of this article.

Scheme 95

Scheme 96

Loss of IPC display and messages can result from the following

1. Loss of 12 volts to IPC
2. Loss of ground to IPC
3. Loss of wake-up signal from BCM
4. Internal BCM or IPC fault NOTE: Test numbers refer to numbers on diagnostic chart. This step checks to see if BCM is awake. If not awake, the cause of blank display is power and ground related. This step checks BCM power and ground sources. This step checks to see which BCM has lost, power or ground. This step checks for ignition source, BCM power and ground to IPC. With ignition on, battery voltage should be present at listed terminals. Checks for short to ground in any of the 640 circuits. This step checks to see if short to ground is inside a component or in circuit itself. This step checks to see if IPC is receiving 7 volts from BCM.

Scheme 97

Scheme 98

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks power supplies to CRT and CRT controller.
2. This step determines if CRT system is awake and operating.
3. This step checks voltage on data line from CRT to CRT controller.
4. This step checks if CRT or CRT controller is causing the low voltage reading on data line.

Scheme 99

CHART C, COLOR CRT UNABLE TO CONTROL ECCP (CRT EQUIPPED TORONADO & TROFEO)

IPC indicates loss of serial data communication by displaying the message ELECTRICAL PROBLEM. All other segments are blank. If display is fully illuminated and ELECTRICAL PROBLEM message is displayed, enter diagnostics and proceed to appropriate code chart. DO NOT use CHART A. (Scheme 98)

Loss of serial data communications can occur for the following reasons

1. Short to ground anywhere in circuit No. 800
2. Short to voltage anywhere in circuit No. 800
3. Two opens in circuit No. 800
4. Internal BCM or IPC fault NOTE: Test numbers refer to numbers on diagnostic chart. If climate control page is accessible, CRTC is receiving ignition power and should be capable of communicating with BCM. This step checks to see if CRTC has lost communications with BCM. This step checks to see if IPC has lost communication with BCM also. This step checks continuity of the data line (circuit No. 800). Since data line is a loop, resistance around it should be close to zero ohms. This step checks for short to ground on data line. This step checks voltage on data line under normal operating conditions. Normal data line voltage is a continual fluctuating between 1.0 and 5.0 volts. Voltage above 5.0 volts or non-fluctuating voltage indicates a problem in data line causing entire communications network to be inoperative.

Scheme 100

CHART D, SERIAL DATA CIRCUIT OPEN (CRT EQUIPPED TORONADO & TROFEO)

Serial data line is redundant at most of the devices it communicates with. Only one line need be connected to each device for that system to function. When making repairs, both lines should be made operational. If when ALDL cover is removed the system shuts down, a single open existed before. Now a second open in circuit is being created by removing ALDL cover. (Scheme 98)

Note. Test number refers to test number on diagnostic chart.

1. This test and all subsequent tests are designed to subdivide data line. This is necessary to pinpoint location of open in data line. This is not the only way to pinpoint an open, but in this case, will most quickly lead to the open.

Scheme 101

CHART D-1, GROUNDED SERIAL DATA CIRCUIT (CRT EQUIPPED TORONADO & TROFEO)

With grounded serial data circuit, BCM will not be able to communicate with any other devices in system. Voltage measured anywhere in circuit will be zero. IPC display will show ELECTRICAL PROBLEM, CRT will display normally and HVAC controls will not function. (Scheme 98)

Note. Test number refers to test number on diagnostic chart.

1. This step splits the system in half to help isolate source of grounded serial data line. If taking PCM out of the system allows both halves of serial data line to rise above one volt, source of short has been located. The remaining steps follow same pattern in that components are removed from serial data line until normal voltage range is again measured. When section of circuit is isolated, a simple check of wire for short to ground will differentiate between grounded wire or ground in a device remote.

Scheme 102

Scheme 103

With a shorted serial data circuit, BCM will not be able to communicate with other devices in system. Voltage measured anywhere in circuit will be above 5 volts. IPC display showing only ELECTRICAL PROBLEM.

Note. Test number refers to test number on diagnostic chart.

1. This step splits system in half to help isolate the source of short in the serial data line. If taking PCM out of system allows both halves of serial data line to fall below 5 volts, source of short was in PCM.

The remaining steps follow the same pattern in that components are removed from serial data line until normal voltage range is again measured.

Scheme 104

CHART E, COLOR CRT ABNORMALITIES (CRT EQUIPPED TORONADO & TROFEO)

Note. If you have not performed the SELF-DIAGNOSTIC SYSTEM CHECK, start there first.

Scheme 105

Scheme 106

The gear selector switch (or PRN D D21) is mounted on transaxle assembly. It is a multi-signal switch sending information relative to gear selector position to BCM and PCM. The PRN D D21 switch assembly also contains the neutral safety switch. The PRN D D21 switch uses 4 discrete circuits to pull 4 BCM voltages low in various combinations to indicate each gear range. The voltage level of each of circuits is represented in BD41 display as "0" equals grounded and "1" equals not grounded. The 4 digits displayed represent decoder "A", "B", "C" and parity inputs in sequence.

Switch status can be seen by monitoring BD41 or ED31 data parameter, where switch status is displayed as a 4-digit binary number. Code B127 will set if following conditions

1. BCM PRN D D21 switch data input indicates park/neutral, but PCM input does not
2. BCM PRN D D21 switch data input does not indicate park/neutral, but PCM input does
3. Decoder and parity data inputs do not agree with possible combinations per BD41 display values found in DATA DISPLAY in introduction portion of this article.

If Code B127 is set, Driver Information Center (DIC) will display warning message GEAR SELECT FAULT and IPC PRN D D21 display will flash.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks BCM data inputs when only decoder "A" and parity circuit are pulled low.
2. Checks BCM data inputs when only decoder "B" and "C" inputs are pulled low. This will complete HI/LO check of all 4 BCM inputs.
3. This step is to check gear selector switch problem in other gear ranges.
4. Checks to see if code is due to discrepancy between PCM and BCM inputs.

Note On Intermittents

Manipulate related PCM and BCM wiring in each gear range while looking for a change in ED31 or BD41 display values to check for intermittent open or short to ground. Also check terminals for good contact at PCM, BCM and PRN D D21 switch. If good contact exists and Code B127 continues to set, replace PRN D D21 switch.

Scheme 107

Note. Test numbers refer to numbers on diagnostic chart.

Scheme 108

1. Checks for short to ground in gear selector switch to BCM circuits.
2. Checks for open in circuits to BCM.
3. Circuit shorted to ground will result in a "0" for that circuit.
4. Checks for open PRN D D21 sensor ground.
5. Circuit with open will result in a "1" for that circuit.
6. The snap data value of DB41 display will be the value that BCM recognized when code was set. This is used to determine whether this value is valid. If it is a valid, PCM park/neutral input must be incorrect to set code.

Scheme 109

CODE 23, MAT SENSOR SIGNAL VOLTAGE HIGH

CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if code is the result of a hard failure or an intermittent condition. Code 23 will set if engine has been running for a precalibrated period of time, has reached operating temperature and signal voltage indicates a MAT temperature less than -22°F (-30°C).
2. This simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the control module and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits. If ground circuit is shared by other sensors and ground circuit is open, accompanying codes related to those sensors may be present.

If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a "Scan" tester. A Code 23 will result if signal and ground circuits become open. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.

Scheme 110

TEMPERATURE-TO-RESISTANCE VALUES

Scheme 111

Scheme 112

Scheme 113

Scheme 114

Scheme 115

Scheme 116

Scheme 117

Scheme 118

Scheme 119

Scheme 120

Scheme 121

Scheme 122

The speed sensor, which is a Permanent Magnet (PM) generator, provides the control module with vehicle speed information. The PM generator, mounted on the transmission, produces a pulsing voltage signal whenever the vehicle speed is more than 3 MPH. The voltage level and pulses increase with vehicle speed. The control module converts the pulsing voltage to MPH, which is used by the control module in calculations to determine vehicle adjustments.

Note. Test numbers refer to test numbers on diagnostic chart.

1. A Code 24 will set when MPH reads zero, transmission is not in Park or Neutral, engine speed indicates vehicle is in a cruise mode (1200-4400) RPM, TPS indicates closed throttle and high manifold vacuum is sensed by the MAP sensor. All of these conditions must be met for 2-5 seconds. The PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
2. Before replacing the control module, PROM/MEM-CAL should be checked for correct application.

A faulty or misadjusted park/neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter. Code 24 may set if vehicle is power braked (brakes applied and throttle depressed) for more than 10 seconds.

Scheme 123

Scheme 124

Note. Test number refers to test number on diagnostic chart.

1. Code 24 will set if vehicle is less than 2 MPH with engine speed at 1125-4400 RPM, transmission is not in Park or Neutral, MAP indicates a road load deceleration condition and these conditions have been met for 5 seconds.

A faulty or misadjusted park/neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter.

Scheme 125

Scheme 126

Scheme 127

Scheme 128

Scheme 129

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 24 will set if VSS signal circuit voltage is constant, engine speed is 1200-4400 RPM. TPS indicates throttle is near idle position, transmission is not in Park or Neutral and all conditions have been met for at least 4 seconds.
2. A voltage of less than one volt at the instrument panel connector indicates that VSS signal circuit wire may be shorted to ground. Disconnect signal wire at the VSS buffer. If voltage remains less than 10 volts, circuit is shorted to ground or open. If circuit is not open or shorted to ground, check for poor control module connection or faulty control module.

A faulty or misadjusted park/neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter.

Scheme 130

Scheme 131

Vehicle speed values are provided by vehicle speed sensor (VSS). Sensor is a permanent magnet (PM) generator mounted on differential areas of transmission converter housing. PM generator produces a pulsing voltage whenever vehicle speed above 3 MPH. These pulses occur 16 times per each revolution of differential housing and are translated to provide vehicle speed in miles per hour.

Code 24 will set if VSS indication is less than 1 MPH, engine speed is above idle, MAP indication is less tan .5 volts or vehicle is not in park or neutral.

When attempting to diagnose an intermittent problem, use a Scan tester to review diagnostic information. Check connectors for poor connections. Speedometer will not work if VSS subsystem has hailed. If VSS reading is accurate and speedometer is incorrect, problem may exist in instrument cluster.

Scheme 132

Scheme 133

CODE 25 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if the code is a hard failure or an intermittent condition. Code 25 will set if a MAT temperature greater than 266°F (130°C) is sensed for more than a precalibrated period.
2. This simulates condition for Code 23, if control module recognizes the open circuit and "Scan" tester displays a temperature of less than -30°C, control module and wiring are okay.

If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor should read close to each other, when measured with a "Scan" tester. A Code 25 will result if sensor signal circuit is shorted to ground. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.

Scheme 134

CODE 26, DIAGNOSTIC CIRCUIT CHECK

The Diagnostic Circuit Check determines

1. If SERVICE ENGINE SOON light works
2. If ECM is operating and can recognize a fault
3. If any codes are stored Check operation of SERVICE ENGINE SOON light. Turn ignition on with engine off. SERVICE ENGINE SOON light should be on steady. If light illuminates and stays on steady, go to next step. If light does not illuminate, go to SERVICE ENGINE SOON LIGHT INOPERATIVE. If light flashes, go to step 3. Grounding ALDL "test" terminal "B" at this time should cause SERVICE ENGINE SOON light to flash a Code 12, followed by any codes stored in ECM memory. If light foes from bright to dim, this is not considered a code. If light dims or remains on, and does not flash Code, go to SERVICE ENGINE SOON LIGHT WON'T GO OUT OR FLASH CODE 12. If light begins to flash as soon as ignition is turned on, check for a short to ground on the diagnostic test terminal wire between ALDL terminal "B" and ECM terminal No. 5. If circuit is okay, replace ECM.

SERVICE ENGINE SOON LIGHT INOPERATIVE

1. If SERVICE ENGINE SOON light does not illuminate with ignition on and engine off, attempt to start engine. If engine starts, go to step 3. If engine does not start, check fusible links at battery and ECM fuse. If fusible links or ECM fuse are blown, repair short to ground.
2. If fusible links and ECM fuse are okay, turn ignition on and check power circuits to ECM, including keep alive memory and ignition feed. If power is not available to power terminals of ECM, check for opens in power circuits. If power is available to ECM power terminals, check for poor ECM ground circuits, or replace faulty ECM.
3. If engine starts and SERVICE ENGINE SOON light does not illuminate, turn ignition off. Disconnect ECM connectors. Turn ignition on and jumper ECM SERVICE ENGINE SOON light driver terminal to ground using a test light.
4. If light is now on, repair light driver terminal connections at ECM or replace faulty ECM. If light stays off when test light is used to ground light driver terminal, check for blown instrument panel fuse, faulty bulb, open in light driver circuit between ECM and bulb, driver circuit shorted to voltage, or an open in the ignition feed to the SERVICE ENGINE SOON light.

SERVICE ENGINE SOON LIGHT WON'T GO OUT OR FLASH CODE 12

1. Turn ignition off. Disconnect ECM connectors. Turn ignition on. If SERVICE ENGINE SOON light is on, check for short to ground in light driver circuit between light and ECM driver terminal.
2. If light is off with ECM connectors disconnected, turn ignition off. Reconnect ECM connectors. Turn ignition on with engine off. Using a DVOM, check voltage at ALDL "test" terminal "B". If voltage is greater than 9 volts, check for a short to voltage on ALDL terminal "B" wire between ECM and ALDL connector. If voltage is 5-6 volts, proceed to next step. If voltage is less than 5 volts, backprobe appropriate ECM terminal with DVOM. If 5-6 volts is now present, repair open or short in wire between ECM and ALDL terminal "B".
3. If voltage at terminal "B" of ALDL connector is 5-6 volts, jumper that wire terminal at ECM to ground. If SERVICE ENGINE SOON light flashes a Code 12, and terminal "A" of ALDL was used when grounding terminal "B" the first time, check for open between ALDL connector terminal "A" and ground. If SERVICE ENGINE SOON light does not flash when ECM end of terminal "B" wire is jumpered to ground, check PROM/MEM-CAL for proper installation. If installed correctly, replace ECM, using original PROM/MEM-CAL. Repeat diagnostic circuit check. If Code 12 still does not flash, replace PROM/MEM-CAL. Replace PROM/MEM-CAL only after replacing ECM, as PROM/MEM-CAL is not likely to be faulty.

Scheme 135

Each ECM Quad Driver has a fault line which is monitored by the ECM. The ECM compares voltage values of the fault line with acceptable values in ECM memory. If the ECM senses other than accepted values, a Code 26 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. The ECM does not know which controlled circuit set the Code 26 so this chart will go through each circuit to determine which is at fault. On vehicles equipped with an ECM controlled hot light, this test check hot light driver and hot light circuit.
2. QDM SYMPTOMS TCC inoperative, Code 39 Hot light on all the time, off during bulb check Cooling fans on low speed all of the time or will not come on at all Poor driveability due to 100 percent canister purge

The coolant temperature sensor, in rare cases, may fail to indicate the correct coolant temperature without setting a malfunction code (Code 14 or 15). This could result in turning on the hot light without having an overheating condition. It could also result in engine overheating without turning on the hot light. Check coolant sensor temperature-to-resistance values.

Hot Light Diagnosis

Note. These checks assume vehicle is not overheating. Verify proper operation of cooling system prior to diagnosing hot light.

Hot light is powered by the 10-amp INDIC fuse. Light will turn on when ECM provides a ground for the circuit. If circuit grounds between light and ECM, light will illuminate any time the ignition is turned on.

1. Turn ignition on with engine off (bulb test position). If hot light illuminates, go to step 3. If hot light does not illuminate, check the following: 10-amp INDIC fuse Faulty instrument cluster bulb Open circuit between fuse and hot light
2. Backprobe terminal B4 at ECM with a test light to battery voltage. Turn ignition on. If test light does not illuminate. ECM terminal connection is bad or ECM is faulty. If test light illuminates, turn ignition off. Disconnect ECM connectors. Jumper terminal B4 to ground. turn ignition on. If hot light does not illuminate, check for open circuit between hot light and ECM. If light does not illuminate and all circuits are intact and power is available to light, instrument cluster must be replaced.
3. Start engine. If test light goes off, no problem is evident. See DIAGNOSTIC AIDS. If test light is on, turn ignition off. Disconnect ECM connector. Probe ECM harness terminal B4 with a test light to battery voltage. If light is off, replace ECM. IF light is on, repair short to ground in circuit No. 35. If no short is present, replace instrument cluster.

Scheme 136

A) (3.8L W Body With ECM-Controlled Temp Light) This determines which circuit is out of specification.

Monitor the voltage of each terminal while moving related harness connectors, including ECM harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace ECM.

Scheme 137

B) (3.8L W Body Without ECM-Controlled Temp Light) This determines which circuit is out of specification. If all circuits check out okay, the in-line resistor (used in place of the hot light) and related wiring should be checked. In-line resistor is taped into engine harness between right power center and ECM pigtail, about 2 inches from ECM pigtail junction.

Monitor the voltage of each terminal while moving related harness connectors, including ECM harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace ECM.

Scheme 138

1. This determines if the problem is the circuit or the component. Factory-installed ECM has an internal fuse and it's unlikely ECM needs replacing.

Scheme 139

Scheme 140

Each ECM Quad-Driver has a fault line which is monitored by the ECM. The ECM compares voltage values of the fault line with acceptable values in ECM memory. If the ECM senses other than accepted values, a Code 26 will set.

Some QDM circuits will normally cycle between high and low, such as depressing the brake pedal, cooling fans cycling on and off. QDM 2 will not set a Code 26. Some "Scan" testers may set a false Code 26 if engine is running, tester is installed and brake pedal is depressed for more than 30 seconds.

Note. Test numbers refer to test numbers on diagnostic charts.

1. The ECM does not know which controlled circuit set Code 26 so this chart will go through each circuit to determine which is at fault. On vehicles equipped with an ECM controlled hot light, this test checks hot light driver and hot light circuit.
2. QDM SYMPTOMS: TCC inoperative, Code 39 EGR inoperative, Codes 63, 64 and 65 Hot light on all the time, off during bulb check Cooling fan on low speed all of the time or will not come on at all Poor driveability due to 100 percent canister purge

The coolant temperature sensor, in rare cases, may fail to indicate the correct coolant temperature without setting a malfunction code (Code 14 or 15). This could result in turning on the hot light without having an overheating condition. It could also result in engine overheating without turning on the hot light. Check coolant sensor temperature-to-resistance values in SENSOR RANGE CHARTS article.

Note. These checks assume vehicle is not actually overheating. Verify proper operation of cooling system prior to diagnosing hot light.

Hot light is powered by the 10-amp INDIC fuse. Light will turn on when ECM provides a ground for the circuit. If circuit grounds between light and ECM, light will illuminate any time the ignition is turned on.

1. Turn ignition on with engine off (bulb test position). If hot light illuminates, go to step 3. If hot light toes not illuminate, check the following: 10-amp INDIC fuse Faulty instrument cluster bulb Open circuit between fuse and hot light
2. Backprobe terminal YC9 at ECM with a test light to battery voltage. Turn ignition on. If test light does not illuminate, ECM terminal connection is bad or ECM is faulty. If test light illuminates, turn ignition off. Disconnect ECM connectors. Jumper terminal YC9 to ground. Turn ignition on. If hot light does not illuminate, check for open circuit between hot light and ECM. If light does not illuminate and all circuits are intact and power is available to light, instrument cluster must be replaced.
3. Start engine. If test light goes off, no problem is evident. See DIAGNOSTIC AIDS. If test light is on, turn ignition off. Disconnect ECM connector. Probe ECM harness terminal TC9 with a test light to battery voltage. If light is off, replace ECM. If light is on, repair short to ground in circuit No. 35. If no short is present, replace instrument cluster.

Scheme 141

This determines which circuit is out of specification. All circuits except SERVICE ENGINE SOON light and hot light should have battery voltage with engine running and ALDL test terminal "B" not grounded.

Monitor the voltage of each terminal while moving related harness connectors, including control module harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace control module.

Scheme 142

1. This determines if the problem is the circuit or the component. Factory-installing ECM has an internal fuse and it's unlikely ECM needs replacing.

Scheme 143

Scheme 144

Scheme 145

Each ECM Quad-Driver has a fault line which is monitored by the ECM. The ECM compares voltage values of the fault line with acceptable values in ECM memory. If the ECM senses other than accepted values, a Code 26 will set. QDM B will not set a Code 26.

Note. Test numbers refer to test numbers on diagnostic charts.

1. The ECM does not know which controlled circuit set the Code 26. This tests SERVICE ENGINE SOON light driver and circuit.
2. QDM SYMPTOMS TCC inoperative Hot light on all the time, off during bulb check Cooling fan on all the time or will not come on at all Poor driveability due to 100 percent canister purge

The coolant temperature sensor, in rare cases, may fail to indicate the correct coolant temperature without setting a malfunction code (Code 14 or 15). This could result in turning on the hot light without having an overheating condition. It could also result in engine overheating without turning on the hot light.

Note. These checks assume vehicle is not overheating. Verify proper operation of cooling system prior to diagnosing hot light.

Hot light is powered by the 10-amp GAGES fuse. Light will turn on when ECM provides a ground for the circuit. If circuit grounds between light and ECM, light will illuminate any time the ignition is turned on.

1. Turn ignition on with engine off (bulb test position). If hot light illuminates, go to step 3. If hot light toes not illuminate, check the following: 10-amp GAGES fuse Faulty instrument cluster bulb Open circuit between fuse and hot light
2. Backprobe terminal YE2 (A body) or YF2 (N body) at ECM with a test light to battery voltage. Turn ignition on. If test light does not illuminate, ECM terminal connection is bad or ECM is faulty. If test light illuminates, turn ignition off. Disconnect ECM connectors. Jumper terminal YE2 (A body) or YF2 (N body) to ground. Turn ignition on. If hot light does not illuminate, check for open circuit between hot light and ECM. If light does not illuminate and all circuits are intact and power is available to light, instrument cluster must be replaced.
3. Start engine. If test light goes off, no problem is evident. See DIAGNOSTIC AIDS. If test light is on, turn ignition off. Disconnect ECM connector. Probe ECM harness terminal YE2 (A body) or YF2 (N body) with a test light to battery voltage. If light is off, replace ECM. If light is on, repair short to ground in circuit No. 35. If no short is present, replace instrument cluster.

Scheme 146

On vehicle with a 3T40 transmission, QDM A fault status on "Scan" tester should read HIGH until 2nd gear switch is closed or brake is applied. To simulate driving in 2nd gear and change status to LOW, disconnect TCC connector and connect test light between harness terminals A and D.

Note. Test number refers to test number on diagnostic chart.

1. This step determines which circuit is out of specification.

Monitor the voltage of each terminal while moving related harness connectors, including control module harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace control module.

Scheme 147

Note. Test number refers to test number on diagnostic chart.

1. This step determines which circuit is out of specification.

Monitor the voltage of each terminal while moving related harness connectors, including control module harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace control module.

Scheme 148

Note. Test numbers refer to test numbers on diagnostic chart.

1. This step determines if the problem is the circuit or the component. Factory-installed ECM has an internal fuse and it's unlikely ECM needs replacing.

Scheme 149

Scheme 150

Scheme 151

Scheme 152

Each PCM Quad-Driver has a fault line which is monitored by the PCM. The PCM compares voltage values of the fault line with acceptable values in PCM memory. If the PCM senses other than accepted values, a Code 26 will set.

Some QDM circuits will normally cycle between high and low, such as depressing the brake pedal. Some "Scan" testers may set a false Code 26 if engine is running, tester is installed and brake pedal is depressed for more than 30 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. The PCM does not know which controlled circuit set the Code 26 so this chart will go through each of the circuits to determine which is at fault. This step tests SERVICE ENGINE SOON light driver and circuit.

QDM CIRCUITS

1. Hot light (C body) on all the time, off during bulb check
2. Cooling fan on low speed all of the time or will not come on at all
3. Poor driveability due to 100 percent canister purge

The Coolant temperature sensor, in rare cases, may fail to indicate the correct coolant temperature without setting a malfunction code (Code 14 or 15). This could result in turning on the hot light without having an overheating condition. It could also result in engine overheating without turning on the hot light.

Hot Light Diagnosis (C Body)

Note. These checks assume vehicle is not overheating. Verify proper operation of cooling system prior to diagnosing hot light.

Hot light is powered by a 15 or 20-amp fuse No. 6 (depending upon instrument cluster option). Light will turn on when PCM provides a ground for the circuit. If circuit grounds between light and PCM, light will illuminate any time the ignition is turned on.

1. Turn ignition on with engine off (bulb test position). If hot light illuminates, go to step 3. If hot light does not illuminate, check the following: 15 or 20-amp fuse No. 6 in I.P. fuse panel Faulty instrument cluster bulb Open circuit between fuse and hot light
2. Backprobe terminal GC7 at PCM with a test light to battery voltage. Turn ignition on. If test light does not illuminate, PCM terminal connection is bad or PCM is faulty. If test light illuminates, turn ignition off. Disconnect ECM connectors. Jumper terminal GC7 to ground. Turn ignition on. If hot light does not illuminate, check for open circuit between hot light and PCM. If light does not illuminate and all circuits are intact and power is available to light, instrument cluster must be replaced.
3. Start engine. If test light goes off, no problem is evident. See DIAGNOSTIC AIDS. If test light is on, turn ignition off. Disconnect PCM connector. Probe ECM harness terminal GC7 with a test light to battery voltage. If light is off, replace PCM. If light is on, repair short to ground in circuit No. 1268. If no short is present, replace instrument cluster. NOTE: Test numbers refer to test numbers on diagnostic chart.
4. This determines which circuit is out of specification. All circuits except GD16 and GD15 should have battery voltage with ignition on, engine not running and ALDL test terminal not grounded.

Monitor the voltage of each terminal while moving related harness connectors, including PCM harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace PCM.

Scheme 153

1. This determines if the problem is the circuit or the component. Factory-installed PCM has an internal fuse and it's unlikely PCM needs replacing.

Scheme 154

CODE 27 OR 28, GEAR SWITCH CIRCUITS - 3.3L WITH 3T40 TRANSAXLE

The gear switches are located inside the transaxle. Switches are normally closed. As road speed increases, hydraulic pressure applies the specific gear clutches and the gear switch opens. The ECM uses the gear switches to control fuel delivery and TCC operation. (Scheme 170)and (Scheme 190).

Code 27 will set if circuit No. 108 indicates ground or closed switch for 12 seconds when the vehicle is in 3rd gear or if circuit No. 108 indicates an open when engine is first stared.

Code 28 will set if circuit No. 581 indicates ground or closed switch for 10 seconds when the vehicle is in 3rd gear or if circuit No. 581 indicates an open when engine is first started.

Note. Test numbers refer to test numbers on diagnostic chart.

1. A digital volt-ohmmeter must be used in this test. A test light will not work due to the low voltage supplied by the ECM.
2. Checks if the circuit is grounded through the switch.
3. Checks for a good, properly operating switch and checks circuit within transaxle for an improper ground.

Check for poor connections at ECM pins. Inspect harness for incorrect routing (too close to high voltage wiring) or chafing. Monitor the voltage of each terminal while moving related harness connectors. If the failure is induced, the voltage reading will change.

Scheme 155

CODE 28 OR 29, GEAR SWITCH CIRCUITS - 3.3L A BODY WITH 4T60

TRANSAXLE

The gear switches are located inside the transaxle. Switches are normally closed. As road speed increases, hydraulic pressure applies the specific gear clutches and the gear switch opens. The ECM uses the gear switches to help determine control of fuel delivery and TCC operation. Code 28 will set if circuit No. 108 indicates ground or closed switch for 12 seconds when the vehicle is in 4th gear or if circuit No. 108 indicates an open when engine is first started. (Scheme 171)

Code 29 will set if circuit No. 446 indicates ground or closed switch for 10 seconds when the vehicle is in 4th gear or if circuit No. 446 indicates an open when engine is first started.

Note. Test numbers refer to test numbers on diagnostic chart.

1. A digital volt-ohmmeter must be used in this test. A test light will not work due to the low voltage supplied by the ECM
2. Checks if the circuit is grounded through the switch
3. Checks for a good, properly operating switch and checks circuit within transaxle for an improper ground.

Check for poor connections at ECM pins. Inspect harness for incorrect routing (too close to high voltage wiring) or chafing. Monitor the voltage of each terminal while moving related harness connectors. If the failure is induced, the voltage reading will change.

Scheme 156

Scheme 157

Scheme 158

The gear switches are located inside the transaxle. Switches are normally closed. As road speed increases, hydraulic pressure applies the specific gear clutches and gear switch opens. The ECM uses the gear switches to help determine control of fuel delivery and TCC operation.

Code 27 will set if no Code 29 is present, circuit No. 581 indicates ground or closed switch for 10 seconds when vehicle is in 4th gear, or circuits No. 581 or No. 446 indicate an open when the engine is first started.

Code 28 will set if circuit No. 108 indicates ground or closed switch for indicate an open when engine is first started.

Code 29 will set if circuit No. 446 indicates ground or closed switch for 10 seconds when the vehicle is in 4th gear (TCC locked and brake not applied) or if circuit No. 446 indicates an open when engine is first started.

Note. Test numbers refer to test numbers on diagnostic chart.

1. A digital volt-ohmmeter must be used in this test. A test light will not work due to the low voltage supplied by the ECM.
2. Checks if the circuit is grounded through the switch.
3. Checks for a good, properly operating switch and checks circuit within transaxle for an improper ground.

Check for poor connections at ECM pins. Inspect harness for incorrect routing (too close to high voltage wiring) or chafing. Monitor the voltage of each terminal while moving related harness connectors. If the failure is induced, the voltage reading will change.

Scheme 159

CODE 31, PARK/NEUTRAL SWITCH CIRCUIT - 3.3L & 3.8L

Note. Complete diagnosis of Code 31 for 3.8L (VIN L) C and E body vehicles requires the use of a GM Tech 1 "Scan" tester which is capable of indicating status of all 4 park/neutral switch positions. This chart does not apply to 3.8L (VIN L) C and E body vehicles.

Scheme 160

The park/neutral switch contacts are part of the neutral start switch. The contacts close to ground in Park or Neutral and open in Drive. Code 31 will set if park/neutral signal circuit indicates an open for 3-4 consecutive starts or if conditions occur as follows

1. No Code 38 (3.3L) or 29 (3.8L) exists
2. Circuit No. 434 indicates ground
3. Transmission is in high gear
4. TCC is locked on 4T60 trans. only
5. Vehicle speed is greater than 45 MPH and TPS is less than 15 percent (.94 volt) on 3T40 trans. only
6. All above conditions have been met for at least 12 seconds NOTE: Test numbers refer to test numbers on diagnostic chart. This tests for a closed switch to ground in Park. This test for an open switch in Drive. Be sure "Scan" tester indicates Drive, even when wiggling shifter.

Scheme 161

Scheme 162

PRNDL switch is mounted on transaxle assembly. Switch sends information relative to gear selector position to PCM. PRNDL assembly also contains neutral safety switch. PRNDL switch uses 4 discrete circuits to pull 4 PCCM voltages low in various combinations to indicate each gear range. Voltage level of each of the circuits is represented as LO being grounded and HI being open. The 4 digits displayed represent Code 31 sets under the following conditions

1. Ignition is on and engine is off
2. Vehicle speed is 0 MPH
3. PRNDL indicates a gear other than Park or Neutral
4. All conditions met for 13 seconds for 3 consecutive starts.
5. PRNDL status indicators (A, B, C and P) indicate an incorrect combination of LO and HI for more than 10 seconds.
6. When a Code 31 is present, PCM assumes the D4 position until a correct combination is received by PCM. Some gear select positions may not be possible until fault is repaired. NOTE: Test numbers refer to test numbers on diagnostic chart. This test checks for proper operation of PRNDL. This test checks for proper operation of PRNDL. This test checks for open or grounded circuit.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broaden inside the insulation. check for the following

Poor Connection or Damaged Harness

Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.

Intermittent Test

Monitor a scan tool while moving related connectors and wiring harness. If failure is induced, scan data will change from LO to HI or HI to LO. Moving gear selector slowly through each gear position while monitoring PRNDL switch with scan tool may locate a faulty switch. This procedure may help isolate location of malfunction.

Scheme 163

Scheme 164

Scheme 165

Scheme 166

ECM supplies ignition voltage through a current limiting resistor to circuit No. 434 and senses a closed switch when voltage on circuit No. 434 drops to less than one volt. ECM uses Park/Neutral signal as one of the inputs to control Idle Speed Control (IAC), spark timing and Vehicle Speed Sensor (VSS). Code 31 will set under the following conditions

1. Circuit No. 434 indicates ground.
2. No Code 38.
3. Transmission is in high gear
4. VSS greater than 45 MPH and TPS is less than .94 volt.
5. Above conditions are met for 12 seconds.

Code 31 does not illuminate "Service Engine Soon" light.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a closed switch to ground in park position.
2. This test checks for a faulty ECM.
3. This test checks for an ungrounded Park/Neutral input in drive range. Ensure scan tool indicates drive, even while wiggling shifter. If scan tool does not indicate as previously described, check for a misadjusted neutral safety switch or and intermittent problem.

Scheme 167

Scheme 168

The 4T60E transaxle is electronically shifted. Within transaxle are 4 solenoids. Solenoid A is used for 1st and 4th gear operation only. Solenoid B is used for 1st and 2nd gear operation. The remaining 2 solenoids are for TCC operation only. All PRNDL indications are ignored as far as transaxle shifting is concerned except manual low. Code 36 will set if solenoid B failed in the off position, which will cause the transaxle to be in 3rd gear and desired gear is 1st. TPS is greater than 5 percent, VSS is greater than 5 MPG and no code 21, 22 or 24 is present. Code will also set if solenoid B failed in the on position, which will cause transaxle to be in 1st gear and desired gear is 4th, PRNDL is in 3rd or 4th, TPS is greater than 10 percent and no Code 31, 21 or 22 is present.

When Code 36 is set, transaxle will be forced into 3rd gear. If code sets due to a grounded circuit No. 1223, 1st and 2nd gear operation will be available only. If circuit No. 1223 is open, 3rd and 4th gear operation will be available only. If fault goes away, normal operation will be resumed for the duration of the key cycle.

Scheme 169

Scheme 170

Scheme 171

Scheme 172

Scheme 173

Scheme 174

Scheme 175

Code 38 will set if no Code 24 is present, status at brake input terminal of control module has not changed from high to low and vehicle speed has been greater than 35 MPH and back to zero MPH a precalibrated number of times.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Jumpering the brake switch determines if the ECM and wiring for the brake switch are okay.
2. Determines if brake switch is out of adjustment or is faulty.
3. Verifies the voltage to the brake switch.

A Code 38 in conjunction with a Code 39 or 26 would mean a problem with one or more of the following components.

1. Fuse or power supply circuit, brake switch or wire before the splice.
2. Code 38 shown is the result of a wire or circuit problem between the splice and the control module, poor connection to the control module, or possibly the control module, itself.

If brake switch has failed in an open state, TCC will not engage. Code 38 does not turn on SERVICE ENGINE SOON light on all models.

Scheme 176

CODE 39, TCC CIRCUIT 3.3L WITH 3T40 TRANSAXLE

Code 39 will set when no Code 28 is set, brake is not applied, TCC is commanded by ECM, transmission is in high gear and engine speed-to-vehicle speed ration does not indicate TCC has engaged. All these conditions must have been met for more than 15 seconds. (Scheme 170)and see scheme 223.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests TCC power supply (GAGES fuse) and brake switch.
2. Test ECM for proper operation.
3. Tests internal transaxle switches.

A poor connection can cause an intermittent Code 39. Using a digital volt-ohmmeter connected to the circuit, move the related wiring and connectors. An intermittent condition would cause a voltage reading change.

Scheme 177

CODE 39, TCC CIRCUIT 3.3L & 3.8L WITH 4T60 (440-T4) TRANSAXLE

Code 39 will set when no Code 28 or 29 is set, brake is not applied, TCC is commanded by ECM, transmission is in high gear and engine speed-to-vehicle speed ratio does not indicate TCC has engaged. All these conditions must have been bet for more than 15 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests fuse, brake switch and battery power circuit to the TCC solenoid.
2. Tests for ECM driver operation at TCC harness connector.
3. Tests for ECM driver operation at ECM terminal.

A Code 39 in conjunction with a Code 38 would mean a problem with one or more of the following components.

1. Fuse or power circuit, brake switch or wire before the splice.

Code 39 alone indicates a problem at

1. Circuit No. 420 between splice and TCC solenoid.
2. Circuit No. 422 between TCC solenoid and ECM.
3. Poor connection to the ECM or ECM, itself.

Scheme 178

Scheme 179

ECM monitors status of brake switch circuits through ECM terminal BC4. Code 38 will set under the following conditions

1. No. Code 24 is present.
2. Vehicle speed has been above 35 MPH and back to 0 MPH 23 times.
3. Status of circuit No. 420 has not changed state. NOTE: Test numbers refer to test numbers on diagnostic chart. Jumpering brake switch will determine if ECM and wiring for brake circuit are okay. This test verifies supply voltage to brake switch.

A Code 38 in combination with a Code 39 would mean a problem with one or more of the following components. Fuse, circuit No. 439, brake switch or wire before splice. A singe Code 38 is the result of wire or circuit No. 420 problem between splice and ECM, poor connection to ECM connector or possibly the ECM.

Scheme 180

Scheme 181

Park/Neutral switch contacts are a part of neutral safety switch and are closed to ground in park or neutral, and open in drive ranges. ECM supplies ignition voltage through a current limiting resistor to signal wire and senses a closed switch when voltage on signal wire drops to less than one volt. ECM uses P/N signal as one of the inputs to control idle air and spark timing.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a closed switch to ground in park position. Different scan tools will read P/N differently. Check tester operators manual for type of display used.
2. This test checks for an open switch in drive range.
3. Ensure scan tool indicates drive, even while wiggling shifter to test for an intermittent or misadjusted switch in drive range.

Scheme 182

Scheme 183

Scheme 184

Park/Neutral switch contacts are a part of neutral safety switch and are closed to ground in park or neutral, and open in drive ranges. ECM supplies ignition voltage through a current limiting resistor to signal wire and senses a closed switch when voltage on signal wire drops to less than one volt. ECM uses P/N signal as one of the inputs to control idle air and spark timing.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a closed switch to ground in park position. Different scan tools will read P/N differently. Check tester operator manual for type of display used.
2. This test checks for an open switch in drive range.
3. Ensure scan tool indicates drive, even while wiggling shifter to test for an intermittent or misadjusted switch in drive range.

Scheme 185

Scheme 186

ECM supplies ignition voltage through a current limiting resistor to circuit No. 434 and senses a closed switch when voltage on circuit No. 434 drops to less than one volt. ECM uses Park/Neutral signal as one of the inputs to control Idle Air Control (IAC) and Vehicle Speed Sensor (VSS).

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a closed switch to ground in park position. Use an ohmmeter instead of a test light. Resistance will be low indicating continuity to ground.
2. This test checks for an open switch in drive range.
3. Checks to this point indicate the Park/Neutral switch and wiring are okay. ECM signal voltage on circuit No. 434 may be mission. Ensure ECM connector is connected. Disconnect Park/Neutral switch and probe harness connector at circuit No. 434 with voltmeter to ground.

Scheme 187

Scheme 188

Scheme 189

Scheme 190

Scheme 191

Scheme 192

Scheme 193

Scheme 194

ECM supplies ignition voltage through a current limiting resistor to circuit No. 434 and senses a closed switch when voltage on circuit No. 434 drops to less than one volt. ECM uses Park/Neutral signal as one of the inputs to control Idle Air Control (IAC), Vehicle Speed Sensor (VSS) and Exhaust Gas Recirculation (EGR).

If circuit No. 434 indicates while in drive range, EGR would be inoperative, resulting in possible detonation. If circuit No. 434 indicates open, a drop in idle may occur when gear selector is moved into drive range.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a closed switch to ground in park position. Different makes of scan tools will read Park/Neutral differently. Check scan tool operator's manual for type of display used.
2. This test checks for an open switch in drive range.
3. Ensure scan tool indicates drive, even while wiggling shifter to test for an intermittent or misadjusted switch in drive or overdrive range.

Scheme 195

Scheme 196

Voltage is supplied to TCC solenoid through brake switch and transaxle 3rd gear apply switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage when

1. Vehicle speed is above 24 MPH.
2. Engine is at normal operating temperature.
3. Throttle position not changing, indication a steady speed.
4. Brake switch and transmission 3rd gear switch are closed. NOTE: Test numbers refer to test numbers in diagnostic chart. Light off confirms transmission 3rd gear apply switch is open. At 30 MPH, transmission 3rd gear switch should open. Test light will light and confirm battery supply and closed brake switch. Grounding diagnostic terminal with ignition switch on, engine off, should turn on test light. This test checks capability of ECM to control the solenoid.

An engine coolant thermostat that is stuck open or opens at too low a temperature may result in a delayed TCC operation.

Scheme 197

Scheme 198

Scheme 199

Voltage is supplied to TCC solenoid through brake switch and transaxle 3rd gear apply switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage when

1. Vehicle speed is above 24 MPH>
2. Engine is at normal operating temperature.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch is closed and transmission is in 3rd or 4th gear. NOTE: Test numbers refer to test numbers in diagnostic chart. This test should be performed on a level road with the aid of a helper to operate scan tool and observe RPM fluctuation. Entering Field Service Mode forces ECM to close TCC control driver circuit completing path for current which should light test light. Perform this test with aid of a helper. The 3rd gear switch must be engaged while measuring TCC solenoid resistance.

An engine coolant thermostat that is stuck open or opens at too low a temperature may result in a delayed TCC operation.

Scheme 200

Scheme 201

Scheme 202

Voltage is supplied to TCC solenoid through brake switch and transaxle second gear apply switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage when

1. Vehicle speed is above 34 MPH.
2. Engine is at normal operating temperature.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch and transmission 2nd gear switch are closed. NOTE: Test numbers refer to test numbers in diagnostic chart. Light off confirms transmission 2nd gear apply switch is open. By 25 MPH, transmission 2nd gear TCC switch should close. Test light will light and confirm battery supply and closed brake switch. Grounding diagnostic terminal with ignition switch on, engine off, should turn on test light. This test checks capability of ECM to supply a ground to the TCC solenoid. Test light will light when connected from 12 volts to ALDL terminal "F" as circuit No. 422 is grounded.

A scan tool only indicates when ECM has turned on TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, engine RPM should decrease when scan tool indicates TCC driver has turned on.

Scheme 203

Scheme 204

Voltage is supplied to TCC solenoid through brake switch and transaxle third gear apply switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage when

1. Vehicle speed is above 34 MPH with air conditioning off, or above 44 MPH with air conditioning on.
2. Engine is at normal operating temperature.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch and transmission 3rd gear switch are closed. NOTE: Test numbers refer to test numbers in diagnostic chart. Light off confirms transmission 3rd gear apply switch is open. At approximately 30 MPH, transmission 3rd gear switch should close. This depends on throttle position. Minimum speed at which 3rd gear switch will closed is approximately 21 MPH. Test light should light and confirm ignition voltage in circuit and a closed brake switch. Grounding diagnostic terminal with ignition switch on, engine off, should turn on test light. This test checks capability of ECM to control the solenoid.

An engine coolant thermostat that is stuck open or opens at too low a temperature may result in an inoperative TCC.

Scheme 205

Scheme 206

Voltage is supplied to TCC solenoid through brake switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage when

1. Vehicle speed is above 22 MPH.
2. Engine is at normal operating temperature.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch is closed. NOTE: Test numbers refer to test numbers in diagnostic chart. This test should be performed on a level road with the aid of a helper to operate scan tool and observe RPM fluctuation. Entering Field Service Mode forces ECM to close TCC control driver circuit completing path for current which should light test light. Perform this test with aid of a helper. the 3rd gear switch must be engaged while measuring TCC solenoid resistance.

Scheme 207

Scheme 208

Scheme 209

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. TCC will engage on a warm engine under given road load in 3rd gear. TCC will engage under the following conditions

1. Brake switch is closed.
2. Coolant temperature is greater than 149°F (65°C).
3. Vehicle speed above a calibrated value about 28 MPH.
4. Throttle position not changing, indicating a steady speed. NOTE: Test numbers refer to test numbers on diagnostic chart.

1. This test checks continuity of TCC circuit from fuse to ALDL connector.
2. Then brake pedal is released, light should come back on and then go off when diagnostic terminal is grounded. This tests circuit No. 422 and TCC driver in ECM.

A scan tool only indicates when ECM has turned on TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when scan tool indicates TCC driver has turned on. An engine coolant thermostat stuck open or opens at too low a temperature may result in an inoperative TCC.

Scheme 210

CHART C-8, TORQUE CONVERTER CLUTCH ELECTRICAL DIAGNOSIS - 3.1L PFI A & J BODIES (2 OF 2)

The 2nd gear signal switch in this vehicle should be open in 2nd and 3rd gear. ECM uses this 2nd gear signal to disengage TCC when going into a downshift. The 3rd gear switch should be open in 3rd gear. (Scheme 207)through (Scheme 228).

1st Gear

2nd gear switch opens

2nd gear signal switch closed

3rd gear signal switch closed

2nd Gear

2nd gear switch closed

2nd gear signal switch opens

3rd gear signal switch closed

3rd Gear

2nd gear switch closed

2nd gear signal switch opens

3rd gear signal switch opens

Note. Test numbers refer to test numbers on diagnostic chart.

1. Some scan tools display state of these switches in different ways. Be familiar with type of tool being used. since both switches should be in closed state during this test, tool should read the same for either 2nd or 3rd gear switch.
2. This test determines whether switch or signal circuit is open. Circuit can be checked for an open by measuring voltage with a DVOM at TCC connector. voltage should be about 12 volts.
3. Because switch(s) should be grounded in this step, disconnecting TCC connector should cause scan tool switch state to change.
4. Switch state should change when vehicle shifts into 2nd gear.

If vehicle is road tested because of a TCC related problem, ensure switch states do not change while in 3rd gear because TCC will disengage. If switches change state, carefully check wire routing and connections.

Scheme 211

Scheme 212

Scheme 213

The purpose of the Torque Converter Clutch (TCC) feature is to eliminate power loss of the torque converter when the vehicle is in a cruise conditions. When the solenoid coil is activated by the ECM, the torque converter clutch is applied, resulting in a straight through mechanical coupling from the engine to the wheels. When the transmission solenoid is deactivated, the TCC is released, which allows the torque converter to operate in the conventional manner (fluidic coupling between engine and transmission).

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks the functional operation of the TCC circuit.
2. This test checks the TCC control driver in the ECM.
3. This test will confirm there is battery voltage to terminal "A".
4. This test will confirm that ECM has the ability to turn on TCC.

The "Scan" tester only indicates when the ECM has turned on the TCC driver. This does not confirm that the TCC has engaged. To determine if TCC is functioning properly, observe engine RPM. RPM should decrease when "Scan" tester indicates TCC driver has turned.

Scheme 214

Note. Test numbers refer to test numbers on diagnostic chart.

1. Some "Scan" testers display the gear switch state in different manners. Refer to manufacturer's operating manual for additional information. Since both switches should be in the closed state during this test, scan should read the same for either 3rd or 4th gear switch.
2. Determines whether the switch or signal circuit is open. The circuit can be checked for an open by measuring voltage at the TCC connector. Reading should be about 12 volts.
3. Because switch should be grounded in this step, disconnecting TCC connector should cause "Scan" tester display to change state.
4. Switch state should change when vehicle shifts into 3rd gear.

Scheme 215

If vehicle is road tested for a TCC related problem, ensure the switch state does not change while in 4th gear because the TCC will disengage. If switch changes state, carefully inspect wire harness/routing and connections.

Scheme 216

Scheme 217

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. ECM turns on TCC when engine coolant temperature is greater than 149°F (65°C), TPS not changing and vehicle speed is greater than a specified value.

Note. Test numbers refer to test numbers on diagnostic chart.

1. When test light is connected from ALDL terminal "F" to ground, a test light on indicates battery voltage is okay and TCC solenoid is disengaged.
2. When diagnostic terminal is grounded, ECM should energize TCC solenoid and test light should go out.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engage. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when Scan tool indicates TCC driver has turned on.

To determine if 4th gear switch is functioning properly, perform checks is CHART C-8 (2 OF 2). Switches will not prevent TCC from functioning but will affect TCC lock and unlock points.

Scheme 218

A 4th gear switch mounted inside transmission closes when transmission shifts into 4th gear and this switch is used by ECM to modify TCC lock and unlock points when in a 4-3 downshift maneuver.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Unless switch of circuit No. 446 is closed, scan tool should display NO, indicating transmission is not in 4th gear. The 4th gear switch should only be closed while in 4th gear.
2. This test checks operation of 4th gear switch. When transmission shifts into 4th gear, switch should close and scan tool should display YES.
3. Disconnecting TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or problem is in transmission.
4. This step determines if ECM and wiring are okay. Grounding circuit No. 446 should cause scan tool to display YES indicating transmission is in 4th gear.

A road test may be necessary to verify customer complaint. If scan tool indicates TCC is turning on and off erratically, check state of 4th gear switch to be sure it is not changing states under a steady throttle position. If switch is changing states, check connections and wire routing carefully. Also, if 4th gear switch is always closed, TCC may engage as soon as sufficient oil pressure is reached.

Scheme 219

Scheme 220

When solenoid coil is activated. TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. TCC will engage on a warm engine under given road load in 2nd and 3rd gear. TCC will engage under the following conditions

1. Brake switch is closed.
2. Coolant temperature is greater than 149°F (65°C).
3. Vehicle speed above a calibrator value about 28 MPH.
4. Throttle position not changing, indicating a steady speed. NOTE: Test numbers refer to test numbers on diagnostic chart. This test checks functional operation of TCC circuit. This test checks TCC control driver in ECM. This test will confirm there is voltage to terminal "A". This test confirms ECM has ability to turn TCC on.

A scan tool only indicates when ECM has turned on TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when scan tool indicates TCC driver has turned on. An engine coolant thermostat stuck open or opens at too low a temperature may result in an inoperative TCC.

Scheme 221

The 2nd gear switch will be open in 2nd and 3rd gear. ECM uses this signal to disengage TCC when going into a downshift. The 3rd gear switch will open in 3rd gear.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Different makes of scan tools will read differently. Be familiar with type of tool being used. All switches should be in closed state during this test and tool should read the same for 2nd and 3rd gear switches.
2. This test determines whether switch of signal circuit is open. circuit can be checked for an open by measuring voltage with a DVOM at TCC connector. Voltage should be about 12 volts.
3. Because switches should be grounded in this step, disconnecting TCC connector should cause displayed switch states to change.
4. Switch state should change when vehicle shifts into 2nd gear.

If vehicle is road tested because of a TCC related problem, ensure switch states do not change while in 3rd gear. TCC will engage. If switches change state, carefully check wire routing and connections. If ECM, TCC solenoid and control circuitry are functioning correctly and TCC still does not apply properly, problem may be hydraulic or mechanical.

Scheme 222

Scheme 223

Voltage is supplied to TCC solenoid through brake switch and transmission 3rd gear apply switch. ECM engages TCC by grounding circuit No. 422 to energize the solenoid. TCC will engage when

1. Vehicle speed is above 33 MPH.
2. Engine at normal operating temperature.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch and transmission 3rd gear switch are closed. NOTE: Test numbers refer to test numbers in diagnostic chart. This test checks fuse and brake switch. This test checks ECM for proper operation. This test checks internal switches in transmission.

Scan tool only indicates when ECM has turned on TCC driver and does not confirm TCC engagement. To determine if TCC if functioning properly, road test vehicle. Engine RPM should decrease when scan tool indicates TCC driver has turned on.

If ECM TCC solenoid and control circuitry are functioning correctly but TCC still does not apply properly, problem may be hydraulic or mechanical. See appropriate AUTO TRANS OVERHAUL article.

Scheme 224

Scheme 225

Each gear switch opens when appropriate clutch is applied. All gear switches are open in 4th gear. TCC will engage under the following conditions

1. Engine is warmed up.
2. Vehicle speed above a calibrated value of about 35 MPH.
3. Throttle position not changing, indicating a steady speed.
4. Transmission 3rd gear switch closed.
5. Brake switch closed. NOTE: Test numbers refer to test numbers on diagnostic chart. This test checks integrity of total circuit minus TCC solenoid. This test checks for faulty brake switch or open circuit No. 422.

If vehicle is road tested because of a TCC related problem, ensure switch states do not change while in 4th gear. TCC will engage. If switches change state, carefully check wire routing and connections. If ECM, TCC solenoid and control circuitry are functioning correctly and TCC still does not apply properly, problem may be hydraulic or mechanical.

Each gear switch opens when appropriate clutch is applied. All gear switches are open in 4th gear.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Some scan tools display state of these switches in different ways. Be familiar with type of tool being used. All switches should be in closed state during this test and tool should read the same for 3rd and 4th gear switches.
2. This test determines whether switch of signal circuit is open. Circuit can be checked for an open by measuring voltage with a DVOM at TCC connector. Voltage should be about 12 volts.
3. Because switch(s) should be grounded in this step, disconnecting TCC connector should cause displayed switch states to change.
4. Switch state should change when vehicle shifts into 3rd gear.

If vehicle is road tested because of a TCC related problem, ensure switch states do not change while in 4th gear. TCC will engage. If switches change state, carefully check wire routing and connections. If ECM, TCC solenoid and control circuitry are functioning correctly and TCC still does not apply properly, problem may be hydraulic or mechanical. See appropriate AUTO TRANS OVERHAUL article.

Scheme 226

Scheme 227

Scheme 228

Fused battery voltage is supplied to TCC solenoid through brake switch and transmission 3rd gear apply switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage under the following conditions

1. Engine is warmed up.
2. Vehicle speed above a calibrated value of about 33 MPH.
3. Throttle position not changing, indicating a steady speed.
4. Transmission 3rd gear switch closed.
5. Brake switch closed.
6. TCC apply switch closed. NOTE: Test numbers refer to test numbers on diagnostic chart. This test checks gage fuse and brake switch. this test checks ECM for proper operation. This test checks internal switches in transmission.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engaged. To determine if TCC if functioning properly, test drive vehicle. Engine RPM should decrease when Scan tool indicates TCC driver has turned on.

If ECM, TCC solenoid and control circuitry are functioning correctly and TCC still does not apply properly, problem may be hydraulic or mechanical.

Scheme 229

Transmission uses 2 solenoids to control shifting. ECM monitors the TPS and vehicle speed to determine correct shift points.

Note. Test number refers to test number on diagnostic chart.

1. This test checks to see if transaxle mode switch is transmitting command to ECM to turn off solenoid "A" which enables 2nd gear. If solenoid "A" does not go off and transaxle mode switch is on, refer to CHART C-8A (4 OF 4) - TRANSAXLE SHIFT SOLENOID.

A scan tool only displays if transaxle mode switch is on or off. If information displayed is correct, solenoid "A" is off and transaxle is not in 2nd gear, refer to appropriate automatic transmission article for hydraulic or mechanical diagnosis.

Scheme 230

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. TCC will engage on a warm engine under given road load in 2nd, 3rd and 4th gears. TCC will engage under the following conditions

1. Engine is warmed up.
2. Vehicle speed above a calibrated value of about 28 MPH.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch is closed. NOTE: Test number refers to test number on diagnostic chart. This test checks functional operation of TCC circuit. This test checks TCC control driver in ECM. This test determines if voltage is present at terminal "A". If there is no voltage, check 10 amp fuse. This test confirms ECM has ability to turn TCC on.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when Scan tool indicates TCC driver has turned on.

Scheme 231

Note. Test number refers to test number on diagnostic chart.

1. If there are any codes present, those codes must be repaired first. Transaxle will default to 3rd gear if Codes 21, 22, 24, 53 and 55 are present.
2. This test checks 1st gear switch status.
3. This test verifies 1st gear switch status change.
4. This test checks 4th gear switch status.

Scheme 232

The 4T60 transaxle uses 2 solenoids to control shifting: Shift solenoid "A" and shift solenoid "B". ECM monitors TPS and vehicle speed versus engine speed to determine correct shift points.

SHIFT SOLENOID OPERATING SEQUENCE

Scheme 233

CHART C-8C, 4T60 TRANSAXLE 1ST GEAR SWITCH ELECTRICAL DIAGNOSIS - 3.4L PFI W BODY

The 4T60 transaxle uses 2 solenoids to control shifting: Shift solenoid "A" and shift solenoid "B". ECM monitors TPS and vehicle speed verses engine speed to determine correct shift points.

Scheme 234

Scheme 235

Scheme 236

The TCC will engage when the engine is warmed up, vehicle speed is greater than 28 MPH, throttle position sensor output is not changing (indicating a steady road speed) and the brake switch is closed.

Note. Test numbers refer to test numbers on diagnostic charts.

1. This test checks the continuity of the TCC circuit from the fuse to the ALDL connector.
2. When the brake pedal is released and the diagnostic terminal is grounded, the light should come back on and then go off. This tests circuit No. 422 and the TCC driver in the ECM.
3. TCC electrical circuits have checked out properly. Fault may be an internal hydraulic problem.

The "Scan" tester only indicates when the ECM has turned on the TCC driver. This does not confirm that the TCC has engaged. To determine if TCC is functioning properly, monitor engine RPM. Engine RPM should decrease when the "Scan" tester indicates the TCC driver has turned on.

Scheme 237

Note. Test numbers refer to test numbers on diagnostic charts.

1. Some "Scan" testers display the state of these switches in different ways. Be familiar with the type of tester being used. All switches should be in the closed state during this test, the tester should read the same for 2nd, 3rd and 4th gear switches.
2. Determines whether the switch or signal circuit is open. The circuit can be checked for an open by measuring the voltage (with a voltmeter) at the TCC connector. Voltage should be about 12 volts.
3. Because the switch should be grounded in this test, disconnecting the TCC connector should cause the "Scan" test switch state to change.
4. The switch state should change when the vehicle shifts into 2nd, 3rd and 4th gear.

If vehicle is road tested because of a TCC related problem, be sure the switch states do not change while in 4th gear, because the TCC will disengage. If switches change state, carefully check wire routing and connections.

Scheme 238

Scheme 239

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. PWN solenoid varies hydraulic pressure to make locking and unlocking of TCC smoother. TCC will engage under the following conditions

1. Engine is warmed up.
2. Vehicle speed above a calibrated value of about 28 MPH.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch is closed. NOTE: Test numbers refer to test numbers on diagnostic chart. This test checks fuse, brake switch and B+ circuit to TCC solenoid. This test checks availability of voltage on circuit No. 420. This test checks PCM's capability to pulse PWM solenoid on and off. Electrical circuits have checked out okay. See appropriate AUTO TRANS OVERHAUL article for hydraulic and mechanical diagnosis.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when Scan tool indicates TCC driver has turned on.

Scheme 240

CHART C-8B, 4T60 TRANSAXLE SHIFT SOLENOID FUNCTIONAL CHECK - 1991 E BODY (EXCEPT ELDORADO) 3.8L PFI (VIN L)

The 4T60E transmission uses two solenoids to control shifting. Shift solenoid "A" and shift solenoid "B". See 4T60 SHIFT SOLENOID OPERATING SEQUENCE table for solenoid operating sequence.

PCM monitors TPS and vehicle speed versus engine speed to determine the correct shift points. If PRNDL position indicates manual low position, PCM will keep both solenoids energized until 54 RPM at which point PCM will force a shift by de-energizing solenoid "A". Manual 2nd and manual 3rd are controlled hydraulically within transmission.

If PCM, control circuitry and shift solenoids are functioning properly and transmission still does not shift correctly, problem may be hydraulic or mechanical.

4T60 SHIFT SOLENOID OPERATING SEQUENCE

Scheme 241

Scheme 242

Scheme 243

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. PWN solenoid varies hydraulic pressure to make locking and unlocking of TCC smoother. TCC will engage under the following conditions

1. Engine is warmed up.
2. Vehicle speed above a calibrated value of about 28 MPH.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch is closed. NOTE: Test numbers refer to test numbers on diagnostic chart. This test ensures transmission test box J-38791 is working properly, ignition power is being supplied to transmission solenoids and circuits No. 422, 584 and 1350 are not grounded. This test ensures PCM can control TCC apply solenoid circuit No. 422 properly. This test ensures PCM can control TCC apply solenoid circuit No. 584 and 1350. DO NOT measure resistance until ignition has been off for at least 10 seconds to ensure PCM has powered down completely.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when Scan tool indicates TCC driver has turned on.

Transaxle Prefunctional Check

Before using Test Box J-38791 in functional checks, perform prefunctional check to ensure proper diagnosis and avoid replacement of good parts.

1. Check for stored codes. If codes are present, refer to code charts and repair as necessary before preforming functional check.
2. Connect Test Box J-38791 to vehicle. with ignition on, observe test box indicator lights. Battery, Ignition, Brake, Shift "A", and Shift "B" indicator lights should be on. Converter Clutch Apply and MOD indicator lights should be off. Disregard Lo Discrete 2-3 and 3-4 indicator lights.
3. If indicator lights do not function as described in previous step, check all connections. If all connections are okay and light indicators still do not function properly, check the following symptoms.

No Indicator Lights On Or Ignition Indicator Light Only On

Check connection at lighter receptacle. If okay, fuse for lighter or lighter receptacle is faulty.

Battery Indicator Light On, All Other Indicator Lights Off

Check 10 amp cooling fan fuse. If fuse blown, repair short to ground in circuit No. 250. If fuse is okay, check for open in circuits No. 250 or 300.

Brake Indicator Light Not On

See Code 38 in code charts.

Shift "A" And/Or Shift "B" Indicator lights Not On

Use Chart C-8B, TRANSAXLE SHIFT SOLENOID FUNCTIONAL CHECK.

Converter Clutch Apply Indicator Light On All The Time

Disconnect PCM Green connector and check circuit No. 422 for short to ground. If okay, PCM is faulty.

Converter Clutch MOD Indicator Light On All the Time

Disconnect PCM Green connector and check circuit No. 1350 for short to ground. If okay, PCM is faulty.

Scheme 244

CHART C-8B, 4T60 TRANSAXLE SHIFT SOLENOID FUNCTIONAL CHECK - 3.8L PFI (VIN L & 1) 1992

The 4T60 transmission uses two solenoids to control shifting. Shift solenoid "A" and shift solenoid "B". See 4T60 SHIFT SOLENOID OPERATING SEQUENCE for solenoid operating sequence.

PCM monitors TPS and vehicle speed versus engine speed to determine the correct shift points. If PRNDL position indicates manual low position, PCM will keep both solenoids energized until 54 RPM at which point PCM will force a shift a de-energizing solenoid "A". Manual 2nd and manual 3rd are controlled hydraulically within transmission. (Scheme 242)and (Scheme 254).

Note. Test numbers refer to test numbers on diagnostic chart.

1. With key on and engine off, PCM should be commanding 1st gear. Both shift solenoid control circuits will be grounded.
2. This test ensures PCM can control ground to shift solenoid "A" properly.
3. This test ensures PCM can control ground to shift solenoid "B" properly.
4. DO NOT measure resistance until ignition has been off for at least 10 seconds to ensure PCM has powered down completely.

If PCM, control circuitry and shift solenoids are functioning properly and transmission still does not shift correctly, problem may be hydraulic or mechanical.

Before using Test Box J-38791 in functional checks, perform prefunctional check to ensure proper diagnosis and avoid replacement of good parts.

1. Check for stored codes. If codes are present, refer to code charts and repair as necessary before performing functional check.
2. Connect Test Box J-38791 to vehicle. With ignition on, observe test box indicator lights. Battery, Ignition, Brake, Shift "A", and Shift "B" indicator lights should be on. Converter Clutch Apply and MOD indicator lights should be off. Disregard Lo Discrete 2-3 and 3-4 indicator lights.
3. If indicator lights do not function as described in previous step, check all connections. If all connections are okay and light indicators still do not function properly, check the following symptoms.

Check connection at lighter receptacle. If okay, fuse for lighter or lighter receptacle is faulty.

Check 10 amp cooling fan fuse. If fuse blown, repair short to ground in circuit No. 250. If fuse is okay, check for open in circuits No. 250 or 300.

See Code 38 in code charts.

Use Chart C-8B, TRANSAXLE SHIFT SOLENOID FUNCTIONAL CHECK.

Disconnect PCM Green connector and check circuit No. 422 for short to ground. If okay, PCM is faulty.

Disconnect PCM Green connector and check circuit No. 1350 for short to ground. If okay, PCM is faulty.

Scheme 245

CHART C-8A, TORQUE CONVERTER CLUTCH FUNCTIONAL CHECK - 1992 3.8L PFI (VIN 1)

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. PWN solenoid varies hydraulic pressure to make locking and unlocking of TCC smoother. TCC will engage under the following conditions

1. Engine is warmed up.
2. Vehicle speed above a calibrated value of about 28 MPH.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch is closed. NOTE: Test numbers refer to test numbers on diagnostic chart. This test determines whether fault is caused by a bad solenoid or control circuit. This test checks availability of voltage on circuit No. 420. This test checks PCM's capability to pulse PWM solenoid on and off. Electrical circuits have checked out okay. Refer to appropriate AUTO TRANS DIAGNOSIS article for hydraulic and mechanical diagnosis.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when Scan tool indicated TCC driver has turned on.

Scheme 246

CHART C-8B, 4T60 TRANSAXLE SHIFT SOLENOID FUNCTIONAL CHECK - 1992 3.8L PFI (VIN 1)

The 4T60E transmission uses two solenoids to control shifting. Shift solenoid "A" and shift solenoid "B". See SHIFT SOLENOID OPERATING SEQUENCE table for solenoid operating sequence.

PCM monitors TPS and vehicle speed versus engine speed to determine the correct shift points. If PRNDL position indicates manual low position, PCM will keep both solenoids energized until 54 RPM at which point PCM will force a shift by de-energized solenoid "A". Manual 2nd and manual 3rd are controlled hydraulically within transmission. (Scheme 243)

Scheme 247

Scheme 248

Each gear switch opens when appropriate clutch is applied. All gear switches are open in 4th gear. TCC will engage when

1. Vehicle speed is above 40 MPH.
2. Engine is at normal operating temperature.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch closed.
5. 3rd and 4th gear switch open. NOTE: Test numbers refer to test numbers on diagnostic chart. This test checks TCC solenoid for open. This test checks fuse, brake switch and circuits No. 39 and 420 to TCC solenoid.

Scan tool only indicates when ECM has turned on TCC driver and does not confirm TCC engagement. To determine if TCC is functioning properly, road test vehicle. Engine RPM should decrease when scan tool indicates TCC driver has turned on.

Each gear switch opens when appropriate clutch is applied. All gear switches are open in 4th gear.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Scan tool should display NO for 2nd, 3rd and 4th gear, indicating all switches are closed.
2. This test determines whether switch of signal circuit is open. circuit can be checked for an open by measuring voltage with a DVOM at TCC connector. Voltage should be about 12 volts.
3. Because switch should be grounded in this step, disconnecting TCC connector should cause displayed switch states to change.
4. Switch state should change when vehicle shifts into 3rd gear.

If vehicle is road tested because of a TCC related problem, ensure switch states do not change while in 4th gear. TCC will engage. If switches change state, carefully check wire routing and connections. If ECM, TCC solenoid and control circuitry are functioning correctly and TCC still does not apply properly, problem may be hydraulic or mechanical. See appropriate AUTO TRANS OVERHAUL article.

Scheme 249

The purpose of automatic transmission torque converter clutch feature is to eliminate power loss of torque converter when vehicle is in cruise condition. This allows convenience of a automatic transmission and fuel economy of a manual transmission. Fused battery ignition is supplied to TCC solenoid through brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the solenoid. TCC will engage solenoid when vehicle speed is greater than 24 MPH, engine is at normal operating temperature, throttle position sensor output is not changing (indicating a steady road speed), and brake switch is closed.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Confirms 12-volt supply as well as continuity of TCC circuit.
2. Grounding the diagnostic test terminal with ignition on and engine off should energize the capability of the ECM to control the solenoid.
3. Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure to the ECM "driver". Using an ohmmeter, check the solenoid coil resistance of all ECM controlled solenoids and relays before installing a replacement ECM. Replace any solenoid or relay that measures less than 20 ohms.

An engine coolant thermostat that is stuck open, or opens at too low a temperature, may result in an inoperative TCC. If TCC engages before "Scan" indicates, solenoid or sump overtemperature switch may be faulty.

Scheme 250

Note. Test numbers refer to test numbers on diagnostic chart.

1. Unless switch or circuit No. 446 is open, "Scan" tester should display "NO", indicating transmission is not in 4th gear. The 4th gear switch should only be open while in 4th gear.
2. This test determines if the ECM and wiring are okay. Grounding circuit NO. 446 should cause "Scan" tester to display "NO", indicating the transmission is not in 4th gear.
3. Checks operation of the 4th gear switch. When the transmission shifts into 4th gear the switch should open and "Scan" tester should display "YES".
4. Disconnecting the TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or the problem is in the transmission.

A road test may be necessary to verify the customer complaint. If the "Scan" tester indicates TCC is turning "ON" and "OFF" erratically, check the state of the 4th gear switch to be sure it is not changing state under a steady throttle position. If the switch is changing state, check connections and wire routing carefully. Also if 4th gear switch is always open, the TCC may engage as soon as sufficient oil pressure is reached.

Scheme 251

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. ECM turns on TCC when engine coolant temperature is greater than 149°F (65°C), TPS not changing and vehicle speed is greater than a specified value.

Note. Test numbers refer to test numbers on diagnostic chart.

1. When test light is connected from ALDL terminal "F to ground, a test light on indicates battery voltage is okay and TCC solenoid is disengaged.
2. When diagnostic terminal is grounded, ECM should energize TCC solenoid and test light should go out.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when scan tool indicates TCC driver has turned on.

To determine if 4th gear switch is functioning properly, perform checks in CHART C-8 (2 of 2). Switches will not prevent TCC from functioning but will affect TCC lock and unlock points. If 4th gear switch is always closed, TCC may engage as soon as sufficient oil pressure is reached.

A 4th gear switch mounted inside transmission closes when transmission shifts into 4th gear. This switch is used by ECM to modify TCC lock and unlock points when in a 4-3 downshift maneuver.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Unless switch is stuck closed or circuit No. 446 is shorted to ground scan tool should display NO, indicating transmission is not in 4th gear. The 4th gear switch should only be closed while in 4th gear.
2. This test checks operation of 4th gear switch. When transmission shifts into 4th gear, switch should close and scan tool should display YES.
3. Disconnecting TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or problem is in transmission.
4. This step determines if ECM and wiring are okay. Grounding circuit No. 446 should cause scan tool to display TES indicating transmission is in 4th gear.

A road test may be necessary to verify customer complaint. If scan tool indicates TCC is turning on and off erratically, check state of 4th gear switch to be sure it is not changing states under a steady throttle position. If switch is changing states, check connections and wire routing carefully. Also, if 4th gear switch is always closed, TCC may engage as soon as sufficient oil pressure is reached.

Scheme 252

The purpose of Torque Converter Clutch (TCC) feature is to eliminate power loss of torque converter stage when vehicle is in cruise condition. This allows convenience of an automatic transmission and fuel economy of a manual transmission. Fused battery ignition is supplied to TCC solenoid through brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the solenoid. TCC will engage solenoid when vehicle speed is greater than 24 MPH, engine is at normal operating temperature, throttle position sensor output is not changing (indicating a steady road speed), and brake switch is closed.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Confirms 12-volt supply as well as continuity of TCC circuit.
2. Grounding the ALDL "test" terminal with ignition on and engine off should energize the capability of the ECM to control the solenoid.
3. Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure to the ECM "quad-driver". Using an ohmmeter, check the solenoid coil resistance of all ECM controlled solenoids and relays before installing a replacement ECM. Replace any solenoid or relay that measures less than 20 ohms.

Diagnostics Aids

Scan tool only indicates when ECM has turned on TCC driver but does not confirm TCC has engaged. To determine if TCC is functioning properly, engine RPM should decrease when scan tool indicates TCC driver has turned on. To determine if gear switches are functioning properly, perform checks in CHART C-8A (2 OF 2). Switches will not prevent TCC from functioning, but will effect TCC lock and unlock points. If 4th gear switch circuit is always open, TCC may engage as soon as sufficient oil pressure is reached.

A 4th gear switch, mounted in the transmission, opens when the transmission shifts into 4th gear. This switch is used by the ECM to modify TCC lock and unlock points when in a 4-3 downshift maneuver.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Unless switch or circuit No. 446 is open, "Scan" tester should display "NO" or "OFF", indicating transmission is not in 4th gear. The 4th gear switch should only be open while in 4th gear.
2. This test determines if the ECM and wiring are okay. Grounding circuit No. 446 should cause "Scan" tester to display "NO" or "OFF", indicating the transmission is not in 4th gear.
3. Checks operation of the 4th gear switch. When the transmission shifts into 4th gear the switch should open and "Scan" tester should display "YES" or "ON".
4. Disconnecting the TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or the problem is in the transmission.

A road test may be necessary to verify the customer complaint. If the "Scan" tester indicates TCC is turning on and off erratically, check the state of the 4th gear switch to be sure it is not changing state under a steady throttle position. If the switch is changing state, check connections and wire routing carefully. Also, if 4th gear switch is always open, the TCC may engage as soon as sufficient oil pressure is reached.

Scheme 253

When solenoid coil is activated, TCC is applied which results in a straight through mechanical coupling from engine to wheels. When solenoid coil is deactivated, TCC is released which allows torque converter to operate in conventional manner. ECM turns on TCC when engine coolant temperature is greater than 149°F (65°C). TPS not changing and vehicle speed is greater than a specified value.

Note. Test numbers refer to test numbers in diagnostic chart.

1. This test confirms 12 volt supply as well as continuity of TCC circuit.
2. Grounding diagnostic test terminal with engine off should energize TCC solenoid.
3. TCC solenoid resistance must measure more than 20 ohms. Using an ohmmeter, check solenoid coil resistance of all ECM controlled solenoids and relays before installing a replacement ECM. Replace any solenoid or relay that measures less than 20 ohms.

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when scan tool indicates TCC driver has turned on.

To determine if gear switches are functioning properly, perform checks in all CHART C-8 charts. Switches will not prevent TCC from functioning but will affect TCC lock and unlock points. If 4th gear switch is always open, TCC may engage as soon as sufficient oil pressure is reached.

Scheme 254

A 2nd gear switch mounted inside transmission, is normally closed and opens when transmission shifts into 2nd gear. ECM uses gear switch signals to adjust ignition timing based on transmission shift points. Adjusting ignition timing after a transmission shift will allow for higher shift speeds.

Note. Test numbers refer to test numbers in diagnostic chart.

1. Unless switch is stuck open or circuit No. 232 is open, scan tool should display NO, indicating transmission is not in 2nd gear. The 2nd gear switch should only be open while in 2nd gear.
2. The ASR system must be disabled when performing this step. Whenever ignition key is cycled to off position and then cycled back on. ASR system will default on . Check operation of 2nd gear switch. When transmission shifts into 2nd gear, switch should open and scan tool should display YES.
3. This step determines if ECM and wiring are okay. Grounding circuit No. 232 should cause scan tool to display YES, indicating transmission is in 2nd gear.
4. Disconnecting TCC connector simulates an open switch to determine if circuit No. 232 is shorted to ground or problem is in transmission.

A faulty 2nd and 3rd gear switch circuit can cause an ECM code 72. If 2nd gear switch is changing states, check connections and wire routing.

Scheme 255

A 3rd gear switch mounted inside transmission, is normally closed and opens when transmission shifts into 3rd gear. ECM uses gear switch signals to adjust ignition timing based on transmission shift points. Adjusting ignition timing after a transmission shift will allow for higher shift speeds.

Note. Test numbers refer to test numbers in diagnostic chart.

1. Unless switch is stuck open or circuit No. 108 is open, scan tool should display NO, indicating transmission is not in 3rd gear. The 3rd gear switch should only be open while in 2nd gear.
2. The ASR system must be disabled when performing this step. Whenever ignition key is cycled to off position and then cycled back on, ASR system will default on. Check operation of 2nd gear switch. When transmission shifts into 3rd gear, switch should open and scan tool should display YES.
3. This step determines if ECM and wiring are okay. Grounding circuit No. 108 should cause scan tool to display YES, indicating transmission is in 3rd gear.
4. Disconnecting TCC connector simulates an open switch to determine if circuit No. 108 is shorted to ground or problem is in transmission.

A faulty 2nd and 3rd gear switch circuit can cause an ECM code 72. If 3rd gear switch is changing states, check connections and wire routing.

Scheme 256

A 4th gear switch mounted inside transmission, is normally closed and opens when transmission shifts into 4th gear. Switch is used by ECM to modify TCC lock and unlock points, when in a 4-3 downshift maneuver. Temperature switch is normally open. Transmission must be in 4th gear for temperature switch to apply TCC to cool transmission fluid. Temperature switch lock at 279°F (155°C) and unlocks at about 259°F (143°C).

Note. Test numbers refer to test numbers in diagnostic chart.

1. Unless switch is stuck closed or circuit No. 446 is shorted to ground, scan tool should display NO, indicating transmission is not in 4th gear. The 4th gear switch should only be closed while in 4th gear.
2. The ASR system must be disabled when performing this step. Whenever ignition key is cycled to off position and then cycled back on, ASR system will default on. Check operation of 4th gear switch. When transmission shifts into 4th gear, switch should close and scan tool should display YES.
3. Disconnecting TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or problem is in transmission.
4. This step determines if ECM and wiring are okay. Grounding circuit No. 446 should cause scan tool to display YES, indicating transmission is in 4th gear.

A road test may be necessary to verify customer complaint. If scan tool indicates TCC is turning on and off erratically, check state of 4th gear switch to ensure it is not changing states under a steady throttle position. If switch is changing states, check connections and wire routing carefully. If 4th gear switch is always closed, TCC may engage as soon as sufficient oil pressure is reached.

Scheme 257

CODE 23, MAT SENSOR TEMP. LOW - 1991 2.5L & 4.3L TURBO

CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

The ECM supplies and monitors a voltage signal (4.6 volts) to the sensor. When temperatures are low, sensor resistance is high and the ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by the ECM drops.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks is Code 23 is a hard failure or an intermittent condition. Code 23 will set if engine is running for one minute, MAT sensor temperature is less than -22°F (-30°C) for 12 seconds and speed sensor signal is not present.
2. This simulates conditions for a Code 25. If the Scan tester displays a high temperature, the ECM and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits.

If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a Scan tester. A Code 23 will also result if signal and ground circuits become open.

TEMPERATURE-TO-RESISTANCE VALUES

Scheme 258

Note. Test numbers refer to test numbers in diagnostic chart.

1. This step will determine if Code 23 is the result of a hard failure or an intermittent condition. Code 23 will set if a signal voltage indicates a intake air temperature below minus 22 degrees for 12 seconds, time since engine start is one minute or longer or no vehicle speed signal present.
2. Code 23 will set due to an open sensor, wire or connection. This test will determine if wiring and ECM are okay.
3. This test will determine if signal from circuit No. 472 or the 5 volt return in circuit No. 469/452 is open.

A scan tester reads temperature of air entering engine and should read close to ambient air temperature when engine is cold, and rises as underhood temperature increases. Carefully check harness and connections for possible open in circuit No. 472 or 469/452.

Temperature to resistance value scale may be used to test sensor at various temperature levels to evaluate the possibility of malfunctioning sensor. This condition could cause a driveability problem.

Scheme 259

Scheme 260

Scheme 261

Scheme 262

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the Vehicle Speed Sensor (VSS). Scan test reading should closely match speedometer reading.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 24 will set if voltage on circuit No. 437 is constant, engine speed is greater than 1200 RPM, vehicle speed is less than 2 MPH vehicle is not in Park or Neutral and all conditions have been met for at least 5 seconds. These conditions are met during a road load deceleration.
2. This determines if DRAC is receiving the AC signal from the VSS.
3. This test monitors DRAC voltage on circuit No. 437. With wheels turning, pulsing voltage should be present. Voltage variation will be greater at a low speed to an average of 4-6 volts at about 20 MPH.

Scan tester reading should closely match speedometer reading with drive wheels turning. If vehicle is equipped with an automatic transmission, check park/neutral switch adjustment.

Scheme 263

Vehicle speed sensor (VSS) circuit consists of a magnetic induction type sensor, VSS buffer (DRAC) module and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor. On 2WD vehicles, this current is transmitted to a DRAC module where it is passed on to ECM. DRAC compensates for various axle ratios and converts signal to a square wave for use by the speedometer, cruise control and anti-lock brake system.

Since vehicle speed is taken from transfer case on 4wd vehicles, transmission vehicle speed sensor signal on these units goes directly to PCM. Code 24 will set when vehicle speed is less than 200 RPM for 1.5 seconds while engine speed is greater than 3000 RPM. Park/Neutral is not selected. If input speed sensor is not operational at start up, vehicle speed sensor may read zero.

Note. Test numbers refer to test numbers in diagnostic chart.

1. This test checks vehicle speed sensor signal to PCM.
2. This test checks vehicle speed sensor signal to DRAC.
3. This test verifies vehicle speed sensor signal.

Check all electrical connections, especially at transmission pass-through connector.

Scheme 264

Scheme 265

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the Digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 24 will set if voltage on circuit No. 437 is constant, engine speed is greater than 1200 RPM, vehicle speed is less than 2 MPH vehicle is not in Park or Neutral and all conditions have been met for at least 10 seconds. These conditions are met during a road load deceleration.
2. This test monitors DRAC voltage on circuit No. 437. With wheels turning, pulsing voltage should be present. Voltage variation will be greater at a low speed to an average of 4-6 volts at about 20 MPH.

Scan tester reading should closely match speedometer reading with drive wheels turning. If vehicle is equipped with an automatic transmission, check park/neutral switch adjustment.

Scheme 266

Scheme 267

VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth cut in the outside diameter of the rear internal gear induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the PCM, cruise control, speedometer, and anti-lock brake system. Code 24 will set if gear selector is not in Park or Neutral, input speed is at least 3000 RPM and output speed is less than 200 RPM for at least one second. If input speed sensor is not operational at start up, this can cause VSS to read zero.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at PCM.
2. Test verifies operation of VSS.
3. Test checks circuits No. 821 and 822 up to the instrument cluster.

Check all connections, especially those at the transmission pass-through connector. While Code 24 is set, Scan tester will display an RPM derived from input speed.

Scheme 268

Scheme 269

Vehicle speed sensor (VSS) circuit consists of a magnetic induction type sensor, VSS buffer (DRAC) module and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor. On 2WD vehicles, current is transmitted to a DRAC module where it is passed on to ECM. DRAC compensates for various axle ratios and converts signal to a square wave for use by the speedometer, cruise control and anti-lock brake system.

Since vehicle speed is taken from transfer case on 4WD vehicles, transmission vehicle speed sensor signal on these units goes directly to PCM. Code 24 will set when vehicle speed is less than 200 RPM for 1.5 seconds while engine speed is greater than 3000 RPM. Park/Neutral is not selected. If input speed sensor is not operational at start up, vehicle speed sensor may read zero.

Note. Test numbers refer to test numbers in diagnostic chart.

1. This test checks vehicle speed sensor signal to PCM.
2. This test checks vehicle speed sensor signal to DRAC.
3. This test verifies vehicle speed sensor signal.

Check all electrical connectors, especially at transmission pass-through connector.

Scheme 270

Scheme 271

VSS output sensor circuit consists of magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the PCM, cruise control, speedometer, and anti-lock brake system. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at lease 3000 RPM and output speed is less than 200 RPM for at least 1.5 seconds. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at PCM.
2. Test verifies VSS signal to DRAC.
3. Test verifies VSS signal at sensor.

Check all connections, especially those at the transmission pass-through connector. While Code 24 is set, Scan tester will display an RPM derived from input speed.

Scheme 272

Scheme 273

VSS output sensor is a magnetic induction type. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor when drive wheels are turning. Since vehicle speed is taken from the transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to PCM. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 200 RPM for at lease 1.5 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at PCM.
2. Test verifies VSS signal at sensor.

Check all connections, especially those at the transmission pass-through connector. While Code 24 is set, Scan tester will display an RPM derived from input speed. If input speed is not operational at start-up, this can cause VSS to read zero.

Scheme 274

Scheme 275

ECM applies and monitors a 12-volt signal on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS is connected to speed sensor buffer which alternately grounds and opens circuit No. 437 when wheels are turning. This pulsing action takes place about 2000 times per mile. The voltage level and pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH. The ECM uses VSS information in calculations to determine vehicle adjustments. Scan tester reading should closely match speedometer reading when wheels are turning.

Note. Test numbers refer to test numbers on diagnostic chart.

1. A Code 24 sets when MPH is less than 2 MPH, transmission is not in Park or Neutral, engine speed is greater than 1400 RPM, TPS is greater than 5 percent, circuit No. 437 voltage is constant and all of these conditions are met for 30 seconds. These conditions are met during a road load deceleration.
2. A steady 8-12 volts at ECM connector indicates VSS circuit is open or speed sensor is faulty. A voltage of less than one volt at IP connector indicates circuit No. 437 wire is shorted to ground. Disconnect vehicle speed sensor connector. If voltage is now greater than 10 volts, vehicle speed sensor buffer is faulty. If voltage remains less than 8 volts, circuit is grounded. If circuit is not grounded, check for faulty ECM connector or ECM. Before replacing the ECM, PROM should be checked for correct application.

A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use Scan tester and check for proper signal in Drive, while wiggling shifter. Scan tester MPH reading should closely match speedometer when vehicle is moving.

Scheme 276

Scheme 277

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 connects to the Digital Ratio Adapter Controller (DRAC), which alternately grounds and opens circuit No. 437 when drive wheels are turning. The circuit is opened and closed at approximately 2000 times per mile when drive wheels are turning. The ECM calculates vehicle speed based on the time between pulses. Scan tester reading should closely match speedometer reading with drive wheels turning.

Code 24 will set under the following conditions are met for at least 10 seconds: Circuit No. 437 is constant, engine speed is more than 2000 RPM, vehicle speed signal at ECM terminal A9 is less than 5 MPH (8 Km/H).

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test monitors the ECM voltage on circuit No. 437. With wheels turning, the pulsating action causes varying voltage. the voltage variation will be greater at low wheel speeds, and an average of 4-6 volts at about 20 MPH (32 Km/H).
2. Less than one volt at ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect circuit No. 437 at DRAC. If voltage now reads more than 10 volts, the DRAC is faulty. If voltage remains less than 10 volts, then circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
3. A steady 8-12 volts reading at the ECM connector indicates that circuit No. 437 is open or DRAC is faulty.
4. Normal voltage indicates a possible intermittent condition.

With drive wheels turning, Scan tester and speedometer reading should closely match.

Scheme 278

Scheme 279

ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 connects to vehicle speed sensor (VSS), which alternately grounds circuit No. 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and ECM will calculate vehicle speed based on time between pulses.

Code 24 will set if circuit No. 437 voltage is constant, engine speed is more than 2000 RPM, or vehicle speed signal is less than 5 MPH. All conditions must be met for 40 seconds.

Note. Test numbers refer to test numbers in diagnostic chart.

1. This test monitors ECM voltage on circuit No. 437. With wheels turning, pulsing action will result in varying voltage. Variation will be greater at low wheel speeds to an average of 4 to 6 volts at about 20 MPH.
2. A voltage of less than one volt at ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect circuit No. 437 at vehicle speed sensor. If voltage reads above 10 volts, vehicle speed sensor is faulty. If voltage remains less than 10 volts, circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
3. A steady 8 to 12 volts at ECM connector indicates circuit No. 437 is open or a faulty vehicle speed sensor.
4. This is a normal voltage which indicates a possible intermittent condition.

A scan tester reading should closely match speedometer reading. (Scheme 278)

CODE 25, MAT SENSOR TEMP. HIGH - 1991 2.5L & 4.3L TURBO

CODE 25 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

The ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, the sensor resistance is high and ECM sees a high signal voltage. As air warms, resistance decreases and voltage sensed by the ECM drops. Sensor resistance can be measured at sensor terminals with harness disconnected.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if the code is a hard failure or an intermittent condition. Code 25 will set if a MAT temperature greater than 266°F (130°C) is sensed for more than a precalibrated period.
2. This simulates condition for code 23. If control module recognizes the open circuit and "Scan" tester displays a temperature of less than -30°C, control module and wiring are okay.

If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a Scan tester. A Code 25 will also result if signal and ground circuits become open. Refer to TEMPERATURE-TO-RESISTANCE VALUES table.

CODE 25, MAT SENSOR TEMP. HIGH - 1992 2.5L & 4.3L TURBO

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 25 will set if signal voltage indicates a intake air temperature above 302° for 2 seconds, time since engine start is one minute or longer or vehicle speed signal is present.

A scan tester reads temperature of air entering engine and should read close to ambient air temperature when engine is cold, and rises as underhood temperature increases. Carefully check harness and connections for possible short to ground in circuit No. 472.

Temperature to resistance value scale may be used to test sensor at various temperature levels to evaluate the possibility of malfunctioning sensor. This condition could cause a driveability problem.

Scheme 280

Scheme 281

The Pressure Switch Manifold (PSM) is actually 5 pressure switches combined into one unit and mounted on the transmission valve body. The PCM/TCM supplies battery voltage to the PSM on 3 separate wires. By grounding one or more of these circuits through various combinations of the pressure switches inside the pressure switch manifold, the PCM/CM detects what gear range has been selected by the vehicle operator.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test compares the indicated range to the range actually selected.
2. This test checks for correct voltage from the PCM/TCM to the transmission pass-thru connector.
3. This test will detect a short to ground in any one of the 3 PSM range circuits.

Code 28 will set if PCM/TCM detects one of 2 "illegal" PSM combinations. See VALID PSM COMBINATION CHART table for various combinations. Check pass-thru connector for good contact.

VALID PSM COMBINATION CHART

Scheme 282

Scheme 283

The purpose of the automatic transmission Torque Converter Clutch (TCC) feature is to eliminate power loss of the torque converter stage when vehicle is in a cruise condition. This allows the convenience of the automatic transmission, and fuel economy of a manual transmission.

Fused battery ignition voltage is supplied to the TCC solenoid, which is used inside the valve body to shift a spool valve to modulate pressure to the TCC. This modulated pressure normally allows some slight slippage of TCC.

The PCM/TCM will engage TCC by grounding circuit No. 422, energizing the solenoid.

Code 39 will set under the following conditions

1. TCC is engaged.
2. TCC "slip" is greater than 65 RPM for 2 seconds.
3. 2nd or 3rd gear is selected or indicated. NOTE: Test numbers refer to test number on diagnostic chart. This test determines if transmission is receiving a TCC Command from the pass-thru connector. If bidirectional Scan tester is not available, TCC may be activated by grounding ALDL test terminal B with ignition on and engine off. This test checks for power to transmission. This test determines if PCM/TCM is commanding TCC to be on.

Clear codes and re-check for Code 39. if code resets, problem could be internal to the transmission. Code 39 will only set in 3rd gear. TCC slip in 4th gear will set Code 68.

Scheme 284

CODE 53, SYSTEM VOLTAGE HIGH - LT. TRUCK

Code 53 will set when ignition is on and PCM terminal D1 voltage is greater than 19.5 volts for about 2 seconds. During the time the failure is present, the force motor is turned off, transmission immediately shifts to 2nd gear, and TCC operation is inhibited. The sensing of additional codes may result.

Note. Test number refer to test numbers on diagnostic chart.

1. Normal battery voltage is 10-17.0 volts.
2. This test checks if high voltage reading is due to the alternator circuit No. 440 or PCM. With engine running, check voltage at battery. If voltage is greater then 19.5 volts, the PCM is okay.
3. This test checks if alternator is faulty under load condition. If voltage is greater than 19.5 volts, check alternator.

Scheme 285

Notes On Intermittents

Jump-starting engine, and charging battery with a battery charger may set Code 53. If code is set when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link.

Scheme 286

Scheme 287

The transmission temperature sensor is a thermistor which controls the signal voltage to the PCM/TCM. The PCM/TCM applies and monitors voltage on circuit No. 1227 to the sensor. When transmission is cold sensor resistance is high; therefore, the PCM/TCM will see high signal voltage. As the transmission warms up, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission temperature greater than 309°F (154°C) for one second.
2. This test determines if circuit No. 1227 is shorted to ground, which will result in conditions for Code 58.

Check harness routing for a potential short to ground in circuit No. 1227. Scan tester displays transmission temperature in degrees Centigrade. After transmission is running, the temperature display should rise steadily to about 100°C then stabilize. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration See TRANSMISSION SENSOR - TEMPERATURE TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts; or TCC enabled complaint.

Scheme 288

CODE 59, TRANSMISSION TEMPERATURE SENSOR CIRCUIT (LOW TEMPERATURE INDICATED) - LT. TRUCK

The transmission temperature sensor is a thermistor which controls signal voltage to PCM/TCM. The PCM/TCM applies and monitors 5 volts to the sensor on circuit No. 1227. When transmission is cold, sensor resistance is high; therefore, the PCM/TCM will see high signal voltage. As transmission temperature warms up, sensor resistance and voltage drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts. (Scheme 287)

Note. The number refer to test numbers on diagnostic chart.

1. Code 59 will set if signal voltage indicates a transmission temperature less than -54°F (-48°C) for one second.
2. This test simulates Code 58. If PCM/TCM recognizes the low signal voltage (high temperature) and Scan tester reads 304°F (151°C) or greater, the PCM/TCM and wiring are okay.
3. This test determines if circuit No. 1227 is open. There should be 5 volts present at the sensor connector if measuring with a DVOM.

Scan tester displays transmission temperature in degrees centigrade After transmission is running, the displayed temperature should rise steadily to about 212°F (100°C) then stabilize. A faulty connection or an open in circuit No. 455 or circuit No. 1227 will result in a Code 59. Test the coolant sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMPERATURE TO RESISTANCE chart. An out-of-calibration sensor could result in firm shifts, or TCC enabled complaint.

Scheme 289

Scheme 290

The PCM/TCM monitors the difference in engine RPM and input shaft RPM. With transmission in Drive, Scan tester reading should show engine speed closely matching input speed.

Note. Test numbers refer to test number on diagnostic chart.

1. Code 68 with set under the following conditions: engine speed is 200 RPM greater than input speed, 4th gear is indicated, TCC is enabled, or If all conditions are met for 2 seconds. 2 This test monitors signal sent to the PCM/TCM from transmission.
2. This test confirms engine RPM signal to PCM/TCM.

Check for spread connectors at pass-through connector. Code 68 will set when going to default (2nd gear).

Scheme 291

Scheme 292

Note. This flow chart requires the use of a bidirectional (Tech 1) Scan tester.

The force motor is a PCM/TCM-controlled device used to regulate transmission line pressure. The PCM/TCM looks at TPS voltage, engine RPM and other inputs to determine appropriate line pressure for a given load, then regulates the pressure by applying a varying amperage. The applied amperage can vary from 1-1.1 amps.

The PCM/TCM then monitors amperage at the return line. If the return amperage varies more than .16 amp from the commanded amperage or the duration of at least one second, Code 73 will set. Once Code 73 is set, the force motor is disabled and full line pressure will be applied until the next time the ignition switch is cycled. Code 73 will remain stored, but the force motor will resume normal function until the conditions for Code 73 re-occur.

Note. The numbers refer to test numbers on diagnostic chart.

1. This procedure verifies an amperage difference of .16 amp or more, and tests the PCM/TCM's ability to control amperage to the force motor.
2. This test checks the force motor for internal shorts

Check for poor connections at PCM/TCM and transmission pass-thru connector.

Scheme 293

CODE 75, SYSTEM VOLTAGE LOW - LT. TRUCK

Code 75 will set when ignition is on and PCM terminal D1 voltage is less than 8.6 volts for about 4 seconds. During the time the failure is present, the force motor is turned off, maintaining only 2nd gear, and inhibiting 4th gear and TCC operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Normal battery voltage is 10-17.0 volts.
2. This test checks if the low voltage reading is due to the alternator, circuit No. 440 or PCM/TCM . With engine running, check voltage at the battery. If voltage is less than 8.6 volts, PCM/TCM is okay.

If Code sets when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link. Minimum voltage allowed or Code 75 to set is on a graduated scale, and will change with temperature. Minimum voltage at -40°F (-40°C) is 6.7 volts. Minimum voltage at 304°F 150°C) is 10.5 volts, and minimum voltage at 194°F (90°C) is 8.6 volts.

Scheme 294

Scheme 295

The PCM/TCM continually monitors voltage on each circuit connected to the quad driver, looking for either low or high voltage, depending on the commanded state of the devices connected to it. Code 81 will set if a fault has been detected on the shift solenoid "B" circuit. For example, if shift solenoid a is commanded on by this PCM/TCM, voltage on that circuit should drop when solenoid is grounded. If voltage stays up for at least .5 second, Code 61 will set. The opposite is also true. If shift solenoid "B" is off, voltage on the circuit should remain high. If voltage drops for more than .5 second, Code 81 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This procedure checks shift solenoid "B" and internal transmission wiring for shorts.
2. This test checks for power, from the ignition through transmission fuse, to shift solenoid "B".
3. This test ensures circuit No. 1223 is not shorted to ground.
4. This test checks PCM/TCM's ability to ground or control shift solenoid "B". If bidirectional Scan tester is not available, solenoid may be activated by grounding ALDL test terminal "B" with ignition on and engine off.
5. This test ensures circuit No. 1223 is not shorted to ground.

Check all connections, especially those at the transmission pass-through connector.

Scheme 296

Scheme 297

The PCM/TCM continually monitors each circuit connected to the quad-driver or either low or high voltage, depending on the commanded state of the device connected to it. Code 82 will set if there is a fault detected on the shift solenoid "A" circuit. For example, if shift solenoid "A" is commanded on by the PCM/TCM, then voltage on circuit should drop as soon as solenoid is grounded.

However, it voltage remains high for 2 seconds after the "on" command is given, Code 82 will set. The opposite is also true. If shift solenoid "A" is off, voltage on the circuit should be high. If voltage drops for .5 second or longer, Code 82 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This procedure checks shift solenoid "A" and internal transmission wiring harness for shorts.
2. This test checks for power, from the ignition through the fuse, to the shift solenoid.
3. This test checks circuit No. 1222 for short to ground.
4. This test checks PCM/TCM's ability to ground or control shift solenoid "A". If bidirectional Scan tester is not available, solenoid may be activated by grounding ALDL test terminal with ignition on and engine off.

Check all connections, especially those at the transmission pass-through connector.

Scheme 298

CODE 83, TCC QDM FAULT - LT. TRUCK

The PCM/TCM continually monitors voltage on each circuit connected to the quad-driver for either low or high voltage, depending on the commanded state of the device connected to it. Code 83 will set if PCM/TCM detects an inappropriate reading on the TCC circuit. For example, if the TCC duty cycle is zero, but voltage on the TCC circuit drops as if the solenoid were on, then Code 83 will set. The TCC solenoid, because of its large current draw, is connected to 2 terminals of a single quad driver.

Note. Test number refers to test number on diagnostic chart.

1. This test checks for low resistance in the solenoid or internal transmission harness. This test also determines which circuit triggered the fault. If bidirectional Scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

Check all connections. especially those at the transmission pass-through connector.

Scheme 299

Scheme 300

The PCM/TCM calculates the actual gear ratio from input and output speed readings while in each gear, then compares these to what the gear ratio should be, taking into consideration the selected gear range. This monitor includes Reverse gear, but does not include Overdrive gear.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test verifies the proper function of the input sensor.
2. This test verifies the proper function of the output sensor.

The PCM/TCM relies on the Power Switch Module (PSM) to indicate what gear range has been selected. PSM must be functioning properly or Code 85 may be set. Check all connections, especially those at the transmission pass-through connector. Compare Scan tester gear ratio reading to specifications in GEAR RATIO table.

Scheme 301

GEAR RATIO

CODE 86, SHIFT SOLENOID "B" STUCK "ON" - LT. TRUCK

The shift solenoids are used inside the valve body to control spool valves, which determine the transmission gear.

Fused ignition power is supplied to solenoid "B". The PCM/TCM will engage solenoid B by grounding circuit No. 1223 to energize the solenoid. (Scheme 295)

Code 86 will set under the following conditions

1. Vehicle speed is greater than 7 MPH.
2. TPS is more than 25 percent.
3. PCM/TCM commands 1st or 2nd gear.
4. Transmission ratio indicates transmission is in 3rd or 4th gear.
5. Conditions are met or 6 seconds. NOTE: Test number refers to test number on diagnostic chart. This test determines if transmission is receiving a solenoid "B" on command.

Check all connections, especially those at the transmission pass-through connector.

Scheme 302

CODE 87, SHIFT SOLENOID "B" STUCK "OFF" - LT. TRUCK

The shift solenoids are used inside the valve body to control spool valve position, which determines the transmission gear.

Fused ignition power is supplied to solenoid "B". The PCM/TCM will engage solenoid "B" by grounding circuit No. 1223 to energize the solenoid. (Scheme 295)

Code 37 will set under the following conditions

1. Vehicle speed is greater than 7 MPH.
2. TPS is more than 25 percent.
3. PCM/TCM commands 3rd or 4th gear.
4. Transmission ratio indicates transmission is in 1st or 2nd gear.
5. Conditions are met for 6 seconds. NOTE: Test numbers refer to test numbers on diagnostic chart. This test checks PCM/TCM's ability to ground or control solenoid "B". If bidirectional Scan tester is not available, solenoid may be activated by grounding ALDL test terminal -E with ignition on and engine off. This test checks the power supply and circuit No. 1223. This test checks the internal transmission harness and solenoid "B".

Check all connections. especially those at the transmission pass-thru connector. Fault may be an internal transmission problem.

Scheme 303

Scheme 304

The Park/Neutral switch contacts are closed to ground in Park or Neutral, and open in all Drive ranges. The ECM supplies ignition voltage, through a current limiting resistor, to circuit No. 434. The ECM senses a closed switch when voltage on circuit No. 434 drops to less than one volt. The ECM uses Park/Neutral signal as one of the inputs to control idle air, VSS diagnostics, and EGR flow.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks or closed switch to ground in Park position. Different makes of "SCAN" testers wild read Park/Neutral signal differently. Refer to testers instruction manual for type of display used for specific tester.
2. Checks for an open switch in Drive or Reverse.
3. Be sure "SCAN" tester indicates Drive, even while wiggling shifter, to test for an intermittent or misadjusted switch in Drive.

If circuit No. 434 indicates Park/Neutral switch grounded while in Drive, EGR would be inoperative, resulting in possible detonation.

If circuit No. 434 always indicates Drive (open), a drop in idle may exist when gear selector is moved into Drive.

Scheme 305

Scheme 306

Scheme 307

The purpose of TCC is to eliminate the power loss of torque converter stage when vehicle is in cruise condition. This allows the use of an automatic transmission with the fuel economy of a manual transmission.

Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch.

The ECM will engage TCC by grounding circuit No. 422 to energize the TCC solenoid.

Engagement of torque convener clutch will be executed under the following conditions

1. Vehicle speed greater than 24 MPH
2. Engine temperature above 149°F (65°C)
3. Steady TPS reading (not changing-steady road speed)
4. Brake switch closed.
5. Transmission in 3rd or 4th gear NOTE: Test numbers refer to test numbers on diagnostic chart. Check continuity through the brake switch and TCC solenoid. Check ability of ECM to energize the TCC solenoid. Grounding the diagnostic connector should energize the relay and cause the light to go out. This test by-passes the TCC solenoid and checks for an open or short in circuit No. 422

Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "DRIVER". Using an ohmmeter, check solenoid coil resistance of all ECM-controlled solenoids and relays before replacing ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

Scheme 308

Scheme 309

Fused battery voltage is supplied to TCC solenoid through TCC brake switch and 3rd gear apply switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid. TCC will engage when

1. Vehicle speed is above 30 MPH.
2. Engine is at normal operating temperature.
3. Throttle position not changing, indicating a steady speed.
4. Brake switch and 3rd gear switch are closed. NOTE: Test numbers refer to test numbers in diagnostic chart. If test light does not light, 3rd gear apply switch is open. At 25 MPH, 3rd gear apply switch should close. Test light should glow and confirm battery voltage exist and closed brake switch. Grounding diagnostic terminal with ignition switch on and engine off, should energize TCC solenoid by grounding circuit No. 422. This test checks ability of ECM to supply a ground to the TCC solenoid. Test light will glow if connected from 12 volts to ALDL terminal "F" when circuit No. 422 is grounded.

A scan tool only indicates when ECM has turned on the TCC driver, and does not confirm that TCC has engaged. To determine if TCC is functioning properly, engine RPM should decrease when scan tool indicates TCC driver has turned on.

Scheme 310

Scheme 311

Fused battery voltage is supplied to TCC solenoid through TCC brake switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid. TCC will engage when

1. Vehicle speed is above 30 MPH.
2. Engine is at normal operating temperature.
3. Throttle position not changing, indicating a steady speed
4. Brake switch is closed and transmission is in 3rd or 4th gear. NOTE: Test numbers refer to test numbers in diagnostic chart. If test light glows, battery voltage and continuity through TCC solenoid. Using a scan tool, check for vehicle speed sensor signal (VSS) to ECM.

Solenoid Coil resistance must measure more than 20 ohms. Less resistance will cause early failure of ECM driver. Before installing a replacement ECM, use a DVOM to check solenoid coil resistance of all ECM controlled solenoids and relays. Replace any solenoid or relay that measures less than 20 ohms resistance.

Scheme 312

Scheme 313

The purpose of TCC is to eliminate the power loss of torque converter stage when vehicle is in cruise condition. This allows the use of an automatic transmission with the fuel economy of a manual transmission.

Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch.

The ECM will engage TCC by grounding circuit No. 422 to energize the TCC solenoid.

Engagement of torque converter Clutch will be executed under the following conditions

1. Vehicle speed greater than 24 MPH.
2. Engine temperature above 149°F (65°C).
3. Steady TPS reading (not changing-steady road speed).
4. Brake switch closed.
5. Transmission in 3rd or 4th gear. NOTE: Test numbers refer to test numbers on diagnostic chart. Glowing test light indicates that battery voltage and continuity through TCC solenoid are okay. Checks for vehicle speed sensor (VSS) signal to ECM, using a "SCAN" tester. Checks for 4th gear signal to ECM. This signal will not prevent TCC engagement, but could cause a change in engage/disengage speed points.

Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "DRIVER". Using an ohmmeter, check solenoid coil resistance of all ECM-controlled solenoids and relays before replacing ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

Scheme 314

Scheme 315

Scheme 316

The purpose of the Torque Converter Clutch (TCC is to eliminate the power loss of the torque converter stage when the vehicle is in a cruise condition. This allows the convenience of an automatic transmission and the fuel economy of a manual transmission.

Fused battery voltage is supplied to the TCC solenoid through the TCC brake switch The ECM will engage TCC by grounding circuit No. 422 to energize the TCC apply solenoid inside the transmission. The ECM completes circuit whenever TPS exceeds a calibrated value for throttle opening.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a complete circuit, from ignition switch, through solenoid, and up to test point. Test light should normally be on, since the ECM has not completed the circuit.
2. This test checks for continuity through TCC brake switch and TCC apply solenoid.
3. This test checks for ECM to complete circuit to ground, to energize TCC apply solenoid and engage TCC. Test light should normally go out when ECM completes circuit.
4. This test checks for TPS signal 1 signal to ECM is correct fault is in ECM connection or ECM. TPS voltage signal to ECM is incorrect proper operation will not occur.
5. This test checks for ground in circuit to ECM terminal "C5". Test light should normally be off.
6. This test checks for voltage to terminal "A" of TCC connector. Test light should normally be on.
7. This test checks for a complete circuit from voltage to ground, via TCC test terminal in ALDL connector. Normally. test light should come on, if harness is good.

Solenoid coil resistance must measure more than 20 ohms. A lower resistance value will cause early failure to the ECM driver Check the solenoid coil resistance of all ECM controlled solenoids and relays, before Installing replacement ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

Scheme 317

Scheme 318

The output sensor circuit consists of a magnetic induction-type sensor, digital ratio adapter, and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor

This current is transmitted to a digital ratio adapter, where it is passed on to the PCM/TCM. The digital ratio adapter compensates for various axle ratios, and converts the signal to a square wave for use by the speedometer, cruise control and anti-lock brake system.

It the input sensor is not operational at start up, it can cause the output sensor to read zero.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks the speed signal on circuit No. 437 at the PCM/TCM from the digital ratio adapter.
2. This test checks for a speed signal from the output speed sensor to the digital ratio adapter.
3. This test checks the output speed sensor.

Check all connections, especially those at the transmission pass-through connector.

Scheme 319

Scheme 320

The output sensor circuit consists of a magnetic induction-type sensor, digital ratio adapter located in the instrument cluster, and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor.

This current is transmitted to the instrument cluster, where it is passed on to the PCM. The digital ratio adapter compensates for various axle ratios, and converts the signal to a square wave for use by the speedometer, cruise control and anti-lock brake system.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test verifies output speed sensor voltage at PCM.
2. This test directly verifies operation of output speed sensor.
3. This test checks circuits No. 821 and 822 up to the instrument cluster.

Check all connections, especially those at the transmission pass-thru connector. If the input sensor is not functioning at start up, it will cause the output sensor to read zero. While Code 24 is set, the Scan tester will display an RPM derived from input speed.

Scheme 321

Scheme 322

The output sensor is a magnetic induction type. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor. Since vehicle speed is taken from the transfer case on 4WD drive vehicles, the transmission output sensor signal on these units goes directly to the PCM/TCM. If input sensor is not operational at start up, it can cause the output sensor to read zero.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks for a speed signal at the PCM/TCM.
2. This test checks the output sensor directly.

Check an connections, especially those at the transmission pass-through connector.

Scheme 323

Scheme 324

The input sensor is of the magnetic induction type and is located on the left side of the transmission, forward of center. Serrations in the forward clutch housing induce a small A/C current as they pass by the input sensor. While there is no specific code for an input sensor problem, the PCM/TCM uses input sensor readings to calculate gear ratio, turbine speed, TCC slip, and determine if the engine is running.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks the input sensor circuit up to the PCM/TCM.
2. This test checks the sensor output.

Check all connections, especially those at the transmission pass-through connector.

Scheme 325

Scheme 326

The normally closed brake switch supplies a 12-volt signal on circuit No. 420 to the PCM/TCM. The signal voltage is removed when brakes are applied. An incorrect brake signal may affect TCC operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for voltage at brake switch.
2. This test simulates closed brake switch (brakes off).
3. This test checks circuit No. 420 from brake switch to PCM/TCM.
4. This test opens circuit No. 420 and simulates condition of brakes being applied.

Check all connections, especially those at the transmission pass-through connector. Check customer driving habits and/or unusual traffic conditions (i.e. stop and go expressway traffic.)

Scheme 327

TORQUE CONVERTER CLUTCH CIRCUIT CHECK - LT. TRUCK

The automatic transmission TCC eliminates power loss of torque converter stage when vehicle is in a cruise condition. This allows the convenience of the automatic transmission, and fuel economy of a manual transmission.

Fused battery ignition is supplied to the TCC solenoid, located inside valve body, to shift a spool valve in order to modulate pressure to the TCC.

The PCM/TCM will engage TCC by grounding circuit No. 422 to energize the solenoid.

TCC will engage under the following conditions

1. Vehicle speed is greater than 30 MPH (48 km/h).
2. Engine at normal operating temperature greater than 149°F (65°C).
3. Transmission at normal operating temperature of 195°F 91°C).
4. Throttle position sensor output is not changing, indicating a steady road speed.
5. Brake switch is closed.
6. 4th gear is indicated.
7. No codes are stored. NOTE: Test numbers refer to test numbers on diagnostic chart. This step checks for a shorted internal transmission harness or TCC solenoid . This step verifies power supply to the TCC solenoid. This step checks circuit No. 422 for a short to ground. This step checks PCM/TCM s ability to control the TCC solenoid. If bidirectional Scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

Check all connections, especially those at the transmission pass-through connector. The TCC is pulse width modulated, and is designed to keep the TCC right at the point of engagement. Therefore, some slight slippage is normal.

Scheme 328

SHIFT SOLENOID "A" CIRCUIT CHECK - LT. TRUCK

Shift solenoid "A" is attached to the valve body and is a normally open exhaust valve. The PCM/TCM activates he solenoid by grounding it through an internal quad driver. Solenoid "A" (Blue) is on in 1st and 4th gears, but is off in 2nd and 3rd gears. When solenoid is on, it redirects fluid to act on the shift valves.

Note. Test number refer to test numbers on diagnostic charts.

1. This test checks shift solenoid "A" and the internal transmission wiring harness for shorts.
2. This test checks for power, from the ignition switch through the fuse, to the shift solenoid.
3. This test checks circuit No. 1222 for a short to ground.
4. This test checks PCM/TCM's ability to ground or control the shift solenoid "A". If bidirectional Scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" With ignition on and engine off.

Check all connections, especially those at the transmission pass-through connector.

Scheme 329

SHIFT SOLENOID "B" CIRCUIT CHECK - LT. TRUCK

The shift solenoids are used inside the valve body to control the spool valves, which determine the transmission gear. Fused ignition is supplied to solenoid "B". The PCM/TCM will engage solenoid "B" by grounding circuit No. 1223 to energize the solenoid.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks shift solenoid "B" and the internal transmission wiring for shorts.
2. This test checks for power, from the ignition through the transmission fuse, to the shift solenoid
3. This test checks PCM/TCM's ability to ground or control the shift solenoid.
4. This test ensures circuit No. 1223 is not shorted to ground.
5. This test checks PCM/TCM's ability to ground or control the shift solenoid "B". If bidirectional Scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine Off.
6. This test ensures circuit No. 1223 is not shorted to ground.

Check all connections, especially those at the transmission pass-through connector.

Scheme 330

FORCE MOTOR CIRCUIT CHECK (LT. TRUCK)

Note. This flow chart requires the use of a bidirectional (Tech 1) Scan tester.

The force motor is a PCM/TCM-controlled device used to regulate transmission line pressure. The PCM/TCM looks at TPS voltage, engine RPM, and other inputs to determine the appropriate line pressure or a given load, then regulates the pressure by applying a variable amperage to the force motor. The applied amperage varies from .1-1.1 amps.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test verifies an amperage difference of .16 amp or more, and tests the PCM/TCM's ability to control amperage to the force motor.
2. This test checks the force motor for internal shorts.

Check or poor connections at PCM/TCM, especially at transmission pass-thru connector.

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See also:
• COMPUTER RELEARN PROCEDURES
• THM 4L80-E SERIES CODE DESIGNATION
• TROUBLE SHOOTING
• RETRIEVING TROUBLE CODES
• DISPLAYING TROUBLE CODES
• TEST TYPE SELECTION
• ENTERING SELF-DIAGNOSTICS
• SYSTEM SELECTION
• SPECIFIC TEST SELECTION
• BRAKE SWITCH TEST
• TCC APPLY SOLENOID TEST
• GROUNDING "D" WIRE AT TRANSMISSION USING ALDL
• COOLER LINE PRESSURE TEST
• ELECTRICAL SWITCHES TEST