Diagnosis - A/t: Other GMC Pickup C2500

THM 4L80-E SERIES CODE DESIGNATION

THM 4L80-E SERIES CODE DESIGNATION

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 358

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.

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.

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.

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.

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»(ref-55809-S24337413792001011700000). 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. see scheme 4
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.

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 359)- (Scheme 366).

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.

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»(ref-55809-S28665223992001011700000). Display trouble codes stored in memory. See «DISPLAYING TROUBLE CODES»(ref-55809-S24337413792001011700000). Select BCM system for testing. See «SYSTEM SELECTION»(ref-55809-S04865896712001011700000). Select SNAPSHOT test type. See «TEST TYPE SELECTION»(ref-55809-S34038263672001011700000). 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 361), (Scheme 362), (Scheme 365) and (Scheme 366).
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»(ref-55809-S24337413792001011700000) 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»(ref-55809-S35419470012001011700000).
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»(ref-55809-S28665223992001011700000). Display trouble codes stored in memory. See «DISPLAYING TROUBLE CODES»(ref-55809-S24337413792001011700000). Select ECM system for testing. See «SYSTEM SELECTION»(ref-55809-S04865896712001011700000). Select SNAPSHOT test type. See «TEST TYPE SELECTION»(ref-55809-S34038263672001011700000). 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 360)and (Scheme 364). 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»(ref-55809-S35419470012001011700000).
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 360)and (Scheme 362).

ECM

The following are brief summaries of ECM status light indicators. (Scheme 360)

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 360)

BCM

The following are brief summaries of BCM system status light indicators. (Scheme 362)

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 362)

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 359

Scheme 360

Scheme 361

Scheme 362

Scheme 363

Scheme 364

Scheme 365

Scheme 366

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»(ref-55809-S09227795652001011700000). If brake switch is okay, check TCC apply solenoid. See «TCC APPLY SOLENOID TEST»(ref-55809-S20148740762001011700000). 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. see scheme 2 If short is found, check apply solenoid. See «TCC APPLY SOLENOID TEST»(ref-55809-S20148740762001011700000). If short is not found, check for computer system problem. See CHECKING SELF-DIAGNOSTIC SYSTEM.

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»(ref-55809-S09227795652001011700000) . 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»(ref-55809-S40951970482001011700000) . 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»(ref-55809-S06032750402001011700000) .
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»(ref-55809-S20148740762001011700000) and «ELECTRICAL SWITCHES TEST»(ref-55809-S14638924332001011700000) . 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. see scheme 2 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»(ref-55809-S16979392082001011700000) . 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. see scheme 2With 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»(ref-55809-S40951970482001011700000). 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»(ref-55809-S16979392082001011700000) . 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).

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. see scheme 2 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.

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 367

Scheme 368

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 369

Scheme 370

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 371

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 372

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 373

Scheme 374

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 375

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 374)

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 376

Scheme 377

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 378

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.

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 378)

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 379

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 380

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 381

Scheme 382

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 383

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 384

Scheme 385

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 386

Scheme 387

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 388

Scheme 389

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 390

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 389)

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 391

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 389)

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 392

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 389)

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 393

Scheme 394

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 395

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

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

Scheme 396

Scheme 397

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 398

Note. Test numbers refer to numbers on diagnostic chart.

Scheme 399

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 400

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.

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 401

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

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

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. see scheme 126and see scheme 150.

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.

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. see scheme 127

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.

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 402

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 403

Scheme 404

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.

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. see scheme 126and 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.

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.

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.

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.

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 405

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. see scheme 208and see scheme 228.

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.

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 406

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. see scheme 209

Scheme 407

Scheme 408

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.

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.

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.

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.

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 409

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 410

Scheme 411

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.

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 411)

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.

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 412

Scheme 413

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.

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.

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 413)

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.

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 413)

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".

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.

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.

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.