Diagnosis - Torque Converter Clutch Cadillac Brougham I

APPLICATIONS

THM 200-4R APPLICATIONS (1989)

THM 3T40 APPLICATION (1989)

THM 4L60 APPLICATION (1989)

THM 4T60 APPLICATION (1989)

THM 200-R4 APPLICATION (1990)

THM 3T40 APPLICATION (1990)

THM 3T40-A APPLICATION (1990)

THM 4L60 APPLICATION (1990)

THM 4T60 APPLICATION (1990)

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. For complete information on General Motors Computerized Engine Control systems see appropriate COMPUTERIZED ENGINE CONTROLS article in ENGINE PERFORMANCE section.

OVERVIEW

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 clutch to operate in a conventional manner.

TCC CONTROL COMPONENTS

The following components are used to engage/disengage torque converter clutch. 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 110

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. On the most sophisticated vehicles, these devices may include the vehicle speed sensor, coolant temperature sensor, throttle position sensor, 3rd or 4th gear switch and brake switch. The ECM controls application of torque converter clutch by providing a ground circuit for the TCC solenoid circuit.

TCC Solenoid Assembly

Energized by ECM to redirect transaxle 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 Switch

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 following conditions can be caused by one or more of the following factors: (1) Incorrect fluid level, (2) Contaminated fluid, (3) Improperly adjusted linkage, or (4) Damaged or worn linkage. When diagnosing Converter Clutch problems, ensure engine and vacuum systems are in perfect operating order.

Problem in Electronic Control Module

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

Electrical Problems

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

Valve Body Assembly

1. Sticking converter clutch shift and/or apply valve. Clean, service and/or replace valve body as required.
2. Sticking throttle valve. Clean, service and/or replace valve body as required.
3. Inspect valve body and service as required.

Oil Pump Assembly

1. Orifice plugged for converter signal oil in pump. Clean and inspect orifice for blockage.
2. Solenoid "O" ring damaged or missing. Check and replace "O" ring.
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. Orifice cup plug omitted form cooler in passage. Check and install plug.
7. Check and replace converter clutch blow-off check ball if not seated or if damaged. Check and replace torque converter clutch accumulator piston or seal if damaged.

CONVERTER CLUTCH APPLY SLIPS, ROUGH OR SHUDDERS

1. Converter clutch pressure plate faulty. Check plate for damage and service as required.
2. Damaged or missing check ball in end of turbine shaft. Check and replace turbine shaft, if required.
3. Converter clutch regulator valve stuck. Clean, service and/or replace valve body as required.
4. Converter clutch accumulator piston or seal damaged. Check and service as required.
5. Channel plate seals damaged or missing. Check and service as required.

CONVERTER CLUTCH DOES NOT RELEASE

1. Solenoid does not exhaust. Verify Electronic Control Module operation. See appropriate CHART C-8 in this article.
2. Converter clutch apply valve stuck. Clean, service and/or replace valve body as required.
3. Check damaged converter. Replace torque converter.
4. Cup plug missing from pump release passage. Check and replace plug or pump assembly.
5. Turbine shaft end seal damaged or missing. Check and replace end seal or turbine shaft as required.
6. Hole not drilled through turbine shaft. Replace turbine shaft.

DIAGNOSTIC TOOLS

Diagnosis of the TCC system may be performed by following the appropriate C-8 chart for the transmission/engine application in question. When utilizing these charts, specific tools which may be required are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), and 6 jumper wires 6" long (1 wire with female connectors at both ends; 1 wire with male connector at both ends; 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart(s).

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 "CHECK ENGINE" or "SERVICE ENGINE SOON" light. It is NOT necessary to enter diagnostic mode to use TCC test chart(s); 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 111

1. To enter diagnostic mode, turn ignition on but do not start engine. "CHECK ENGINE" or "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 111) CAUTION: Inserting spade lug in terminals of ALDL connector grounds "test" terminal lead. Do not ground ALDL connector until after ignition is on.
2. "CHECK ENGINE" or "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 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's diagnostic circuit, see appropriate COMPUTERIZED ENGINE CONTROLS article in ENGINE PERFORMANCE section.

CLEARING TROUBLE CODES

Trouble codes are cleared by removing battery voltage form ECM for at least 10 seconds. Ensure that "test" terminal is not grounded. Turn ignition off and remove ECM fuse from fuse block for 10 seconds and then reinsert it. Another way of removing battery voltage is to disconnect the battery positive cable (or pigtail) from the battery for 10 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.

OUTPUT DISPLAYS

When trouble shooting a malfunction, the ECM and BCM output cycling can be used to determine if output tests can be actuated regardless of the inputs and normal program instructions. Once a test in outputs is selected, except for ECM IAC, the test will display "HI" or "LO" for 3 seconds in each state to indicate the command and output terminal voltage. When ECM output "EO01" is entered, the torque converter clutch display will be "LO" when TCC is on (energized.)

DIAGNOSTIC CHARTS

Note. Codes not listed are not transmission related. See appropriate article in ENGINE PERFORMANCE.

PFI CHART C-1 - ECM CHECK

Prior to replacing an ECM, vehicle must be tested for

1. Check connector terminals. Ensure they are properly seated and locked in the connector and terminal retainers are properly positioned and locked.
2. Check ECM ground at generator and battery ground at engine and chassis.
3. Check IGN 1-ISO fuse in fuse block.

Notes On Fault Tree

1. The ECM supplies power through a high side driver to Cruise Control Vent and Vacuum Solenoids. An internal short in servo causing a low resistance or short to ground in wiring harness may damage ECM or cause servo-to-solenoids to fail.
2. The ECM supplies power to fuel pump relay through a high side driver. An internal short in relay winding causing a low resistance or short to ground in wiring harness may cause damage to ECM or relay to fail.
3. The ECM supplies power to ISC through an internal switching IC that will change polarity of circuit. An internal short in ISC motor armature windings or short to ground in wiring harness may cause damage to ECM or ISC motor to fail.
4. The ECM provides ground for EGR solenoid, Electric Switching Valve (ESV). Canister Purge solenoid, and Electric Divert Valve (EDV) through a quad driver IC. An internal short in windings or resistor of devices or a short to ground in wiring harness may cause damage to ECM or Emission Control System (ECS) device.
5. The ECM provides ground for the VCC solenoid in transmission through a quad driver IC. An internal short in winding or diode of solenoid or a short to ground in wiring harness may cause damage to ECM or solenoid to fail.

PFI Chart C-1 - ECM Check Flowchart

Note. Always check "MEM-CAL" for the correct application and installation before replacing an "ECM".

ECM Check Flowchart (1 Of 2). Scheme 112

ECM Check Flowchart (2 Of 2). Scheme 113

Scheme 114

Note. Test numbers refer to test numbers on diagnostic chart. Steps 1-3 represent an EGR functional check.

1. This test checks to see if the EGR passages are restricted or if valve is stuck open. Engine should run roughly or stall as valve is opened manually.
2. EGR valve diaphragm should begin to move as engine speed approaches 2000 RPM.
3. This test should result in EGR solenoid being energized, shutting off vacuum to EGR valve. This indicates system is functioning properly.
4. If vacuum is less than 7in. Hg at 2000 RPM, a leak or restriction between the EGR diaphragm and vacuum source is indicated.
5. With ignition on and engine stopped, ECM normally grounds terminal "T" to energize the EGR solenoid. This test checks EGR solenoid and solenoid electrical control circuit.
6. The EGR is normally de-energized with the engine idling at normal operating temperature. This test checks EGR solenoid and solenoid electrical control circuit.
7. This determines whether ECM is providing ground to terminal "T" or circuit No. 435 is shorted to ground.

Diagnostic Aids

Vacuum hoses should be checked for internal restrictions.

Chart C-7C, EGR Check - 5.0L Full Function Flowchart (1 Of 2). Scheme 115

Chart C-7C, EGR Check - 5.0L Full Function Flowchart (2 Of 2). Scheme 116

Scheme 117

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

1. This test confirms a 12-volt supply as well as continuity of TCC circuit.
2. When ALDL "test" terminal is grounded with engine off, ECM capability is energized to control the solenoid.
3. Test light should go off if circuit is okay because grounding terminal "P" shorts out test light. If test light goes off, ECM is faulty.
4. If test light goes off, the Quad-Driver inside the ECM is faulty. ECM must be replaced.
5. Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause continual opening of ECM Quad-Driver. Using an ohmmeter, check solenoid coil resistance of all ECM-controlled solenoids and relays. 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. On 200-4R transmission, if TCC engages before "Scan" tester indicates TCC "ON", wiring may be grounded between TCC solenoid and sump over temperature switch or switch may be faulty.

Chart C-8, Torque Converter Clutch Electrical Diagnosis - 5.0L Full Function Flowchart. Scheme 118

Scheme 119

The ECM applies and monitors 12 volts on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS alternately opens and grounds circuit No. 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and ECM calculates vehicle speed based on time between pulses. "Scan" tester reading should closely match speedometer reading.

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

1. This test monitors ECM voltage on circuit No. 437. With drive wheels turning, pulsing action varies voltage. This variation is greater at low speeds to an average of 4-6 volts at about 20 MPH. Cruise control must be off for accurate test results.
2. A voltage of less than one volt indicates that circuit No. 437 is shorted to ground. Disconnect circuit No. 437 at VSS. The VSS is faulty if voltage now reads more than 10 volts. IF voltage remains less than one volt, circuit No. 437 is grounded. If circuit is not grounded, check for a faulty ECM connector or ECM.
3. A steady 8-12 volts at ECM connector indicates circuit No. 437 is open, or a faulty VSS.
4. This is a normal voltage condition and indicates a possible intermittent condition. See INTERMITTENTS in THEORY & OPERATION section.
5. This step will isolate whether problem is in circuit No. 437 or ECM.

Code 24, Vehicle Speed Sensor - 5.0L Full Function Flowchart. Scheme 120

Scheme 121

The Park/Neutral (P/N) switch is closed when gear selector is in Park or Neutral and open in all other ranges.

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

1. This step separates a faulty switch or switch adjustment from a faulty electrical circuit or ECM. Normal voltage across terminals of connector, when removed from P/N switch, should be approximately battery voltage.

Chart 24B, Park/Neutral Circuit - 5.0L Full Function Flowchart. Scheme 122

Scheme 123

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 close.

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-drive". 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.

Chart C-8A, TCC - 5.0L & 5.7L TBI Flowchart (1 Of 3). Scheme 124

Chart C-8A, TCC - 5.0L & 5.7L TBI Flowchart (2 Of 3). Scheme 125

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.

Chart C-8A, TCC - 5.0L & 5.7L TBI Flowchart (3 Of 3). Scheme 126

Scheme 127

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

1. Code 23 will set if signal voltage indicates manifold air temperature less than -22°F (-30°C) for 12 seconds and elapsed time since engine start is more than 1 minute.
2. Code 23 will set due to an open sensor, wire or connection. This test determines if wiring and ECM are okay.
3. This test determines if signal circuit No. 472 or sensor ground circuit No. 452 is open.

"Scan" tester displays temperature of air entering engine and should read close to ambient temperature when engine is cold. Temperature reading should rise as underhood temperature increases. If vehicle is allowed to sit overnight, MAT temperature and CTS temperature should read close to the same. Carefully check harness and connections for possible open is circuits No. 472 or 452.

MAT SENSOR TEMP VS. RESISTANCE VALUE (1)

Code 23, MAT Sensor Signal Voltage High - 5.0L & 5.7L TBI Flowchart. Scheme 128

Scheme 129

The ECM applies and monitors 12 volts on circuit No. 437. Vehicle Speed Sensor (VSS) alternately grounds and opens circuit No. 437 when vehicle is in motion. This pulsing action takes place about 2000 times per mile. ECM will calculate vehicle speed based on the time between "pluses". With vehicle in motion, "Scan" tester reading should closely match speedometer reading.

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

1. Code 24 will set if vehicle speed equals zero MPH when the engine speed is between 1200-4400 RPM, TPS is less than 2 percent (closed throttle), a low load condition (high vacuum) exists and all conditions met for 5 seconds. These conditions are met during a road load deceleration. Disregard Code 24 that sets when vehicle is not in motion.
2. If 8-12 volts is indicated at IP connector, circuit No. 437 is open between IP connector and the VSS, or there is a faulty vehicle speed sensor. A voltage of less than one volt at the IP connector indicates that circuit NO. 437 is shorted to ground. If, after disconnecting circuit No. 437 at the vehicle speed sensor, voltage reads greater than 10 volts, vehicle speed sensor is faulty. If circuit No. 437 is not grounded, there is a faulty connection at ECM, or ECM is faulty.

A "Scan" tester should indicate vehicle speed whenever vehicle is in motion. A faulty or misadjusted Park/Neutral switch can result in a false Code 24. Use "Scan" tester and check for proper signal while in Drive.

Code 24, VSS - 5.0L & 5.7L TBI (B Body) Flowchart. Scheme 130

CODE 25, MANIFOLD AIR TEMPERATURE SENSOR SIGNAL VOLTAGE LOW - 5.0L & 5.7L TBI

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

1. Code 25 will set if signal voltage indicates manifold air temperature greater than 302°F (150°C) for 2 seconds, vehicle speed is greater than 5 MPH and elapsed time since engine start is more than 2 minutes.

A "Scan" tester displays temperature of air entering engine. Temperature should read close to ambient temperature when engine is cold and rise as underhood temperature increases. Check harness and connections for possible short to ground on circuit No. 472. MAT will rarely exceed 176°F (80°C). If temperature is greater than this, check for proper THERMAC operation. See MAT SENSOR TEMP VS. RESISTANCE VALUE table.

Code 25, MAT Sensor Signal Voltage Low - 5.0L & 5.7L TBI Flowchart. Scheme 131

Scheme 132

The purpose of the Torque Converter Clutch (TCC) feature is to eliminate the power loss of the transmission converter stage when the vehicle is in a cruise condition. Electrical power from the ignition circuit is supplied to the TCC solenoid through the brake switch and transmission 3rd gear switch. The ECM will engage TCC by grounding circuit No. 422 to energize the solenoid.

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

1. If test light is off, this confirms 3rd gear switch in open.
2. At 25 MPH, the 3rd gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding the ALDL "test" terminal with ignition on and engine off should energize the TCC solenoid when ECM grounds circuit No. 422. This test checks the ability of the ECM to supply a ground to the TCC solenoid. The test light connected from 12 volts to ALDL terminal "F" will turn on as circuit No. 422 is grounded.

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, note engine RPM. RPM should decrease when "Scan" tester indicates TCC driver has turned on.

Chart C-8A, TCC - THM 3T40 Trans. - 2.8L A, J & L Bodies Flowchart (1 Of 2). Scheme 133

Chart C-8A, TCC - THM 3T40 Trans. - 2.8L A, J & L Bodies Flowchart (2 Of 2). Scheme 134

CODE 24, VEHICLE SPEED SENSOR - 2.8L PFI A & J BODIES

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the vehicle speed sensor buffer in the instrument cluster. The speed sensor buffer alternately grounds circuit No. 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between pulses. A, and J Bodies use a Hall Effect switch mounted in the speedometer head to signal the VSS buffer.

The "Scan" tester reading should closely match the speedometer reading with drive wheels turning.

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

1. To avoid possible damage to CV joints on FWD vehicles, always support control arms whenever drive wheels are turning off of ground. Code 24 will set if vehicle is stopped when the following conditions occur: Reference signal indicates engine speed is between 1400 and 4400 RPM. Throttle angle is less than 2 percent (closed throttle). Low load condition (low airflow or high vacuum). Transmission not in Park or Neutral. All conditions met for 4-5 seconds. These conditions are met during a road load deceleration. Disregard Code 24 that sets when drive wheels are not turning.
2. A voltage of less than one volt at the IP connector indicates that circuit No. 437 is shorted to ground. If after disconnecting circuit No. 437 at the vehicle speed sensor, the voltage reads greater than 10 volts, the vehicle speed sensor is faulty. If voltage remains less than 10 volts, then it is possible that circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, there is a faulty connection at the ECM, or a faulty ECM.

A "Scan" tester should indicate a vehicle speed whenever the drive wheels are turning greater than 3 MPH. A faulty or misadjusted Park/Neutral switch can result in a false Code 24. Use "Scan" tester and check for proper signal while wiggling shifter in Drive.

Code 24, VSS - 2.8L PFI - A & J Bodies Flowchart. Scheme 135

Scheme 136

Scheme 137

Scheme 138

The ECM monitors the output from the Permanent Magnet (PM) generator mounted in the transaxle on circuits No. 400 and 401. The PM generator produces a pulsating, alternating current whenever vehicle speed is greater than 3 MPH. The AC voltage level and the number of pulses increases with vehicle speed. The ECM will calculate vehicle speed based on the time between pulses. The VSS buffer used on other models is incorporated into the ECM on these models. If the vehicle is equipped with a digital speedometer and odometer and/or cruise control, the ECM provides pulses for operation of these components (2000 and 4000 pulses per mile respectively). The "Scan" tester reading should closely match speedometer reading when drive wheels are turning.

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

1. Code 24 will set if vehicle speed equals zero MPH when engine speed is between 1400 and 3600 RPM, throttle opening is less than 2 percent, a low load condition exists and transmission is not in Park or Neutral. All conditions must be met for 5-10 seconds during a road load deceleration. Disregard Code 24 that sets when drive wheels are not turning.

If "Scan" tester displays vehicle speed, check Park/Neutral switch operation using appropriate C-1 CHART. If switch is okay, check for intermittent connections. Also, check for proper application of PROM or MEM-CAL.

Code 24, VSS - 2.8L J & L Bodies, & 2.8L & 3.1L A & W Bodies Flowchart. Scheme 139

Scheme 140

Scheme 141

Scheme 142

Scheme 143

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

1. Code 25 will set if monitored voltage indicates manifold air temperature is high for 3 seconds and time since engine start is at least 3-8 minutes, depending upon engine application. Due to the conditions necessary to set a Code 25, the "SERVICE ENGINE SOON" light will remain on only while the signal is low and vehicle speed is present.

The "Scan" tester reads temperature of the air entering the engine. Parameter should read close to ambient air temperature when engine is cold, and rise as underhood temperature increases. A short to ground in the MAT signal line will result in a Code 25. Use the MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES table to help determine if sensor calibration has shifted.

MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES (1)

Code 25, MAT Signal Voltage Low - 2.8L A & L Bodies - Flowchart. Scheme 144

Scheme 145

Scheme 146

The Park/Neutral switch contacts are a part of the neutral start switch. Contacts are closed to ground in Park or Neutral. The ECM applies and monitors a 12-volt signal on circuit No. 434 and senses a closed switch when voltage drops to less than one volt.

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

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

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

1. Checks for a switch closed to ground in Park.
2. Checks for an open switch in Drive.
3. To test for an intermittent or misadjusted switch in Drive, be sure "Scan" tester indicates Drive, even while wiggling shifter.

Chart C-1A, P/N Switch - 2.8L PFI A, J, L & W Bodies Flowchart. Scheme 147

CHART C-8, TORQUE CONVERTER CLUTCH - 4.3L TBI B BODY

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 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 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.

Chart C-8, TCC - 4.3L TBI B Body Flowchart (1 Of 2). Scheme 148

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.

Chart C-8, TCC - 4.3L TBI B Body Flowchart (2 Of 2). Scheme 149

Scheme 150

The ECM applies and monitors 12 volts on circuit No. 437. Vehicle Speed Sensor (VSS) alternately grounds and opens circuit No. 437 when vehicle is in motion. This pulsing action takes place about 2000 times per mile. ECM will calculate vehicle speed based on the time between "pulses".

With vehicle in motion, "Scan" tester reading should closely match speedometer reading.

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

1. Code 24 will set if vehicle speed equals zero MPH when the following conditions occur: Engine speed is between 1200-4400 RPM. TPS is less than 2 percent (closed throttle). Low load condition (high vacuum). All conditions met for 5 seconds. These conditions are met during a road load deceleration. Disregard Code 24 that sets when vehicle is not in motion.
2. If 8-12 volts is indicated at IP connector, circuit No. 437 is open between IP connector and the VSS, or there is a faulty vehicle speed sensor. A voltage of less than one volt at the IP connector indicates that circuit No. 437 is shorted to ground. If, after disconnecting circuit No. 437 at the vehicle speed sensor, voltage reads greater than 10 volts, vehicle speed sensor is faulty. If voltage remains less than 8 volts, check circuit No. 437 for short to ground. If circuit No. 437 is not grounded, there is a faulty connection at ECM, or ECM is faulty.

A "Scan" tester should indicate vehicle speed whenever vehicle is in motion. A faulty or misadjusted Park/Neutral switch can result in a false Code 24. Use "Scan" tester and check for proper signal while in Drive.

Code 24, VSS - 4.3L TBI - B Body Flowchart. Scheme 151

Scheme 152

The Park/Neutral switch is closed to ground in Park or Neutral, and open in Drive. The ECM supplies and monitors 12 volts through an internal resistor to circuit No. 434, ECM senses that switch is closed when voltage on circuit No. 434 drops to less than one volt.

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

1. Checks for a closed switch to ground in Park position. Different makes of "Scan" tester will read P/N differently. Refer to "Scan" tester operation manual for type of display used.
2. Checks for an open switch in Drive.
3. Be sure "Scan" tester indicates Drive, even while wiggling shifter to test for an intermittent or misadjusted switch in Drive.

Chart C-1A, P/N Switch - 4.3L TBI - B Body - Flowchart. Scheme 153

CODE 24, VEHICLE SPEED SENSOR - 3.8L PFI - C & H BODIES

The Vehicle Speed Sensor (VSS) consists of a PM generator, buffer, speedometer and ECM. The PM generator is a permanent magnet assembly attached to the transmission or transaxle (in speedometer cluster on "A" body). As the vehicle moves, the generator creates a "sine wave" electrical pulse, which is routed to the buffer. In the buffer, the signal is changed form a "sine wave" to a "square wave" and amplified. The "square wave" is an on/off signal. The length of time between pulses determines vehicle speed. The ECM sends a 12-volt signal out on circuit No. 437. The frequency of the signal, which is pulsed low, is used by the ECM to determine vehicle speed.

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

1. Code 24 will set if vehicle speed equals zero MPH when the following conditions occur: Engine speed is between 1500 and 4000 RPM. TPS voltage reading shows closed throttle. Low load condition (low airflow). Not in Park or Neutral. All conditions met for 20 seconds.
2. Probe terminal "G" on "A" body. This test checks to see if the fault is circuit No. 437, including the ECM or the VSS/Buffer circuit. The ECM is the source of 12 volts via circuit No. 437 to the buffer in a normal working system.

Note. Disregard Code 24 that sets when the drive wheels are not turning.

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation.

Inspect ECM harness connector terminal "A10" for improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness.

If connections and harness check out okay, raise drive wheels (support drive axles to prevent damage to CV joints). Block other wheels and idle engine more than 3 MPH, in low gear. Use "Scan" tester to check the vehicle speed while moving related connectors and wiring harness. If the failure is induced, the vehicle speed display will change. This may help to isolate the location of the malfunction. A shorted cruise control module or a faulty or misadjusted Park/Neutral switch can result in a false Code 24.

Note. Disregard CODE 24, if set while drive wheels are not turning.

Code 24, VSS - 3.8L PFI - C & H Bodies - Flowchart. Scheme 154

CHART C-1A, PARK/NEUTRAL SWITCH - 3.8L PFI - C & H BODIES

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

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" tester will display P/N status differently. Refer to owners manual for display used.
2. This test checks for an open switch in Drive.
3. Be sure "Scan" tester indicates Drive, even while wiggling shifter. This will test for an intermittent condition due to an faulty or misadjusted P/N switch.

Chart C-1A, P/N Switch - 3.8L PFI - C & H Bodies - Flowchart. Scheme 155

Scheme 156

The purpose of the Torque Converter Clutch (TCC) feature is to eliminate the power loss of the transmission converter stage when the vehicle is in a cruise condition. Electrical power from the ignition circuit is supplied to the TCC solenoid through the brake switch and transmission 2nd or 3rd gear switch. The ECM will engage TCC by grounding circuit No. 422 to energize the solenoid.

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

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

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, note engine RPM. RPM should decrease when "Scan" tester indicates TCC driver has turned on.

Chart C-8A, TCC - THM 3T40 - 3.1L A Body Flowchart (1 Of 3). Scheme 157

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

1. Some "Scan" tester display the gear switch status 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 2nd or 3rd 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 2nd gear.

If vehicle is road tested for a TCC related problem, ensure the switch state does not change while in 3rd gear because the TCC will disengage. If switch changes state, carefully inspect wire harness/routing and connections.

Chart C-8A, TCC - THM 3T40 - 3.1L A Body Flowchart (2 Of 3). Scheme 158

Chart C-8A, TCC - THM 3T40 - 3.1L A Body Flowchart (3 Of 3). Scheme 159

Scheme 160

Scheme 161

The TCC will engage when the engine is warmed up, vehicle speed is greater than about 32 MPH, throttle sensor output is not changing (indicating a steady road speed), 3rd gear switch is closed and brake switch is closed. An engine coolant thermostat that is stuck open, or opens at too low a temperature, may result in an inoperative TCC.

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

1. When test light is off, this confirms 3rd gear switch is open.
2. At 25 MPH the 2nd gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding the ALDL "test" terminal with ignition on and engine off, should energize the TCC solenoid by grounding circuit No. 422. This test checks the capability of the ECM to supply a ground for the TCC solenoid. Test light should turn on as circuit No. 422 is grounded.

The "Scan" tester only indicates when the ECM has commanded the TCC driver to engage. This does not confirm that the TCC has engaged. To determine if the TCC has engaged, RPM should drop when the "Scan" tester indicates the TCC driver has turned on.

Chart C-8A, TCC - THM 3T40 - 3.3L A & N Bodies - Flowchart (1 Of 2). Scheme 162

Chart C-8A, TCC - THM 3T40 - 3.3L A & N Bodies - Flowchart (2 Of 2). Scheme 163

CHART C-8, TORQUE CONVERTER CLUTCH - 3.8L PFI (TURBO) G BODY

The purpose of the converter clutch feature is to eliminate the power loss of the torque converter when the vehicle is in a cruise condition. This allows the convenience of an automatic and the fuel economy of a manual transmission. The heart of the system is an ECM-controlled solenoid which is located inside the transmission.

When the solenoid coil is energized, the torque converter clutch is applied, which results in straight through mechanical coupling from the engine to the wheels. When the transmission solenoid is deactivated. the torque converter clutch is released which allow the torque converter to operate in the conventional manner (fluid coupling between engine and transmission). The TCC will engage on a warm engine under road load, in 4th gear only.

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

1. A test light on indicates battery voltage and continuity through TCC solenoid is okay.
2. Vacuum hose on throttle shaft pin increases TPS signal so TCC will engage without excessive wheel speed. Without the hose, it would require vehicle speed in excess of 65 MPH to engage TCC.
3. Checks for vehicle speed signal to ECM. Voltage should vary from 2-9 volts.
4. Checks 3rd and 4th gear signal to ECM. Signals will not prevent TCC engagement, speed points.
5. Solenoids and relays are turned on or off by ECM internal electronic switches called "drives". Each driver is part of a group of 4 called "quad-drivers". Failure of one can damage any other driver within the set.

Note. Before replacing ECM, be sure to check the coil resistance of all solenoids and relays controlled by the ECM.

Chart C-8, TCC - 3.8L PFI (Turbo) - G Body - Flowchart (1 Of 2). Scheme 164

Chart C-8, TCC - 3.8L PFI (Turbo) - G Body - Flowchart (2 Of 2). Scheme 165

Scheme 166

The Vehicle Speed Sensor (VSS) is a PM generator type located in the transmission or transaxle. As the vehicle moves, the generator creates AC voltage signal which is routed to the buffer inside the ECM. In the buffer, the AC signal is changed from a "sine wave" to a "square wave" and amplified. The "square wave" is an on/off signal. The length of time between pulses determines vehicle speed.

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

1. Code 24 will set if vehicle speed equals less than 3 MPH when the following conditions occur: Engine is running. Vehicle is in high gear. No Code 29 or 31 is present. All conditions met for 40 seconds.
2. Before replacing ECM, check MEM-CAL for proper application.

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation.

Inspect ECM harness connectors for improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness. If connections and harness check out okay, raise drive wheels (support drive axles to prevent damage to CV joints). Block other wheels and idle engine at greater than 3 MPH, in low gear. Use "Scan" tester to check the vehicle speed while moving related connectors and wiring harness. If the failure is induced, the vehicle speed display will change. This may help to isolate the location of the malfunction.

Code 24, VSS - 3.3L & 3.8L PFI - Flowchart. Scheme 167

CODE 25, MANIFOLD AIR TEMPERATURE SENSOR SIGNAL VOLTAGE HIGH

3.8L PFI

Scheme 168

Code 25 will set if the following conditions occur

1. Signal voltage indicates manifold air temperature is greater than 275°F (135°C).
2. Vehicle speed is greater than 35 MPH.
3. Both of the above requirements are met for at least 16 seconds. Due to conditions necessary to set a Code 23, the SERVICE ENGINE SOON light will stay on only while the fault is present.

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

1. The "Scan" tester may not be used to diagnose this fault, due to the ECM transmitting "default" values. ECM and wiring are good if voltage is greater than 4 volts.
2. If resistance is less than 185 ohms, replace sensor. INTERMITTENTS is located in THEORY & OPERATION section.

Note. A Scan Tool may not be used to diagnose this fault, due to the ECM transmitting "DEFAULT" (substitute) values when the fault is present.

Code 25, MAT Sensor Signal Voltage High - 3.8L PFI - Flowchart. Scheme 169

Scheme 170

The digital EGR valve is designed to accurately supply EGR to the engine independent of intake manifold vacuum. The valve controls EGR flow from the exhaust to the intake manifold through 3 orifices which increment in size to produce 7 combinations. When solenoid is energized, the armature with attached shaft and swivel pintle is lifted, opening the orifice. Flow accuracy is dependent on metering orifice size only, which results in improved control.

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

1. Disconnect 4-wire EGR connector. Install a fused jumper from battery to terminal "D" of the EGR valve. Start engine and allow to idle. Grounding terminal "A" should result in a small change in RPM, while terminal "C" should result in a large change in RPM. Engine may stall when terminal "C" is grounded. If the EGR valve shows signs of extreme heat (melting), check the exhaust system for blockage. See CHART B-1. If the exhaust system is plugged due to a restricted converter, this may be due to a defective injector or injector circuit which is causing injector to open constantly.

Chart C-7, EGR - 3.8L PFI - Flowchart. Scheme 171

Scheme 172

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

1. With engine idling, a knock signal should not be present at ECM, as detonation is not likely under no-load condition.
2. Tapping on the engine lift hook bracket should simulate a knock condition. This will determine if knock sensor is capable of detecting detonation. If knock is not detected, try tapping on engine block near sensor. If knock is still not detected, replace sensor.
3. If engine has an internal problem that is simulating a knock, the knock sensor may be responding to that condition. Internal problem should be diagnosed and repaired.
4. This test determines if the knock sensor is faulty, or the ESC portion of the MEM-CAL is faulty. If MEM-CAL is at fault, ensure that it is properly installed and latched in place. If not properly installed, repair and retest.

While observing "Scan" tester, a knock should be indicated whenever detonation is present (heard). Detonation is most likely to take place under high engine load condition.

Chart C-5, ESC System Check - 2.3L PFI - Flowchart. Scheme 173

Scheme 174

The TCC will engage under the following conditions

1. Vehicle road speed exceeds calibrated value of about 34 MPH (55km/h).
2. TPS not varying signal, indicating a steady road speed.
3. Transmission 2nd gear switch closed.
4. Brake switch closed.

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

1. If light is not illuminated, this confirms transmission 2nd gear apply switch is open.
2. Transmission 2nd gear apply switch should close at 25 MPH. Test light should illuminate and confirm battery voltage, and closed brake switch.
3. Grounding diagnostic terminal with ignition on and engine off, should energize TCC solenoid. When terminal is grounded, circuit No. 422 is simultaneously grounded. This test ECM's ability to supply a ground to the TCC solenoid. Test light should illuminate when connected to battery voltage and ALDL terminal "F". This illumination occurs as circuit No. 422 is grounded.

"Scan" tester will only indicate whether ECM has turned on TCC driver, and does not confirm that TCC has engaged. Check for RPM drop as "Scan" tester indicates TCC driver has been turned on. If RPM drops occurs, TCC has engaged.

Chart C-8A, TCC (Electrical Diagnosis) 2.3L PFI - Flowchart (1 Of 2). Scheme 175

Chart C-8A, TCC (Electrical Diagnosis) 2.3L PFI - Flowchart (2 Of 2). Scheme 176

Scheme 177

This chart will check 2nd and 3rd gear switch inputs to ECM. Transmission gear input signals to ECM are determined as follows

GEAR INPUT SIGNALS

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

1. This checks for an open or grounded circuit between transmission connector and ECM. Since ECM terminals are normally high, battery voltage should be indicated at both terminals with connector removed. A DVOM must be used for this step.
2. A test light to battery voltage is used in this step to check transmission switches for normal state. This is 1st gear (of Park/Neutral) state. The test light should be "ON" for normally closed (N.C.) 2nd gear switch. Use of Wiring Probe Kit (J-35616) will assist here and step 3).
3. This checks if transmission switches function normally. In 3rd gear both switches should have changed from their original 1st gear state.

A sticky or improperly functioning 2nd or 3rd gear switch may cause Code 26 to set intermittently.

Chart C-8D, Trans. Gear Switches Circuit Check - 2.3L PFI - Flowchart. Scheme 178

CHART C-8, TORQUE CONVERTER CLUTCH 2.3L PFI - N BODY

The purpose of the TCC is to prevent converter slippage during cruise conditions. Once the TCC is engaged, all slippage is eliminated, thus improving fuel economy. Fused battery voltage is supplied to the TCC solenoid through the brake switch, and transmission 3rd gear apply switch. ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

The TCC will engage under the following conditions

1. Vehicle road speed exceeds calibrated value of about 34 MPH (55m/h).
2. TPS not varying signal, indicating a steady road speed.
3. Transmission 2nd gear switch closed.
4. Brake switch closed.

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

1. If light is not illuminated, this confirms transmission 2nd gear apply switch is open.
2. Transmission 2nd gear apply switch should close at 25 MPH. Test light should illuminate and confirm battery voltage, and closed brake switch.
3. Grounding diagnostic terminal with ignition on and engine off, should energize TCC solenoid. When terminal is grounded, circuit No. 422 is simultaneously grounded. This test ECM's ability to supply a ground to the TCC solenoid. Test light should illuminate when connected to battery voltage and ALDL terminal "F". This illumination occurs as circuit No. 422 is grounded.

Scan tester will only indicate whether ECM has turned on TCC driver, and does not confirm that TCC has engaged. Check for RPM drop as "Scan" tester indicates TCC driver has been turned on. If RPM drops occurs, TCC has engaged.

Chart C-8, Torque Converter Clutch 2.3L PFI - N Body Flowchart (1 Of 2). Scheme 179

Chart C-8, Torque Converter Clutch 2.3L PFI - N Body Flowchart (2 Of 2). Scheme 180

Scheme 181

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

1. The following conditions must exist for a Code 23 to set: Signal voltage indicates manifold air temperature is less than -29°F (-34°C). Engine had been running for at least 320 seconds. Vehicle speed is less than 15 MPH.
2. Code 23 is set due to an open sensor, wire or connection. This test determines if wiring and ECM are okay.
3. This test indicates if signal circuit No. 472 or 5-volt return circuit No. 469 is open.

Scan tester should display temperature of air entering engine close to ambient air temperature when engine is cold, and rise as underhood temperature increases. Improper connections or open circuits No. 472 or 469, can set Code 23. Code 23 and 24 stored simultaneously, could be result of an open circuit No. 469. See MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES table.

Code 23, Manifold Air Temperature Sensor Signal Voltage High 2.3L PFI Flowchart. Scheme 182

Scheme 183

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

1. Code 24 will set (with vehicle speed less than 2 MPH) only if the following conditions are met: Engine RPM is between 1600 and 3600. TPS is greater than 7 percent. Gear selector not in Park or Neutral. All conditions met for 20 seconds. No Codes 21 or 22. Above conditions are met during road load deceleration. Ignore Code 24 that sets when drive wheels are turning faster than 3 MPH. PM generator only produces a signal if drive wheels are turning greater than 3 MPH.
2. Ensure that correct PROM is used before replacing ECM.

A problem in circuit No. 434 will not affect VSS input or readings displayed by "Scan" tester. Ensure that circuits No. 400 and 401 have proper connections. Also check for correct harness routing.

Note. On vehicles with automatic transmission, a faulty or improperly adjusted Park/Neutral switch can result in a false Code 24. Use a Scan tester and check for proper signal while in Drive. Refer to CHART C-1A for P/N switch diagnosis.

Note. Disregard CODE 24 if set while drive wheels are not turning.

Code 24, Vehicle Speed Sensor Circuit - 2.3L PFI Flowchart. Scheme 184

CODE 24, VEHICLE SPEED SENSOR - 2.3L PFI - N BODY

The speed sensor, which is a Permanent Magnet (PM) generator, provides ECM with vehicle speed information. The PM generator, mounted in the transmission, produces a pulsing voltage signal whenever vehicle speed is greater than 3 MPH. The AC voltage level and the number of pulses increase with vehicle speed. ECM converts pulsing voltage to MPH, which is used for calculations. MPH can be displayed with a "Scan" tool. Output of generator can be monitored with a digital voltmeter on the AC scale while rotating generator. The function of the VSS buffer used in past models years has been incorporated into the ECM. ECM supplies the necessary signal for the instrument panel (4000 pulses per mile) for operating the speedometer, and for the odometer and cruise control (2000 pulses per mile).

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

1. Code 24 will set only if the following conditions are met: Engine RPM is between 1600 and 3600. TPS is less than 7 percent. Gear selector not in Park or Neutral. All conditions met for 20 seconds. No Codes 21 or 22. Above conditions are met during road load deceleration. Ignore Code 24 that sets when drive wheels are turning faster than 3 MPH.
2. Ensure that correct PROM is used before replacing ECM.

Vehicle speed should be indicated whenever drive wheels are rotating greater than 3 MPH. Ensure that circuits No. 400 and 401 have proper connections. Also check for correct harness routing.

Note. On vehicles with automatic transmission, a faulty or improperly adjusted Park/Neutral switch can result in a false Code 24. Use a Scan tool and check for proper signal while in drive. Refer to CHART C-1A for P/N switch diagnosis check.

Code 24, Vehicle Speed Sensor - 2.3L PFI - N Body Flowchart. Scheme 185

Scheme 186

The Park/Neutral switch contacts are a part of the neutral start switch and are closed to ground in Park of Neutral and open in Drive ranges. The 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. The ECM uses the P/N signal as one of the inputs to control Idle Air Control (IAC) and VSS diagnostics.

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

1. This test checks for a closed switch to ground in Park position. If using an ohmmeter, instead of a test light or Scan tester, the resistance will be low, indicating continuity to ground.
2. This test checks for an open switch in Drive range.
3. Ensure that Scan tester indicates Drive while wiggling shifter to test for a faulty or misadjusted switch.

Chart C-1A, Park/Neutral Switch - 2.3L PFI - N Body Flowchart. Scheme 187

Scheme 188

The purpose of automatic transmission/transaxle Torque Converter Clutch (TCC) feature is to eliminate power loss of torque converter stage when vehicle is in cruise condition. This allows convenience of automatic transmission/transaxle and fuel economy of a manual transmission. Fused battery ignition is supplied to TCC solenoid through brake switch, and transmission 3rd gear apply switch. The ECM will engage TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage when vehicle speed is above 30 MPH, engine at normal operating temperatures above 158°F (70°C), throttle position sensor output not changing (indicating a steady road speed), transmission 3rd gear switch closed, and brake switch closed.

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

1. Light off confirms transmission 3rd gear apply switch is open.
2. At 30 MPH transmission/transaxle 3rd gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding diagnostic terminal with engine off should energize TCC solenoid. This test checks capability of ECM to control solenoid. Solenoids are turned on or off by ECM internal electronic switches called "drivers". Failure of one can damage another driver within a set. Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure of ECM driver. Using an ohmmeter, check solenoid coil resistance before installing a replacement ECM. Check TCC solenoid resistance. Disconnect TCC at transmission. Connect ohmmeter between transmission connector opposite harness connector terminals "A" and "D". Raise drive wheels. Run vehicle in Drive at about 30 MPH to close 3rd gear apply switch. Replace TCC solenoid and ECM if resistance measures less than 20 ohms when switch is close.

Chart C-8, TCC - 2.0L & 2.5L TBI - A, J, & N Bodies Flowchart (1 Of 2). Scheme 189

Chart C-8, TCC - 2.0L & 2.5L TBI - A, J, & N Bodies Flowchart (2 Of 2). Scheme 190

Scheme 191

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

A Code 24 will set when MPH reads zero, transmission is not in Park or Neutral, engine speed is between 1400-4400 RPM, TPS is at 2 percent (closed throttle), and a high manifold vacuum is sensed by the MAP sensor. All of these conditions must be met for 3-5 seconds.

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

1. This test monitors the ECM voltage on circuit No. 993. With wheels turning, the pulsating action created in circuits No. 400 and 401 will result in a voltage signal to the ECM and be displayed as MPH. The PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
2. 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.

Code 24, VSS - 2.0L & 2.5L TBI - A, J, & N Bodies (With PM Generator) Flowchart. Scheme 192

CODE 24, VEHICLE SPEED SENSOR 2.5L TBI - A BODY (WITH IN-DASH SPEED SENSOR)

The ECM applies and monitors a 12-volt signal on circuit No. 437. Circuit No. 437 connects to the vehicle speed sensor and is alternately grounded when the wheels are turning. The pulsing action takes place approximately 2000 times per mile and the ECM will calculate vehicle speed based on the time between pulses.

Code 24 will set if vehicle speed reads zero MPH, transmission is not in Park or Neutral, engine speed is between 1400-4400 RPM, TPS is less than 2 percent, a high manifold vacuum is sensed by the MAP sensor, and these conditions have existed for more than 5 seconds.

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

1. ECM monitors circuit No. 437, counting voltage pulses created by the grounding of the speed sensor. "Scan" reading should closely match speedometer reading with drive wheels turning.
2. 8-12 volts at the I.P. connector indicates circuit No. 437 is open or there is a faulty speed sensor. A voltage of less than one volt at I.P. connector indicates that circuit No. 437 wire is shorted to ground or open between the connector and the ECM. The I.P. connector is located in the center console near the ECM.

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, VSS 2.5L TBI - A Body (With In-Dash Speed Sensor) Flowchart. Scheme 193

Scheme 194

The Park/Neutral switch contacts are part of neutral start switch, are closed to ground in Park or Neutral, and open in Drive ranges. The ECM uses this signal as one of the inputs to control IAC and VSS diagnostics.

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

1. Checks for closed switch to ground in Park position. Use an ohmmeter, instead of a test light to 12 volts. Resistance will be low, indicating continuity to ground.
2. Checks for an open switch in Drive range. Use an ohmmeter, instead of a test light to 12 volts. Resistance will be high or infinity, indicating an open switch.
3. Be sure "Scan" tester indicates Drive. Wiggle shifter to test for an intermittent or misadjusted switch.

Chart C-1A, P/N Switch - 2.0L & 2.5L TBI - A, J, & N Bodies Flowchart. Scheme 195

Scheme 196

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

1. If the "misfire" compliant exists at idle only, use CHART C-4D-1. Spark Tester (ST-125) must be used because it is essential to verify adequate available secondary voltage at the spark plug (25,000 volts). Spark should jump tester gap on all 4 plug wires.
2. If spark jumps the tester gap after grounding the opposite plug wire, it indicates excessive resistance in the plug which was by-passed. A faulty or poor connection at that plug could also result in the misfire condition. Also check for carbon deposits inside the spark plug boot.
3. If carbon tracking is evident, replace coil and be sure spark plug wire to that coil is clean and tight. Excessive wire resistance or faulty connections could have caused the coil to be damaged.
4. If the no spark condition follows the suspected coil, that coil is faulty. Otherwise, the ignition module is the cause of no spark. This test could also be performed by substituting a known good coil for the one causing the no spark condition.

Chart C-4D-2, DIS Misfire Under Load - 2.0L (VIN 1) TBI Flowchart. Scheme 197

CHART C-8A, TRANSMISSION CONVERTER CLUTCH - 2.0L (VIN 1) TBI - J & L BODIES

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

1. Light off confirms transmission 3rd gear apply switch is open.
2. At 30 MPH transmission/transaxle 3rd gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding ALDL "test" terminal with engine off should energize TCC solenoid by grounding circuit No. 422. This test checks capability of ECM to control solenoid.

A thermostat that is stuck in the open position, or opens at too low a temperature, may result in an inoperative TCC.

Chart C-8A, TCC - 2.0L (VIN 1) TBI - J & L Bodies Flowchart (1 Of 2). Scheme 198

Chart C-8A, TCC - 2.0L (VIN 1) TBI - J & L Bodies Flowchart (2 Of 2). Scheme 199

Scheme 200

A/C clutch operation is delayed by ECM for approximately .43 second after A/C is turned on. This allows the IAC to adjust RPM before the A/C clutch engages. The ECM will also disengage the A/C relay during Wide Open Throttle (WOT) operation, during power steering operation greater than 40 psi (2.8 kg/cm 2 ), ECM will engage clutch relay by grounding circuit No. 459. At about 430 psi (30 kg/cm 2 ), the high pressure switch will open to disengage the A/C clutch and prevent system damage.

If complaint was insufficient cooling, the problem may be caused by an inoperative cooling fan or A/C pressure fan switch. Engine cooling fan should turn on when A/C pressure exceeds value to open switch. See CHART C-12 to diagnose cooling fan.

Chart C-10, A/C Clutch Control - 2.0L (VIN 1) TBI J Body Flowchart (1 Of 2). Scheme 201

Chart C-10, A/C Clutch Control - 2.0L (VIN 1) TBI J Body Flowchart (2 Of 2). Scheme 202

Scheme 203

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

1. This test checks to see if Code 23 is the result of a hard failure or an intermittent condition. Code 23 will be displayed if the engine is running for longer than 2 minutes (J Body) or 8.5 minutes (L Body), signal voltage indicates a MAT temperature less than -22°F 30°C) or there is no VSS signal.
2. This test simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the ECM and wiring are okay.
3. This test checks continuity of sensor wiring. If wiring is open there may also be a Code 21.

If the engine has been allowed to cool to an ambient temperature (overnight), coolant and MAT temperature may be checked with the "Scan" tester and should read close to each other. A code 23 will result if either sensor wire becomes open. See MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES table.

Code 23, MAT Sensor - Temp. Too Low - 2.0L (VIN 1) TBI Flowchart. Scheme 204

Scheme 205

Vehicle speed information is provided to the ECM by the vehicle speed sensor Permanent Magnet (PM) generator mounted in the transaxle. The PM generator produces a pulsing voltage whenever vehicle speed is greater than 3 MPH. The A/C voltage level and the number of pulses increases with vehicle speed. The ECM then converts the pulsing voltage to MPH, which is used for calculations. MPH can be displayed with the "Scan" tester and should match speedometer reading.

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

1. Code 24 will set if vehicle speed equals zero MPH when the following conditions occur: Engine speed is between 1400 and 3600 RPM. Throttle angle is less than 2 percent. Low load condition, low MAP voltage, high manifold vacuum. Not in Park or Neutral. All conditions met for 5 seconds. These conditions are met during a road load deceleration. Disregard Code 24 that sets when drive wheels are not turning.
2. Circuits No. 400, 401 and 993 are okay if the speedometer works properly. Code 24 is being caused by a faulty ECM, faulty PROM or an incorrect PROM.

The Scan tester should indicate a vehicle speed whenever the drive wheels are turning more than 3 MPH. Check circuits No. 400 and 401 for proper connections. Make sure connections are clean and tight and the harness is routed correctly. ON vehicles with automatic transmissions, a faulty or misadjusted Park or Neutral safety switch can result in a false Code 24. Use the Scan tester and check for proper signal while in Drive. Scan reading should match speedometer.

Code 24, Vss - 2.0L (VIN 1) TBI Flowchart. Scheme 206

Scheme 207

The Park/Neutral switch contacts are part of neutral start switch. They are closed to ground in Park or Neutral, and open in Drive ranges. The 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. The ECM uses this signal as one of the inputs to control IAC and VSS diagnostics.

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

1. Checks for a closed switch to ground with transmission in Park position. Different makes of "Scan" testers will display P/N status differently. Refer to manufacturer's tester operation manual for type of display used.
2. Checks for an open switch circuit in Drive.
3. Be sure "Scan" indicates Drive, even while wiggling shifter to test for an intermittent or misadjusted switch in Drive.

Chart C-1A, P/N Switch - 2.0L (Vin 1) TBI - J & L Bodies Flowchart. Scheme 208

CHART C-8, TORQUE CONVERTER CLUTCH - 2.0L PFI - J & N BODIES

Fused battery ignition is supplied to the Torque Converter Clutch (TCC) solenoid through the brake switch and transmission/ transaxle third gear apply switch. The ECM will engage TCC by grounding circuit No. 422 to energize the solenoid. TCC will engage when engine is warmed up, vehicle speed is greater than a calibrated value, throttle position sensor output is not changing (indicating a steady road speed), transaxle third gear apply switch is closed and brake switch is closed.

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

1. Light off confirms transmission/transaxle third gear supply switch is open.
2. At 30 MPH the transmission/transaxle third gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding the diagnostic terminal with engine off should energize the TCC solenoid. This test checks the capability of the ECM to control the solenoid.
4. Solenoids and relays are turned on and off by the ECM internal electronic switches called "Drivers". Each Driver is part of a group of 4 called "Quad-Drivers". Failure of one can damage any other Driver within the set.

Check TCC solenoid resistance. Disconnect the TCC at transmission. Connect ohmmeter between transmission connector, opposite harness connector terminal "A" and "D". Raise drive wheels (support drive axles) and run vehicle in Drive to about 30 MPH to close third gear apply switch. Replace TCC solenoid and ECM if resistance measures less than 20 ohms when switch is closed.

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

Chart C-8, TCC - 2.0L PFI - J & N Bodies Flowchart (1 Of 2). Scheme 209

Chart C-8, TCC - 2.0L PFI - J & N Bodies Flowchart (2 Of 2). Scheme 210

CODE 24, VEHICLE SPEED SENSOR 2.0L PFI - J & N BODIES

The Vehicle Speed Sensor (VSS), which is a Permanent Magnet (PM) generator mounted in the transmission, provides the ECM with vehicle speed information. The PM generator produces a pulsing voltage signal whenever the vehicle speed is greater than 3 MPH. The voltage level and the number of pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH, which is used by the ECM in calculations to determine vehicle adjustments.

A Code 24 will set when MPH reads zero, transmission is not in Park or Neutral, engine speed is between 1400-4400 RPM, TPS is at 2 percent (closed throttle), and a low airflow condition is sensed. All of these conditions must be met for at least 3 seconds.

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

1. This test monitors the ECM voltage on circuit No. 389. With wheels turning, the pulsing action created in circuits No. 400 and 401 will result in a voltage signal to the ECM, and be displayed as MPH. The PM generator only produces a voltage signal if drive wheels are turning at speeds greater than 3 MPH.
2. 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.

Note. Disregard CODE 24 if set while drive wheels are not turning.

Code 24, Vehicle Speed Sensor 2.0L PFI - J & N Bodies Flowchart. Scheme 211

CHART C-1A, PARK/NEUTRAL SWITCH - 2.0L PFI - J & N BODIES

The Park/Neutral (P/N) switch contacts are a part of the Neutral/Start switch and are close to ground in Park or Neutral and open in Drive. The ECM supplies ignition voltage through a current limiting resistor to circuit No. 434 and senses a closed switch when the voltage on circuit No. 434 drops to less than one volt. The ECM uses the P/N signal as one of the inputs to control Idle Air Control (IAC) and Vehicle Speed Sensor (VSS) diagnostics.

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

1. This test checks for a closed switch to ground in Park position. If using an ohmmeter instead of a test light or "Scan" tester, the resistance will be low, indicating continuity to ground.
2. This test checks for an open switch in Drive.
3. Be sure "Scan" tester indicates Drive while wiggling shifter to test for a faulty or misadjusted switch.

Chart C-1A, Park/Neutral Switch - 2.0L PFI - J & N Bodies Flowchart. Scheme 212

Chart C-7, EGR Valve Check - 2.5L A & N Bodies Flowchart. Scheme 213

CHART C-8, TORQUE CONVERTER CLUTCH - 2.0L (VIN K) & 2.5L TBI

The purpose of automatic transmission/transaxle Torque Converter Clutch (TCC) feature is to eliminate power loss of torque converter stage when vehicle is in cruise condition. Fused battery ignition is supplied to TCC solenoid through brake switch, and transmission 3rd gear apply switch. The ECM will engage TCC by grounding circuit No. 422 to energize solenoid. TCC will engage when vehicle speed is greater than 30 MPH, engine at normal operating temperatures greater than 158°F (70°C), throttle position sensor output not changing (indicating a steady road speed), transmission 3rd gear switch closed, and brake switch closed.

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

1. Light off confirms transmission 3rd gear apply switch is open.
2. At 30 MPH transmission/transaxle 3rd gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding ALDL "test" terminal with engine off should energize TCC solenoid. This test checks capability of ECM to control solenoid.

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

Chart C-8, TCC - 2.0L (VIN K) & 2.5L TBI Flowchart (1 Of 2). Scheme 214

Chart C-8, TCC - 2.0L (VIN K) & 2.5L TBI Flowchart (2 Of 2). Scheme 215

Scheme 216

The ECM supplies and monitors a voltage signal (4-6 volts) to the sensor through circuit No. 472. When temperatures are low, sensor resistance is high and the ECM will see a high monitored voltage signal. As temperatures increase, sensor resistance decreases and the voltage sensed by the ECM drops.

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

1. This step checks to see if Code 23 is the result of a hard failure or an intermittent condition. Code 23 will set if the engine has been running for more than 58 seconds and the signal voltage indicates a MAT temperature less than -22°F (-30°C).
2. This step simulates conditions for a Code 25. If the Scan tester displays a high temperature, the ECM and wiring are not at fault.
3. This step checks for continuity of circuits No. 472 and 469. If circuit No. 469 is open there may also be a Code 33 stored.

If the engine is allowed to cool overnight, the coolant and MAT sensors, when measured with a Scan tester, should read close to each other. See MAT SENSOR TEMP VS. RESISTANCE VALUE table.

When Code 15 is set, the ECM will turn on the cooling fan. A Code 23 will also result if circuits No. 472 or 469 become open.

Code 23, MAT Sensor Temp. Low - 2.0L (VIN K) & 2.5L TBI Flowchart. Scheme 217

Scheme 218

The speed sensor, which is a Permanent Magnet (PM) generator, provides the ECM with vehicle speed information. The PM generator, mounted in the transmission, produces a pulsing voltage signal whenever the vehicle speed is over 3 MPH. The voltage level and the number of pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH, which is used by the ECM 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 is between 1800-4400 RPM, TPS is at zero percent (closed throttle), and a high manifold vacuum is sensed by the MAP sensor. All of these conditions must be met for 3-5 seconds. The PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
2. 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.

Note. Disregard CODE 24 if set while drive wheels are not turning.

Code 24, VSS - 2.0L (VIN K) & 2.5L TBI - A, J & N Bodies (With PM Generator) Flowchart. Scheme 219

Scheme 220

Fused battery ignition is supplied to the Torque Converter Clutch (TCC) solenoid through the brake switch and transmission/ transaxle 3rd gear apply switch. The ECM will engage TCC by grounding circuit No. 422 to energize the solenoid. TCC will engage when engine is warmed up, vehicle speed is greater than a calibrated value, throttle position sensor output is not changing (indicating a steady road speed), transaxle 3rd gear apply switch is closed and brake switch is closed.

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

1. Light off confirms transmission/transaxle 3rd gear supply switch is open.
2. At 30 MPH the transmission/transaxle 3rd gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding the diagnostic terminal with engine off should energize the TCC solenoid. This test checks the capability of the 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.

Chart C-8, TCC - 2.0L (VIN M) PFI - Flowchart (1 Of 2). Scheme 221

Chart C-8, TCC - 2.0L (VIN M) PFI - Flowchart (2 Of 2). Scheme 222

Scheme 223

The Permanent Magnet (PM) generator mounted in the transmission, produces a pulsing voltage signal whenever the vehicle speed is greater than 3 MPH. The voltage level and the number of pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH, which is used by the ECM in calculations to determine vehicle adjustments.

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

1. Code 24 will set if vehicle speed equals zero MPH when transmission is not in Park or Neutral, engine speed is between 1600-4400 RPM, TPS indicates closed throttle, and a low airflow condition is sensed. All of these conditions must be met for at least 5 seconds.
2. Before replacing the ECM, 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, VSS (PM Generator) - 2.0L (VIN M) PFI Flowchart. Scheme 224

Scheme 225

Code 25 will set if signal voltage indicates a manifold air temperature greater than 275°F (135°C) for at least 30 seconds. Due to the conditions necessary to set Code 25, the "SERVICE ENGINE SOON" light will only stay on while the fault is present.

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

1. A Scan tester may not be used to diagnose this fault due to the "default values" substituted by the ECM. If voltage is greater than 4 volts, the ECM and wiring are not at fault.
2. If the resistance is less than 100 ohms, replace the sensor. See «MAT SENSOR TEMP VS. RESISTANCE VALUE»(/cadillac/brougham/i-1987-1992/remont/automatic-trans/#diagnosis-torque-converter-clutch) table.

Note. INTERMITTENTS is located in THEORY & OPERATION section of this article.

Code 25, MAT Sensor Signal Voltage Low - 2.0L (VIN M) PFI Flowchart. Scheme 226

CHART C-8, TORQUE CONVERTER CLUTCH - 3.8L "3800" PFI BONNEVILLE, ELECTRA & NINETY-EIGHT

The purpose of the converter clutch is to eliminate the power loss of the torque converter when vehicle is in cruise condition. This allows the convenience of automatic transmission and fuel economy of a manual transaxle. The heart of the system is a solenoid located inside the transaxle which is controlled by the ECM.

When solenoid coil is activated (on), the torque converter clutch is applied which results in straight through mechanical coupling from engine to drive wheels. When the transaxle (TCC) solenoid is deactivated, the torque converter clutch is released which allows the torque converter to operate in the conventional manner (fluid coupling between engine and transaxle).

TCC will engage under the following conditions

1. Engine warmed to operating temperature.
2. Vehicle speed greater than a calibrated value (about 28 MPH).
3. TPS output not changing, indicating a steady road speed.
4. Brake switched closed.

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

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

The Scan tester only indicates when ECM has turned on TCC driver, and this does not confirm that TCC has engaged. To determine if TCC is functioning properly, road test vehicle. Engine RPM should decrease when "Scan" tester indicates TCC driver has turned on.

Chart C-8, TCC - 3.8L PFI Bonneville, Electra & Ninety-Eight Flowchart (1 Of 2). Scheme 227

Each gear switch opens when the 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 testers display the state of these switches in different ways. Read instructions on Scan tester usage before proceeding with test procedures. Since both switches should be in closed state during this test, "Scan" tester should indicate same readings for 2nd, 3rd or 4th gear switches.
2. Determines whether switch or signal circuit is open. The circuit can be checked for an open be measuring voltage at TCC connector. Reading should be about battery voltage. Because switch or switches should be grounded at some point in this procedure, unplugging TCC connector should cause "Scan" tester switch state to change. The 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, because TCC will disengage. If switches change state, carefully check wire routing and connections.

Chart C-8, TCC - 3.8L PFI - Bonneville, Electra & Ninety-Eight Flowchart (2 Of 2). Scheme 228

CODE 24, VEHICLE SPEED SENSOR - 3.8L "3800" PFI - BONNEVILLE, ELECTRA & NINETY-EIGHT

Vehicle speed information is provided to the ECM by the vehicle speed sensor, a permanent magnet (PM) generator mounted in the transmission. The PM generator produces a pulsing voltage whenever vehicle speed is greater than about 3 MPH. The AC voltage level and the number of pulses increases with vehicle speed. The ECM converts the pulsing voltage to MPH, and the MPH can be displayed with a Scan tester.

The function of the VSS buffer, used in the past model years, has been incorporated into the ECM. The ECM supplies the necessary signal for the instrument panel (4004 pulses per mile) for operating the speedometer and the odometer.

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

1. Code 24 will set if vehicle speed signal equals zero MPH when: Engine speed is between 1500 and 4000 RPM. TPS reading shows closed throttle. Low load condition (low air flow). Not in Park or Neutral. No Code 29 or 31. When vehicle is in 4th gear. All conditions met for 20 seconds. Disregard Code 24 that sets when drive wheels are not rotating. The PM generator only produces a signal if drive wheels are rotating faster than 3 MPH.
2. Before replacing the ECM, check MEM-CAL for correct application.

Scan tester should indicate a vehicle speed whenever drive wheels are rotating faster than 3 MPH. Check circuit No. 400 and 401 for proper connections. Ensure they are clean and tight, and harness is routed correctly. See INTERMITTENTS in TROUBLE SHOOTING section.

Code 24, VSS - 3.8L PFI - Bonneville, Electra & Ninety-Eight Flowchart. Scheme 229

Scheme 230

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

Code 31 will set under the following conditions

1. If circuit No. 434 indicates P/N(grounded) while in drive range and 4th gear TCC engaged, EGR would be inoperative, resulting in possible detonation.
2. If circuit No. 434 indicates drive (open) at start-up, a drop in idle may exist when gear selector is moved into drive range.
3. If transaxle 4th gear switch has an intermittent open, ECM thinks vehicle is in 4th gear and will set Code 31.

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

1. Checks for a closed switch to ground in "PARK" position. Different types of SCAN testers will read P/N differently. Refer to Scan tester operators manual for type of display used for specific tester.
2. Checks for an open switch in drive range.
3. Ensure Scan tester indicates Drive, even while wiggling shifter. This will test for an intermittent or misadjusted switch in Drive or Overdrive range.

Code 31, P/N Switch - 3.8L PFI - Bonneville, Electra & Ninety-Eight Flowchart. Scheme 231

Scheme 232

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 chart.

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 ground, 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.

Note. Mechanical checks such as linkage, oil level, etc. should be performed prior to using this chart.

Chart C-8A, TCC - THM 4T60 - 3.8L Flowchart (1 Of 2). Scheme 233

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

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 be 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 tester 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.

Chart C-8A, TCC - THM 4T60 - 3.8L Flowchart (2 Of 2). Scheme 234

CHART C-8B, TORQUE CONVERTER CLUTCH - 3.3L "A" BODY W/THM 4T60 TRANSMISSION (1 OF 2)

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

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

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 be measuring the voltage (with a voltmeter) at the TCC connector. Voltage should be about 12 volts.

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.

Chart C-8B, TCC - 3.3L "A" Body W/THM 4T60 - Flowchart (1 Of 2). Scheme 235

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

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 be measuring the voltage (with a voltmeter) at the TCC connector. Voltage should be about 12 volts.
3. Because switch should be grounded in this test, disconnecting TCC connector should cause SCAN tester switch state to change.
4. Switch state should change when vehicle shifts into 3rd 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.

Chart C-8B, TCC - 3.3L "A" Body W/THM 4T60 - Flowchart (2 Of 2). Scheme 236

Scheme 237

The purpose of the Torque Converter Clutch (TCC) feature is to eliminate the power loss of the torque converter when the vehicle is in a cruise condition. When the solenoid coil is activated by the ECM, the torque converter clutch is applied, resulting in a straight through mechanical coupling from 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 continuity of the TCC circuit from the fuse to the ALDL connector.
2. When the brake pedal is released, the light should come back on and then go off when the ALDL "test" terminal is grounded. This will test circuit No. 422 and the TCC driver in the ECM.

A 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 the Scan tester indicates the TCC driver has turned on.

Chart C-8B, TCC - THM 4T60 - 2.8L A Bodies Flowchart (1 Of 3). Scheme 238

The 3rd gear switch is open both in 3rd and 4th gear. The 4th gear switch is open only in 4th gear, which allows for TCC when in 4th gear. The TCC will disengage during a 4-3 downshift.

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

1. Some Scan testers display the state of these switches in different ways. Be familiar with the type of tester being used. Since both switches should be in the closed state during this test, the tester should read the same for either the 3rd and 4th gear switch.
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. Reading should be about 12 volts.
3. Because the switch should be grounded in this step, disconnecting the TCC connector should cause the SCAN tester statue to change.
4. Switch status should change when vehicle shifts into 3rd gear.

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

Chart C-8B, TCC - THM 4T60 - 2.8L A Bodies Flowchart (2 Of 3). Scheme 239

Chart C-8B, TCC - THM 4T60 - 2.8L A Bodies Flowchart (3 Of 3). Scheme 240

Scheme 241

The purpose of the Torque Converter Clutch (TCC) feature is to eliminate the power loss of the torque converter when the vehicle is in a cruise condition. When the solenoid coil is activated by the ECM, the torque converter clutch is applied, resulting in a straight through mechanical coupling from 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.

A 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 the Scan tester indicates the TCC driver has turned on.

Chart C-8B, TCC - THM 4T60 Transmission 2.8L W Bodies Flowchart (1 Of 3). Scheme 242

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

1. Some Scan testers display the gear switch status 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 be 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 status should change when vehicle shifts into 3rd gear.

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.

Chart C-8B, TCC - THM 4T60 Transmission 2.8L W Bodies Flowchart (2 Of 3). Scheme 243

Chart C-8B, TCC - THM 4T60 Transmission 2.8L W Bodies Flowchart (3 Of 3). Scheme 244

Scheme 245

The purpose of the Torque Converter Clutch (TCC) feature is to eliminate the power loss of the torque converter when the vehicle is in a cruise condition. When the solenoid coil is activated by the ECM, the torque converter clutch is applied, resulting in a straight through mechanical coupling from 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.

A 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 the Scan tester indicates the TCC driver has turned on.

Chart C-8B, TCC - THM 4T60 Transmission 3.1L W Bodies Flowchart (1 Of 3). Scheme 246

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

1. Some Scan testers display the gear switch status in different manner. 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.

Note. Checks made in this chart will not prevent the TCC from working, but will affect engagement or disengagement points.

Chart C-8B, TCC - THM 4T60 Transmission 3.1L W Bodies Flowchart (2 Of 3). Scheme 247

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

1. Switch state should change when vehicle shifts into 3rd gear.

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.

Chart C-8B, TCC - THM 4T60 Transmission 3.1L W Bodies Flowchart (3 Of 3). Scheme 248

See also:
• MAT SENSOR TEMP VS. RESISTANCE VALUE