Diagnosis - Torque Converter Clutch GMC Magnavan G3500

APPLICATIONS

THM 3L30 APPLICATIONS (1989 & 1990)

THM 4L60 APPLICATION (1989)

THM 4L60 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 63

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 64

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 64) 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.)

Scheme 65

The purpose of TCC is to eliminate the power loss of torque converter stage when vehicle is in "cruise" condition. This allows the use of an automatic transmission with the fuel economy of a manual transmission. Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the TCC solenoid. Engagement of torque converter clutch will be executed under the following conditions

1. Vehicle speed greater than 24 MPH.
2. Engine temperature above 149°F (65°C).
3. Steady TPS reading (not changing-steady road speed).
4. Brake switch closed.
5. Transmission in 3rd or 4th gear.

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

1. Check continuity through the brake switch and TCC solenoid.
2. Check ability of ECM to energize the TCC solenoid. Grounding the diagnostic connector should energize the relay and cause the light to go out.
3. This test by-passes the TCC solenoid and checks for an open or short in circuit No. 422.

Diagnostic Aids

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

Chart C-8, TCC - 2.5L & 2.8L Flowchart. Scheme 66

Scheme 67

The purpose of TCC is to eliminate the power loss of torque converter stage when vehicle is in "cruise" condition. This allows the use of an automatic transmission with the fuel economy of a manual transmission. Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the TCC solenoid. Engagement of torque converter clutch will be executed under the following conditions

1. Vehicle speed greater than 24 MPH.
2. Engine temperature above 149°F (65°C).
3. Steady TPS reading (not changing-steady road speed).
4. Brake switch closed.
5. Transmission in 3rd or 4th gear.

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

1. Glowing test light indicates that battery voltage and continuity through TCC solenoid are okay.
2. Checks for vehicle speed sensor (VSS) signal to ECM, using a SCAN tester.
3. Checks for 4th gear signal to ECM. This signal will not prevent TCC engagement, but could cause a change in engage/disengage speed points.

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

Chart C-8, TCC - 4.3L, 5.0L 5.7L Flowchart (1 Of 2). Scheme 68

Chart C-8, TCC - 4.3L, 5.0L 5.7L Flowchart (2 Of 2). Scheme 69

CODE 24, VEHICLE SPEED SENSOR

The ECM supplies and monitors 12-volts signal on circuit No. 437. Circuit No. 437 connects Vehicle Speed Sensor (VSS) which alternately grounds circuit No. 437 when drive wheels are turning. This pulsating action takes place about 2000 times per mile, and the ECM calculates vehicle speed based on time between "pulses". SCAN tester reading should closely match speedometer reading with drive wheels turning.

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

Code 24 will be set if the following conditions exist

1. Circuit No. 437 voltage is constant.
2. Engine speed is more than 200 RPM.
3. Vehicle speed signal (voltage on terminal "A9") is less than 10 MPH.
4. All conditions must be met for 10 seconds.

The above conditions are met during a road load deceleration.

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

SCAN tester reading should closely match speedometer reading with drive wheels turning. See PARK/NEUTRAL SWITCH DIAGNOSIS chart if vehicle is equipped with automatic transmission. If Park/Neutral switch is okay, see INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

Code 24, VSS Flowchart. Scheme 70

CHART C-1A, PARK/NEUTRAL SWITCH

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

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

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

If circuit No. 434 indicates Park/Neutral switch grounded while in Drive, EGR would be inoperative, resulting in possible detonation. If circuit No. 434 always indicates Drive (open), a drop in idle may exist when gear selector is moved into Drive.

Chart C-1A, P/N Switch Flowchart. Scheme 71

CODE 24, VEHICLE SPEED SENSOR CIRCUIT FAULT COMPUTER COMMAND CONTROL

The ECM supplies and monitors 12-volt signal on circuit No. 437. Circuit No. 437 connects Vehicle Speed Sensor (VSS) which alternately grounds circuit No. 437 when drive wheels are turning. This pulsating action takes place about 2000 times per mile. The ECM calculates vehicle speed based on time between pulses. "SCAN" tester readings should closely match speedometer reading with drive wheels turning.

Code 24 will set under the following conditions

1. Voltage on circuit No. 437 is constant.
2. Engine speed is more than 200 RPM.
3. Vehicle speed signal is less than 10 MPH.
4. All conditions must be met for at least 10 seconds.

These conditions are met during road load deceleration.

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

1. This test monitors ECM voltage on circuit No. 437. With wheels turning, pulsing action will result in varying voltage. Variation will be greater at low wheel speed to an average of 4-6 volts at about 20 MPH.
2. A voltage of less than one volt at ECM connector indicates that circuit No. 437 is shorted to ground. Disconnect circuit No. 437 at VSS. If voltage now reads above 10 volts, VSS is faulty. If voltage remains less than 10 volts, circuit No. 437 is grounded. If circuit No. 437 is not grounded, check for 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.

SCAN tester reading should closely match speedometer reading with drive wheels turning. See PARK/NEUTRAL SWITCH DIAGNOSIS chart if vehicle is equipped with automatic transmission. If Park/Neutral switch is okay, see INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

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

Code 24, VSS Circuit Fault - Computer Command Control Flowchart. Scheme 72

CODE 25, MANIFOLD AIR TEMPERATURE SENSOR VOLTAGE LOW - COMPUTER COMMAND CONTROL (2.5L ENGINE ONLY)

The Manifold Air Temperature (MAT) sensor uses a thermistor to control signal voltage to ECM. The ECM applies 4-6 volts on circuit No. 472 to sensor. When air is cold, sensor resistance is high, therefore, ECM will see high voltage signal. If air is warm, sensor resistance is low, therefore, ECM will see low voltage.

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

1. Code 25 will be set if the following conditions are met: Signal voltage indicates manifold air temperature below 302°F (150°C) for 2 seconds. Time since engine start is one minute or longer. Vehicle speed sensor reading is present (vehicle moving).

SCAN tester reads temperature of incoming air. This reading should be close to ambient air temperature when engine is cold, and rise as underhood air temperature increases. Carefully check harness and connections for possible short to ground in circuit No. 472. See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section. The MAT SENSOR TEMP VS. RESISTANCE VALUE table may be used to test MAT sensor at various temperatures to evaluate possibility of "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

MAT SENSOR TEMP VS. RESISTANCE VALUE (1)

Code 25, MAT Sensor Voltage Low - Computer Command Control (2.5L) Flowchart. Scheme 73

MAP OUTPUT CHECK - COMPUTER COMMAND CONTROL

The Manifold Absolute Pressure (MAP) sensor measures manifold pressure (vacuum) and sends that signal to the ECM. The MAP sensor is mainly used for fuel calculation, when ECM is running in the throttle body back-up mode. The MAP sensor is also used to determine barometric pressure and to help calculate fuel delivery.

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

1. This test checks MAP sensor output voltage to ECM. With engine off, this voltage represents barometric reading at ECM.
2. Applying 10 in. Hg vacuum to MAP sensor should cause voltage to be about 1.2-2.3 volts less than voltage in step 1. Upon applying vacuum to sensor, the change in voltage should be immediate. A slow voltage change indicates a faulty sensor.
3. Check vacuum hose to sensor for leaks or restrictions. Be sure no other vacuum devices are connected to MAP sensor vacuum hose.

MAP Output Check - Computer Command Control Flowchart. Scheme 74

MAP ALTITUDE-TO-VOLTAGE VALUES

PARK/NEUTRAL SWITCH DIAGNOSIS - COMPUTER COMMAND CONTROL

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

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

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

If circuit No. 434 indicates Park/Neutral switch grounded while in Drive, EGR would be inoperative, resulting in possible detonation. If circuit No. 434 always indicates Drive (open), a drop in idle may exist when gear selector is moved into Drive.

P/N Switch Diagnosis - Computer Command Control Flowchart. Scheme 75

TORQUE CONVERTER CLUTCH ELECTRICAL DIAGNOSIS - COMPUTER COMMAND CONTROL (2.5L & 2.8L ENGINES)

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

Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the TCC solenoid.

Torque converter clutch will engage under the following conditions

1. Vehicle speed greater than 24 MPH.
2. Engine temperature above 149°F (65°C).
3. Steady TPS reading (not changing-steady road speed).
4. Brake switch closed.
5. Transmission in 3rd or 4th gear.

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

1. This test checks continuity through brake switch and TCC solenoid.
2. This test checks ability of ECM to energize TCC solenoid. Grounding diagnostic connector should energize relay and cause light to go out.
3. This test by-passes TCC solenoid and checks for an open or short in circuit No. 422.

Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM driver. Before replacing ECM, check solenoid coil resistance of all ECM controlled solenoids and relays. Replace any solenoid or relay that measures less than 20 ohms resistance.

TCC Electrical Diagnosis - Computer Command Control (2.5L & 2.8L) Flowchart. Scheme 76

TORQUE CONVERTER CLUTCH ELECTRICAL DIAGNOSIS - COMPUTER COMMAND CONTROL (4.3L, 5.0L & 5.7L UNDER 8500 GVW)

The purpose of TCC is to eliminate the power loss of torque converter stage when vehicle is in "cruise" condition. This allows the use of an automatic transmission with the fuel economy of a manual transmission. Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the TCC solenoid. Engagement of torque converter clutch will be executed under the following conditions

1. Vehicle speed greater than 24 MPH.
2. Engine temperature above 149°F (65°C).
3. Steady TPS reading (not changing-steady road speed).
4. Brake switch closed.
5. Transmission in 3rd or 4th gear.

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

1. Glowing test light indicates that battery voltage and continuity through TCC solenoid are okay.
2. Checks for vehicle speed sensor (VSS) signal to ECM, using a SCAN tester.
3. Checks for 4th gear signal to ECM. This signal will not prevent TCC engagement, but could cause a change in engage/disengage speed points.

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

TCC Electrical Diagnosis - Computer Command Control (4.3L, 5.0L & 5.7L Under 8500 GVW). Scheme 77

TCC Electrical Diagnosis - Computer Command Control (4.3L, 5.0L & 5.7L Under 8500 GVW). Scheme 78

Circuit Description

When accelerator pedal is fully depressed, manifold vacuum drops, causing MAP sensor signal voltage to increase to about 4 volts. The ECM responds by grounding circuit No. 422 to energize downshift control relay. The downshift control relay then sends battery voltage to detent solenoid that causes a forced transmission downshift.

If problem is diagnosed as being an internal transmission problem, perform transmission service.

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

Downshift Control Diagnosis Flow Chart - (4L60 CCC TBI). Scheme 79

THM 400 Downshift Control Electrical Diagnosis Schematic. Scheme 80

CODE 23, THROTTLE POSITION SENSOR MISADJUSTED - DIESEL ELECTRONIC CONTROL SYSTEM

The throttle Position Sensor (TPS) is a variable resistor that informs the ECM the degree of throttle opening. The sensor is connected to a 5-volt reference signal and has a high resistance value at closed throttle. At wide open throttle, TPS resistance is at its lowest and output to the ECM will be close to 5 volts. Code 23 means that the ECM has seen the following: voltage not between .25-1.35 volts at ECM terminal "A2" for at least 30 seconds, with engine speed between 550-650 RPM.

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

1. This test confirms Code 23 and that fault is present.
2. This test will determine if sensor signal line is shorted to ground.
3. Adjust Throttle Position Sensor.

Disregard Code 23 if SERVICE ENGINE SOON light goes out as soon as throttle is returned to idle.

Code 23, TPS Misadjusted - Diesel Electronic Control System Flowchart. Scheme 81

CODE 24, VEHICLE SPEED SENSOR CIRCUIT - DIESEL ELECTRONIC

CONTROL SYSTEM

Scheme 82

The ECM applies and monitors 12 volts on circuit No. 437. This circuit connects the Vehicle Speed Sensor (VSS), which alternately grounds circuit No. 437 when drive wheels are turning. This pulsating action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between pulses. SCAN tester reading should closely match speedometer reading with drive wheels turning.

Code 24 will set if: circuit No. 437 is constant, engine speed is more than 200 RPM, vehicle speed signal at terminal ECM "A9" is less than 10 MPH, all conditions are met for at least 10 seconds.

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

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

With drive wheels turning, SCAN tester reading should closely match with speedometer reading.

Code 24, VSS Circuit - Diesel Electronic Control System Flowchart. Scheme 83

Scheme 84

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

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

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

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

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

TCC Electrical Diagnosis - Diesel Electronic Control System Flowchart. Scheme 85

See also:
• TROUBLE SHOOTING
• MAT SENSOR TEMP VS. RESISTANCE VALUE