Engine Controls - Tests W/codes - Electronic Transmission Chevrolet Cab & Chassis C3500

INTRODUCTION

On gasoline vehicles with 4L60-E or 4L80-E transmission, transmission is controlled by the Powertrain Control Module (PCM). PCM controls other vehicle functions as well as the transmission. On diesel vehicles with 4L80-E transmission, electronic transmission is controlled by the Transmission Control Module (TCM). TCM controls no other components. The PCM/TCM monitors a number of engine/vehicle functions and uses the data to control shift solenoid "A", shift solenoid "B", TCC, and the pressure control solenoid to regulate upshift pattern, downshift pattern and line pressure (shift quality).

Most engine/transmission control problems result from mechanical failures, poor electrical connections or damaged vacuum hoses. Before condemning the computer system, perform checks and inspections, including the diagnostic circuit check or diagnostic system check, covered in BASIC TESTING - GASOLINE article in this section. Failure to do so may result in lost diagnostic time. On diesel vehicles, diagnostic system check is covered in TESTS W/CODES - 6.5L DIESEL article in this section.

If no faults were found while performing checks and inspections in BASIC TESTING - GASOLINE article in this section, go to DIAGNOSTIC PROCEDURE under SELF-DIAGNOSTIC SYSTEM. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to TESTS W/O CODES - GASOLINE article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article in this section.

TRANSMISSION APPLICATIONS

DIAGNOSTIC INFORMATION

Note. SERVICE ENGINE SOON light, located on instrument cluster, is also referred to as the Malfunction Indicator Light (MIL).

Gasoline vehicles are equipped with a Powertrain Control Module (PCM). The PCM controls engine/emission functions, and electronic-controlled transmission functions. For additional information on self-diagnostic system for gasoline vehicles, proceed to THEORY/OPERATION - GASOLINE article and appropriate G - GASOLINE TESTS W/ CODES articles in this section.

1. «TESTS W/CODES - 4.3L»(ref-19732)
2. «TESTS W/CODES - 5.0L»(ref-19750)
3. «TESTS W/CODES - 5.7L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-57l)
4. «TESTS W/CODES - 7.4L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l)

Control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate the Malfunction Indicator Light (MIL) located on instrument cluster. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in control module memory. To retrieve stored codes, see ENTERING & EXITING DIAGNOSTIC MODE . Malfunctions are recorded as either hard failures or intermittent failures.

Preliminary Inspection

1. Check all vacuum hoses for correct routing, restrictions, cuts or other damage. Inspect difficult-to-see vacuum hoses beneath the air cleaner assembly and other engine components.
2. Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
3. The preliminary inspection is very important and should be performed carefully and thoroughly, as it can often lead to fixing a problem without further diagnosis.

Diagnostic Procedure

1. Ensure all non-controlled systems not related to engine or transmission control system are operating properly. DO NOT proceed with testing unless all other non-controlled system problems are repaired.
2. ALWAYS begin system diagnosis with checking diagnostic circuit or system. On gasoline vehicles, see DIAGNOSTIC CIRCUIT CHECK in «BASIC TESTING - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-basic-testing-gasoline) article in this section. On diesel vehicles, DIAGNOSTIC SYSTEM CHECK is covered in «TESTS W/CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-65l-diesel) article in this section. The diagnostic circuit or system check determines if computer-controlled system is working properly. If trouble codes are displayed (other than Code 12), determine if codes are hard or intermittent trouble codes.
3. A hard code is one which is present while working on vehicle, and problem still exists. Hard codes will cause the Malfunction Indicator Light (MIL) to come on. NOTE: SERVICE ENGINE SOON light, located on instrument cluster, is also referred to as the Malfunction Indicator Light (MIL).
4. An intermittent code is one which does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. The MIL will go out 10 seconds after fault goes away. For intermittent diagnosis procedures, see appropriate TESTS W/O CODES article in this section. See below. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline) «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

ENTERING & EXITING DIAGNOSTIC MODE

1. With ignition on and engine off, connect a jumper wire between Data Link Connector (DLC) terminal "B" (diagnostic terminal) and terminal "A" (ground). (Scheme 276) This places control module into diagnostic mode.
2. In this mode, PCM will display Code 12 by flashing the MIL once, followed by a short pause, then 2 flashes in quick succession.
3. Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from DLC.

Scheme 276

TROUBLE CODE IDENTIFICATION

Note. Trouble codes retrieved from PCM/TCM may be related to either engine or transmission. Transmission-related codes are covered in this article. For engine-related codes, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE section. See below. See TROUBLE CODE IDENTIFICATION table to determine if code is transmission or engine related.

1. «TESTS W/CODES - 4.3L»(ref-19732)
2. «TESTS W/CODES - 5.0L»(ref-19750)
3. «TESTS W/CODES - 5.7L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-57l)
4. «TESTS W/CODES - 7.4L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l)
5. «TESTS W/CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-65l-diesel)

1. The system codes indicate failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of the control module. Code 51 indicates PROM is installed improperly or has failed.
2. Code 55 indicates PCM/TCM has failed and must be replaced. If the condition is still not corrected after replacing PCM/TCM, the cause may be as follows.

1. An incorrect PCM/TCM or PROM application may cause a malfunction, which may or may not set a code.
2. The PCM/TCM connector may be the problem. Connector terminals may have to be removed from the connector to be properly checked.
3. PROM failure. Although the PROM rarely fails, it could be the cause of the problem.
4. Replacement PCM/TCM may be faulty.
5. Intermittent problem. Make a careful physical inspection of affected sensor/circuit.
6. A shorted solenoid, relay, or harness may be the cause of PCM/TCM failure. Use Short Circuit Tester (J-34636) to check for short circuits.

TROUBLE CODE IDENTIFICATION

CLEARING TROUBLE CODES

Note. To prevent PCM/TCM damage, ensure ignition switch is in OFF position when disconnecting or reconnecting power to PCM/TCM.

Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require that codes be cleared before using diagnostic chart. To clear codes, disconnect PCM/TCM power feed for 30 seconds.

If no hard fault codes are present (only intermittent codes exist), proceed to appropriate TESTS W/O CODES article in this section. See below for intermittent diagnostic procedures.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

PCM/TCM LOCATION

The PCM/TCM is located under behind right side of dash.

SPECIAL TOOLS (DIAGNOSTIC)

Special scan testers, plugged into the DLC, may be used to read trouble codes, and check voltages in system on serial data line. These testers can save a great deal of time. For additional information, see owner's manual included with tester.

SCAN TESTER USAGE

Note. Before connecting scan tester, check diagnostic circuit or system and ensure accurate information is received by scan tester. Perform DIAGNOSTIC CIRCUIT CHECK (GASOLINE) in BASIC TESTING - GASOLINE article in the ENGINE PERFORMANCE section. Perform DIAGNOSTIC SYSTEM CHECK (DIESEL) in TESTS W/CODES - 6.5L DIESEL . If vehicle does not pass diagnostic circuit or system check, information received by scan tester may be invalid.

The scan tester is a specialized tester which can diagnose on-board computer control systems by providing almost instant access to circuit voltage information without crawling under dash or hood to backprobe sensors and connectors. scan testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN TESTER DATA .

Scan testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the PCM/TCM. They do not indicate whether devices respond properly to that signal. This must be verified at output device using a voltmeter or test light.

Note. Code 12 should always exist when DLC test terminal is grounded with key on and engine off, but it may not be indicated by all makes of scan tester.

If trouble codes are not present, a problem may still exist. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. A scan tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See SCAN TESTER DATA .

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground DLC test terminal. If same codes are not flashed by Malfunction Indicator Light (MIL) as were indicated by scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.

SCAN TESTER DATA

Note. Information contained in the following table is typical of readings taken on vehicle with engine idling, upper radiator hose hot, throttle closed, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Not all devices and systems are used on all models. For additional information, see tester owner's manual.

SCAN TESTER DATA

TEST EQUIPMENT

A tachometer, test light, Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, vacuum gauge, and jumper wires are required to test and diagnose electronic transmission control system. A scan tester may also be used to access data parameters. Tester will supply a visual reading of most inputs, and some outputs, to the PCM/TCM. Some diagnostic flow charts require the use of a bidirectional (Tech 1) scan tester.

TROUBLE CODE CHARTS

The following diagnostic flow charts and circuit schematics are provided courtesy of General Motors Corp.

Note. Some charts require the use of a bidirectional (Tech 1) scan tester. If a bidirectional scan tester is not available, the PCM/TCM-controlled relays and solenoids may be energized by grounding DLC diagnostic terminal "B" with ignition on and engine off.

CODE 24, VSS TRANS OUTPUT SIGNAL - "C" & "K" SERIES DIESEL

Note. For Code 24 coverage on gasoline models, see appropriate G - GASOLINE TESTS W/ CODES article below.

1. «TESTS W/CODES - 4.3L»(ref-19732)
2. «TESTS W/CODES - 5.0L»(ref-19750)
3. «TESTS W/CODES - 5.7L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-57l)
4. «TESTS W/CODES - 7.4L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l)

VSS output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning. Since vehicle speed is taken from transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to PCM. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 250 RPM for at least 1.5 seconds.

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

1. Test verifies VSS voltage at PCM.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at sensor.

Diagnostic Aids

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, proceed to INTERMITTENTS in the TESTS W/O CODES - 6.5L DIESEL article in this section. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

Code 24 Schematic ("C", "K", "G" & "P" Series). Scheme 277

Code 24 Flow Chart. Scheme 278

CODE 28, TRANSMISSION RANGE PRESSURE SWITCH

Transmission range pressure switch assembly consists of 5 pressure switches (2 normally closed, 3 normally open), and a fluid temperature sensor combined into one unit and mounted on the valve body. PCM supplies voltage to each range signal. By grounding one or more of these circuits through various combinations of pressure switches, PCM detects which manual valve position has been selected. With ignition on and engine off, Park/Neutral will be indicated. Code 28 will set when range signals "A" and "C" are both zero volts (on) for 2 seconds.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks indicated range signal to valve position signal actually selected.
2. Checks for correct voltage from PCM to transmission external connector.
3. Checks for short to ground in any one of the 3 valve position circuits.

Code will set if PCM detects one of 2 illegal combinations. Check all connections for good contact. An intermittent may be caused by a poor connection, chaffed wire insulation or a broken wire. Monitor voltage of each terminal while moving related harness connectors. If failure is induced, voltage reading will change.

VALID PSM COMBINATION (4L60-E)

VALID PSM COMBINATION (4L80-E)

Code 28 Schematic (4L60-E Transmission (Diesel) Transmission Range Pressure Switch. Scheme 279

Code 28 Schematic (4L60-E Transmission (Gasoline) Transmission Range Pressure Switch. Scheme 280

Code 28 Schematic (4L80-E Transmission (Diesel) Transmission Range Pressure Switch. Scheme 281

Code 28 Schematic (4L80-E Transmission (Gasoline) Transmission Range Pressure Switch. Scheme 282

Code 28 Flow Chart Transmission Range Pressure Switch. Scheme 283

CODES 37/38, TCC BRAKE SWITCH ON/OFF

The normally closed brake switch supplies battery voltage to the PCM. Signal voltage will drop to zero volts when brake pedal is applied.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks if voltage is available at brake switch.
2. Checks brake switch function.
3. Checks circuit from brake switch to PCM.

If code is intermittent, check customer driving habits (driving with foot resting on brake pedal) or for unusual traffic conditions (stop and go expressway traffic).

Codes 37/38 Schematic ("C" & "K" Series) TCC Brake Switch On/Off. Scheme 284

Codes Flow Chart TCC Brake Switch On/Off. Scheme 285

CODE 39, TCC STUCK OFF

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks the mechanical status of the TCC. When PCM commands the TCC solenoid off, TCC slip speed should increase.

If TCC is mechanically stuck on, vehicle speed is zero MPH, brakes are applied and D2 is selected, the TCC fluid will mechanically apply the TCC causing an engine stall.

Code 39 Schematic (Diesel) TCC Stuck Off. Scheme 286

Code 39 Schematic (Gasoline) TCC Stuck Off. Scheme 287

Code 39 Flow Chart TCC Stuck Off. Scheme 288

CODE 52/53, SYSTEM VOLTAGE HIGH

Code 53 will set when ignition is on and PCM/TCM 12-volt battery feed voltage is greater than 19.5 volts for about 2 seconds. During the time the failure is present, the force motor is turned off, transmission immediately shifts to 2nd gear, and TCC operation is inhibited. The setting of additional codes may result.

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

CODE 58, TRANSMISSION FLUID TEMPERATURE HIGH

The transmission fluid temperature sensor is a thermistor which controls the signal voltage to the PCM/TCM. The PCM/TCM applies and monitors voltage to the sensor. When transmission fluid is cold, sensor resistance is high; therefore, the PCM/TCM will see high signal voltage. As the transmission fluid warms up, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission fluid temperature greater than 305°F (151°C) for one second.
2. This test determines if circuit is shorted to ground, which will result in conditions for Code 58.

Check harness routing for a potential short to ground in TFT signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, the temperature display should rise steadily to about 100°C then stabilize. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

Code 58 Schematic (4L80-E Gasoline) Transmission Fluid Temperature High. Scheme 289

Code 58 Flow Chart Transmission Fluid Temperature High. Scheme 290

CODE 59, TRANSMISSION FLUID TEMPERATURE LOW

The transmission fluid temperature sensor is a thermistor which controls the signal voltage to PCM/TCM. The PCM/TCM applies and monitors 5 volts to the sensor. When transmission fluid is cold, sensor resistance is high; therefore, the PCM/TCM will see high signal voltage. As transmission fluid temperature warms up, sensor resistance and voltage drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 59 will set if signal voltage indicates a transmission fluid temperature less than -34°F (-37°C) for one second.
2. This test simulates Code 58. If PCM/TCM recognizes low signal voltage (high temperature) and scan tester reads 305°F (151°C) or greater, PCM/TCM and wiring are okay.
3. This test determines if signal circuit is open. There should be 5 volts present at the sensor connector if measuring with a DVOM.

Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, the displayed temperature should rise steadily to about 100°C then stabilize. A faulty connection or an open in ground circuit or signal circuit will result in a Code 59. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. An out-of-calibration sensor could result in firm shifts or TCC enabled complaint. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See below.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 59 Flow Chart Transmission Fluid Temperature Low. Scheme 291

CODE 66, 3-2 CONTROL SOLENOID CIRCUIT FAULT

The 3-2 control solenoid hydraulically coordinates the apply rate of the 2-4 band with hydraulic release of the 3-4 clutch during a 3-2 downshift. 3-2 circuit duty cycle is continually monitored by the PCM depending on the command state of the circuit. When the transmission is in 1st gear the duty cycle of the solenoid is equal to zero. When the transmission is in 2nd gear or higher, the duty cycle of the solenoid will be about 90 percent. When the transmission downshifts 3-2, the duty cycle of the solenoid will be about 20 percent.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks 3-2 control solenoid and internal transmission harness for short circuits.
2. This test checks for power, from the ignition through the fuse, to the 3-2 control solenoid.

Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See below. Some slight TCC slippage is normal. 3-2 control solenoid feedback normally oscillates on/off when the duty cycle is applied.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 66 Schematic Control Solenoid Circuit Fault. Scheme 292

Code 66 Flow Chart. Scheme 293

CODE 67, TCC SOLENOID CIRCUIT CHECK

The TCC solenoid is a normally open exhaust valve. The PCM will engage the solenoid by grounding circuit with an internal quad-driver.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks the ability of PCM to control solenoid.
2. This test checks for power, from the ignition through the fuse, to the TCC solenoid.

Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See below. Some slight TCC slippage is normal.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 67 Schematic (4L60-E Diesel) TCC Solenoid Circuit Check. Scheme 294

Code 67 Schematic (4L60-E Gasoline) TCC Solenoid Circuit Check. Scheme 295

Code 67 Flow Chart TCC Solenoid Circuit Check. Scheme 296

CODE 68, OVERDRIVE RATIO ERROR

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

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks the indicated range signal to the selected range. A faulty switch could set this code.
2. Checks TCC for slippage while in a commanded lock-up state.

Check for spread connectors at pass-thru connector. Code 68 will set when going to default (2nd gear). If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See below.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 68 Flow Chart Overdrive Ratio Error. Scheme 297

CODE 69, TORQUE CONVERTER CLUTCH STUCK ON

The PCM commands the TCC PWM solenoid on by modulating the signal fluid on the converter clutch shift valve. The TCC apply fluid applies the torque converter clutch.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks the mechanical state of the TCC. When the TCC is commanded off, the slip speed should increase.

If the TCC is mechanically stuck on, TCC fluid will apply the TCC causing an engine stall.

Code 69 Schematic (4L60-E Diesel) Torque Converter Clutch Stuck On. Scheme 298

Code 69 Schematic (4L80-E Gasoline) Torque Converter Clutch Stuck On. Scheme 299

Code 69 Flow Chart Torque Converter Clutch Stuck On. Scheme 300

CODE 72, VSS CKT LOSS TRANSMISSION OUTPUT SIGNAL - DIESEL

Note. For Code 72 coverage on gasoline models, see appropriate G - GASOLINE TESTS W/ CODES article in the ENGINE PERFORMANCE section. See below.

1. «TESTS W/CODES - 4.3L»(ref-19732)
2. «TESTS W/CODES - 5.0L»(ref-19750)
3. «TESTS W/CODES - 5.7L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-57l)
4. «TESTS W/CODES - 7.4L»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l)

VSS output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning.

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

1. Test verifies VSS voltage at PCM.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at sensor.

Code 72 will set when VSS signal is present and is lost. Code 24 will set when no vehicle speed is detected at vehicle start off. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - 6.5L DIESEL article in this section.

Code 72 Schematic ("C" & "K" Series Diesel) VSS Circuit Loss Transmission Output Signal. Scheme 301

CODE 73, PRESSURE CONTROL SOLENOID (CURRENT ERROR)

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

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

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks the ability of the PCM/TCM to command the solenoid.
2. Checks internal transmission harness and solenoid for incorrect resistance.

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

Code 73 Schematic (Diesel) Pressure Control Solenoid (Current Error). Scheme 302

Code 73 Schematic (Gasoline) Pressure Control Solenoid (Current Error). Scheme 303

Code 73 Flow Chart Pressure Control Solenoid (Current Error). Scheme 304

CODE 74, TRANSMISSION INPUT SPEED ERROR

Sensor is a permanent magnet type. When rotating trigger teeth pass sensor, an AC voltage signal is produced. Signal voltage and frequency varies with forward rotational speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks entire circuit for continuity.
2. Checks output of the input speed sensor.

Check for poor connections in PCM/TCM related circuits.

Code 74 Schematic (Diesel) Transmission Input Speed Error. Scheme 305

Code 74 Schematic (Gasoline) Transmission Input Speed Error. Scheme 306

Code 74 Schematic Flow Chart Transmission Input Speed Error. Scheme 307

CODE 75, SYSTEM VOLTAGE LOW

Code 75 will set when ignition is on and PCM/TCM 12-volt battery feed voltage is less than 9 volts for about 4 seconds. During the time failure is present, force motor, also referred to as pressure control solenoid, is turned off, maintaining only 2nd gear, and inhibiting TCC operation.

NOTE ON INTERMITTENTS

If code sets when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link. Minimum voltage allowed for Code 75 to set is on a graduated scale and changes with temperature. Minimum voltage at -40°F (-40°C) is 6.7 volts. Minimum voltage at 304°F (150°C) is 10.5 volts, and minimum voltage at 194°F (90°C) is 9 volts.

CODE 79, TRANSMISSION FLUID TEMPERATURE HIGH

The transmission fluid temperature sensor is a thermistor which controls the signal voltage to the PCM/TCM. The PCM/TCM applies and monitors voltage to the sensor. When transmission fluid is cold, sensor resistance is high; therefore, the PCM/TCM will see high signal voltage. As the transmission fluid warms up, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 79 will set if signal voltage indicates a transmission fluid temperature greater than 151°C for one second.
2. This test determines if circuit is shorted to ground, which will result in conditions for Code 79.

Check harness routing for a potential short to ground in signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, the temperature display should rise steadily to about 100°C then stabilize. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

Code 79 Flow Chart Transmission Fluid Temperature High. Scheme 308

CODE 81, TRANSMISSION 2-3 ERROR

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks function of 2-3 shift solenoid and internal transmission wiring.
2. Checks for power to 2-3 shift solenoid from ignition.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See below.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 81 Schematic (Diesel) Transmission 2-3 Error. Scheme 309

Code 81 Schematic (Gasoline) Transmission 2-3 Error. Scheme 310

Code 81 Flow Chart Transmission 2-3 Error. Scheme 311

CODE 82, TRANSMISSION 1-2 ERROR

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks 1-2 shift solenoid and internal transmission wiring harness for short circuits.
2. This test checks for power, from the ignition through the fuse, to the shift solenoid.

Check all connections, especially at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See below.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 82 Schematic (Diesel) Transmission 1-2 Error. Scheme 312

Code 82 Schematic (Gasoline) Transmission 1-2 Error. Scheme 313

Code 82 Flow Chart Transmission 1-2 Error. Scheme 314

CODE 83, TCC SOLENOID CIRCUIT FAULT

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

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks if PCM/TCM is commanding TCC solenoid on.
2. This test checks for voltage in the solenoid.

Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 83 Schematic (Diesel) TCC Solenoid Circuit Fault. Scheme 315

Code 83 Schematic (Gasoline) TCC Solenoid Circuit Fault. Scheme 316

Code 83 Flow Chart TCC Solenoid Circuit Fault. Scheme 317

CODE 85, UNDEFINED GEAR RATIO

While in each gear, PCM/TCM calculates actual gear ratio from input and output speed readings, also referred to as vehicle speed, then compares these to what gear ratio should be, taking into consideration selected gear range. This monitor includes reverse gear, but does not include overdrive gear.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-calibration transmission range pressure switch could falsely set Code 85.
2. This test verifies the proper ratio.

Code will set when an unknown gear ratio is detected for any gear but 4th. Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See below. Compare scan tester gear ratio reading to specifications in GEAR RATIO SPECIFICATIONS .

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

GEAR RATIO SPECIFICATIONS

Code 85 Schematic (4L80-E Diesel) Undefined Gear Ratio. Scheme 318

Code 85 Schematic (4L80-E Gasoline) Undefined Gear Ratio. Scheme 319

Code 85 Flow Chart (4L80-E Gasoline) Undefined Gear Ratio. Scheme 320

CODE 86, LOW GEAR RATIO ERROR

Control module calculates ratio based on the transmission input speed and output speed sensor readings. Control module compares known ratio to the calculated ratio.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-calibration transmission range pressure switch could falsely indicate actual transmission range.
2. This test verifies the proper ratio.

If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section.

1. «TESTS W/O CODES - GASOLINE»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline)
2. «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-c3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel)

Code 86 Flow Chart (4L60-E Gasoline) Low Gear Ratio Error. Scheme 321

CODE 87, HIGH GEAR RATIO ERROR

Control module calculates ratio based on the transmission input speed and output speed sensor readings. Control module compares known ratio to the calculated ratio.

1. An out-of-calibration transmission range pressure switch could falsely indicate actual transmission range.
2. This test verifies the proper ratio.

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

Code 87 Flow Chart (4L60-E Gasoline) High Gear Ratio Error. Scheme 322

See also:
• BASIC TESTING - GASOLINE
• TESTS W/CODES - 6.5L DIESEL
• TESTS W/O CODES - GASOLINE
• THEORY/OPERATION - GASOLINE
• TESTS W/CODES - 5.7L
• TESTS W/CODES - 7.4L
• TESTS W/O CODES - 6.5L DIESEL
• COMPUTER RELEARN PROCEDURES
• ENTERING & EXITING DIAGNOSTIC MODE
• SCAN TESTER DATA