Automatic Transmission Diagnosis - 4l80-e GMC Suburban I

APPLICATION

4L80-E APPLICATION

SERIES CODE DESIGNATIONS

DESCRIPTION

The 4L80-E transmission uses 2 electric shift solenoids to control transmission upshifts and downshifts. In addition, a pressure control (force motor) solenoid controls hydraulic line pressure, and a Torque Converter Clutch (TCC) Pulse Width Modulated (PWM) solenoid is used to control fluid acting on converter clutch valve, which then controls TCC apply and release. Solenoids are turned on and off by a Powertrain Control Module (PCM) or a Transmission Control Module (TCM).

PCM/TCM receives signals from various transmission sensors. Sensors include engine speed and throttle position, transmission speed, hydraulic pressure and transmission fluid temperature. PCM/TCM has on-board self-diagnostics to help identify any parts or circuits which may need further testing.

OPERATION

Shift solenoid holds hydraulic pressure (solenoid on) or releases hydraulic pressure (solenoid off). This action controls shift valves inside valve body. By switching one or both solenoids on or off, different combinations of clutches, sprags and bands are operated. See CLUTCH AND BAND APPLICATION under TROUBLE SHOOTING.

PCM/TCM

On gasoline models, PCM is located behind glove box on "C" and "K" series vehicles, under driver's seat on "G" series vehicles, or at left inner fenderwell on "P" series vehicles. For series identification, see SERIES CODE DESIGNATIONS table in APPLICATION.

On Diesel models, TCM is located behind right side of dash on "C" and "K" series vehicles, under driver's seat or behind right side of dash on "G" series vehicles or at left inner fenderwell on "P" series vehicles.

PCM/TCM controls TCC, pressure control solenoid, (hydraulic pressure), PWM solenoid and shift solenoids 1-2 and 2-3. In addition, PCM/TCM also controls ignition, fuel and emission devices related to engine.

PCM/TCM receives electronic signals from sensors and switches. These signals help PCM/TCM determine when to operate various relays and solenoids related to engine and transmission components.

SENSORS & SWITCHES

PCM/TCM controls converter clutch lock-up, upshifts and downshifts based on transmission temperature, system voltage, throttle position, transmission oil pressure switches, and transmission output and input (engine) speed sensors. (Scheme 93) System includes several other sensors and switches that are used for engine control (gasoline engines). For additional information and testing of engine components, see appropriate SYSTEM/COMPONENT TESTS article in the ENGINE PERFORMANCE section.

Shift Solenoids 1-2 & 2-3

Transmission is shifted up or down by 2 electric shift solenoids. Both solenoids are located on valve body. (Scheme 93) Ignition power is supplied to each solenoid by transmission fuse. Solenoid 1-2 controls hydraulic pressure to 1-2 and 3-4 shift valves. Solenoid 2-3 controls hydraulic pressure to 2-3 shift valve.

Pressure Control Solenoid

Pressure control (force motor) solenoid has a spool valve and operates pressure regulator valve. (Scheme 93) PCM sends a frequency signal to pressure control solenoid to regulate hydraulic line pressure. Frequency signal (duty cycle) is measured with a dwell meter or lab scope. When duty cycle is zero, line pressure is at maximum, and pressure control solenoid draws zero amps. When duty cycle is 60 percent, line pressure is at minimum, and pressure control solenoid draws 1.1 amps at 4.5 volts.

TCC PWM Solenoid

This solenoid is used to control TCC apply valve. PCM/TCM sends a frequency signal to TCC PWM solenoid to gradually apply or release TCC. (Scheme 93)

Scheme 93

SELF-DIAGNOSTICS

PCM/TCM constantly monitors all electrical circuits. If PCM/TCM detects circuit problems or sensors out of range, it will record a Diagnostic Trouble Code (DTC). If problem continues for a predetermined time, Malfunction Indicator Light (MIL) will glow.

If MIL is on all the time, DTC(s) are currently being detected. If MIL is off, but PCM/TCM had detected a circuit or sensor problem, DTC(s) will be stored in computer memory.

Stored DTCs may be retrieved from PCM/TCM memory. Depending on vehicle, several methods may be used. The most basic method (non-scan) uses a jumper wire to activate MIL, which will flash a series of codes. Other method includes using a factory recommended Tech 1 scan tool or aftermarket scan tool.

Note. Faulty engine sensors and actuators may cause transmission related DTCs or driveability problems. Engine faults and related DTCs must be diagnosed and repaired before transmission codes are repaired. For additional information on diagnosing and repairing engine-related PCM/TCM trouble codes, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE section.

CLUTCH AND BAND APPLICATION

CLUTCH & BAND APPLICATION CHART (4L80-E)

ELECTRONIC SELF-DIAGNOSTICS

Note. To test electronic control of transmission solenoids without using self-diagnostics or if self-diagnostics does not function, go to COMPONENT TESTS under ELECTRONIC TESTING. After repairs are made, trouble codes should be erased from computer memory. See CLEARING TROUBLE CODES under ELECTRONIC SELF-DIAGNOSTICS.

RETRIEVING CODES WITHOUT SCAN TOOL - GASOLINE ENGINES

Note. Following procedure is for vehicles equipped with a 12-pin Data Link Connector (DLC). On vehicles equipped with 16-pin DLC (OBD-II vehicles), DTCs can only be retrieved using scan tool.

Scheme 94

1. Turn ignition on. DO NOT start engine. MIL should glow. Locate Data Link Connector (DLC) attached to control module wiring harness. Most DLCs are located under dash on driver's side of vehicle. Turn ignition on with engine not running. Connect jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of DLC. (Scheme 94) NOTE: Connecting jumper wire into test and ground terminals of DLC with engine running will cause fuel injected vehicles to enter field service mode. MIL will not indicate codes if this is done.
2. MIL should begin to flash codes. Each code will be repeated 3 times. For example, FLASH, FLASH, pause, FLASH, longer pause, identifies Code 21. First series of flashes is first digit of trouble code. Second series of flashes is second digit of trouble code.
3. Trouble codes are displayed starting with lowest numbered code. Codes will continue to repeat as long as DLC test terminal is grounded. If codes are not flashed, or MIL does not glow, self-diagnostics will not work. For diagnosis of this condition, see DIAGNOSTIC CIRCUIT CHECK in appropriate BASIC TESTING article in the ENGINE PERFORMANCE section. To exit diagnostic mode, turn ignition off and remove jumper wire from DLC.

Note. Trouble codes will be recorded at various operating times. Some codes require operation of affected sensor or switch for 5 seconds; others may require operation for 5 minutes or longer at normal operating temperature, road speed and load. Therefore, some codes may not set in a service bay operational mode and may require road testing vehicle in order to duplicate condition under which code will set.

RETRIEVING CODES WITH SCAN TOOL - GASOLINE ENGINES

Note. Plugging scan tool into DLC enables user to read trouble codes and check voltages in system on serial data line.

Scan tools may also furnish information on status of output devices (solenoids and relays). However, status parameters are only an indication that output signals have been sent to devices by control module; they do not indicate if devices have responded properly to signal. Check for proper response at output device using a voltmeter or test light.

If trouble codes are not present, this is not necessarily an indication a problem does not 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. Sensors that are out of specification WILL NOT set a trouble code but WILL cause driveability problems. Using scan tool is the easiest method of checking sensor specifications and other data parameters. Scan tool is also useful in finding intermittent wiring problems by wiggling wiring harness and connections (key on, engine off) while observing scan tool.

Note. If erroneous voltage signals are suspected, it will be necessary to verify scan tool information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off, remove scan tool, turn ignition on and ground DLC test terminal "B". (Scheme 94) Same codes flashed by MIL should be indicated by scan tool.

Preliminary Inspection

1. Check all vacuum hoses for correct routing, restrictions, cuts or other damage. Inspect difficult-to-see vacuum hoses beneath air cleaner assembly and other engine components.
2. Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chafed 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 fix a problem without requiring further diagnosis.

Note. Begin all diagnosis with ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK chart. After any DEC/electronically-controlled transmission systems repair, repeat On-Board Diagnostic (OBD) system check.

Diagnostic Procedure

1. Ensure all non-controlled systems NOT related to the Diesel Electronic Control (DEC)/electronically-controlled transmission systems are operating properly. DO NOT proceed with testing unless all non-DEC/electronically-controlled transmission systems problems are repaired.
2. ALWAYS begin diagnosis with On-Board Diagnostic (OBD) system check to determine if DEC/electronically-controlled transmission systems and PCM are working properly. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK chart. Refer to DIAGNOSTIC CHARTS - DIESEL. If trouble codes, other than Diagnostic Trouble Code (DTC) 12 are displayed, determine if they are hard or intermittent trouble codes.
3. A hard code is present while working on vehicle, and problem persists. Hard codes will cause 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 does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. MIL will go out 10 seconds after fault goes away.

Entering Or 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 94) Diesel Electronic Control (DEC)/electronically-controlled transmission systems will enter diagnostic mode.
2. In this mode, PCM/TCM will display DTC 12 by flashing the MIL once, followed by a short pause, then 2 flashes in quick succession. DTC 12 will be displayed 3 times. If no other codes are stored, DTC 12 will continue to flash until diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from DLC.

TROUBLE CODE DEFINITIONS

Note. Only transmission-related trouble codes are listed. For engine-related DTCs, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE section. Engine-related DTCs must be repaired first, as engine performance and related component signals will affect transmission operation and diagnosis.

PCM/TCM TROUBLE CODE DEFINITIONS

HARD OR INTERMITTENT TROUBLE CODE DETERMINATION - GASOLINE

During any diagnostic procedure, it must be determined if codes are hard failure codes or intermittent failure codes. Diagnostic charts will not usually help analyze intermittent codes. To determine hard codes and intermittent codes, proceed as follows

1. Enter diagnostic mode. See RETRIEVING CODES. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See «CLEARING TROUBLE CODES»(/gmc/suburban/i-1992-1999/remont/automatic-trans/#automatic-transmission-diagnosis-4l80-e__clearing-trouble-codes) .
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. MIL should go out. Run warm engine at specified curb idle for 2 minutes and note MIL.
3. If MIL comes on, enter diagnostic mode. Read and record DTCs. This will reveal hard failure codes. DTCs may require a road test to reset hard failure after clearing DTC. If MIL does not come on, all stored DTCs were intermittent failures.

CLEARING TROUBLE CODES

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

Diesel

To clear codes, use Tech 1 scan tester. If scan tester is not available, perform the following procedure. With ignition off, connect a jumper wire between Data Link Connector (DLC) terminal "B" (diagnostic terminal) and terminal "A" (ground). (Scheme 94) Turn ignition on. Fully apply brake pedal, then fully apply accelerator pedal. Check MIL for DTC 12. Release brake pedal, then release accelerator pedal. Check MIL again for DTC 12. Remove jumper wire from DLC. Turn ignition off.

Gasoline

Turn ignition switch to ON position, and ground diagnostic test terminal "B" at DLC connector. (Scheme 93) Turn ignition switch to OFF position, and remove control module fuse from fuse block for 30 seconds. Replace fuse. Remove diagnostic terminal ground lead. If fuse cannot be located, disconnect pigtail at battery. However, this may result in loss of other on-board memory data, such as preset radio tuning. After power to PCM/TCM is removed, poor driveability may occur until control module "relearns" operating parameters.

Diagnostic Charts

Following charts are DTC specific. For terminal locations, see WIRING DIAGRAMS . For engine-related DTCs, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE section.

Diagnostic Aids

Diagnostic aids located in many trouble code charts are additional tips used to help diagnose trouble codes when circuit checks do not find a problem.

Note. In following diagnostic tests, schematics and illustrations are courtesy of General Motors Corp. Powertrain Control Module (PCM) may also be referred to as Transmission Control Module (TCM) in some diagnostic charts and figures. Terms are used interchangeably.

Note. If specific DTC diagnostic test is not included in gasoline or Diesel diagnostic tests, refer to 4L60-E ELECTRONIC CONTROLS article under DIAGNOSTIC CHARTS for specific DTC. Diagnostic tests not included in gasoline or Diesel diagnostic tests are same as 4L60-E diagnostic test procedures.

Scheme 95

Coolant temperature sensor input is used to determine control of fuel delivery, engine timing, idle speed and converter clutch (TCC) application. As engine warms, sensor resistance reduces. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at coolant sensor signal terminal.

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

1. This tests if code was set because of a hard failure or intermittent condition.
2. This simulates conditions for a Code 15. If scan tester displays a low temperature, control module and wiring are not at fault.

Scheme 96

DTC 14 TERMINAL & CIRCUIT WIRING IDENTIFICATION

After engine is started, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens. Test sensor at various temperature levels to determine if sensor is out of calibration. See TEMPERATURE-TO-RESISTANCE VALUES chart. If engine is allowed to cool overnight, coolant temperature sensor and intake air temperature sensor (if equipped) should read close to each other, when measured with a scan tester.

TEMPERATURE-TO-RESISTANCE VALUES

DTC 15, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE HIGH

As engine warms, sensor resistance reduces and voltage drops. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at control module coolant sensor signal terminal. If sensor signal circuit opens, control module will see -56°F (-49°C) and deliver fuel for this temperature.

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

1. This checks if code was set as a result of a hard failure or intermittent condition.
2. This simulates conditions for a Code 14. If control module recognizes grounded circuit and displays a high temperature, control module and wiring are okay.
3. This determines if problem is control module or wiring. There should be 5 volts present at sensor when measured with a DVOM.

Scheme 97

DTC 15 TERMINAL & CIRCUIT WIRING IDENTIFICATION

After engine starts, temperature should rise steadily to about 194°F (90°C) and stabilize when thermostat opens. Test sensor at various temperature levels to determine if sensor is out of calibration. See TEMPERATURE-TO-RESISTANCE VALUES chart. If engine is allowed to cool overnight, coolant temperature sensor and intake air temperature sensor (if equipped) should read close to each other when measured with a scan tester. Code 15 will also set if sensor signal or ground circuit is open.

TEMPERATURE-TO-RESISTANCE VALUES

Scheme 98

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from about .50 volt at idle to 4 volts at wide open throttle. Each time TPS voltage drops to less than 1.25 volts and stops, control module assumes this is zero degrees throttle angle and measures throttle percentage angle from this point.

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

1. This test confirms Code 21 and checks if fault is a hard failure or an intermittent condition.
2. This test simulates conditions for Code 22. If control module recognizes low voltage signal and sets Code 22, control module and power and signal circuits are not at fault.
3. This step isolates a faulty sensor, control module or an open ground circuit.

Scheme 99

DTC 21 TERMINAL & CIRCUIT WIRING IDENTIFICATION

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 21 will also result if ground circuit is open or TPS signal circuit is shorted to voltage.

DTC 22, TPS SIGNAL VOLTAGE LOW

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from less than about .50 volt at idle to 4 volts at wide open throttle.

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

1. This test confirms Code 22 and tests if fault is a hard failure or an intermittent condition.
2. This simulates Code 21. If control module recognizes a high voltage signal and sets Code 21, control module and wiring are not at fault. Replace TPS.
3. This simulates a high voltage signal to check for on open TPS signal circuit.

Scheme 100

DTC 22 TERMINAL & CIRCUIT WIRING IDENTIFICATION

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

Scheme 101

Scheme 102

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 control module. 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 voltage at buffer module.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at from module.

Scheme 103

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. 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.

Scheme 104

Scheme 105

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 valve body. Control module supplies voltage to each range signal. By grounding one or more of these circuits through various combinations of pressure switches, control module 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 control module to transmission external connector.
3. Checks for short to ground in any one of 3 valve position circuits.

Scheme 106

Code will set if control module 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

Scheme 107

The normally closed brake switch supplies battery voltage to control module. 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 control module.

Scheme 108

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).

Scheme 109

Note. Test number refers to number on diagnostic chart.

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

Scheme 110

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

Scheme 111

Scheme 112

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

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

1. Normal voltage is 9-15 volts.
2. Checks if alternator is faulty under load.

Scheme 113

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.

Scheme 114

Scheme 115

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors voltage to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid warms, 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.

Scheme 116

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, temperature display should rise steadily to about 212°F (100°C) then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TEMPERATURE-TO-RESISTANCE VALUES chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES

DTC 59, TRANS. FLUID TEMP. LOW

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors 5 volts to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid temperature warms, 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 control module recognizes low signal voltage (high temperature) and scan tester reads 305°F (151°C) or greater, control module and wiring are okay.
3. This test determines if signal circuit is open. There should be 5 volts present at sensor connector if measuring with a DVOM.

Scheme 117

Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, displayed temperature should rise steadily to about 212°F (100°C) then stabilize. A faulty connection or an open in ground circuit or signal circuit will result in a Code 59. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TEMPERATURE-TO-RESISTANCE VALUES. An out-of-calibration sensor could result in firm shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES

DTC 68, TRANSMISSION COMPONENT SLIPPING

Control module monitors the difference in engine RPM and input shaft RPM. With transmission in Drive, scan tool reading should show engine speed closely matching input speed.

Note. Test numbers refer to numbers on diagnostic chart.

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

Scheme 118

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

Scheme 119

Control module commands TCC PWM solenoid on by modulating signal fluid on converter clutch shift valve. TCC apply fluid applies torque converter clutch.

Note. Test number refers to number on diagnostic chart.

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

Scheme 120

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

Scheme 121

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 control module.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at sensor.

Scheme 122

Code 72 will set when VSS signal is present and then 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.

Scheme 123

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 input speed sensor.

Scheme 124

Check for poor connections in related circuits.

DTC 79, TRANS. FLUID TEMP. HIGH

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors voltage to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid warms, 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.

Scheme 125

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, temperature display should rise steadily to about 212°F (100°C) then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TEMPERATURE-TO-RESISTANCE VALUES chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES

Scheme 126

While in each gear, control module calculates actual gear ratio from input and output speed readings, also referred to as vehicle speed, then compares these readings 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 proper ratio.

Scheme 127

Code will set when an unknown gear ratio is detected for any gear but 4th. Check all connections, especially those at transmission pass-thru connector. Compare scan tool gear ratio reading to specifications in GEAR RATIO table.

GEAR RATIO

DTC 86, LOW GEAR RATIO ERROR

Control module calculates ratio based on transmission input speed and output speed sensor readings. Control module compares known ratio to 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 ratio.
2. This test verifies proper ratio.

Scheme 128

Check all connections, especially at transmission pass-thru connector.

DTC 87, HIGH GEAR RATIO ERROR

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

Scheme 129

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

Scheme 130

Scheme 131

Scheme 132

The On-Board Diagnostic (OBD) system check is an organized approach to identifying a problem created by a control module system malfunction. This is the starting point for any driveability complaint diagnosis, as it directs technician to the next logical step in diagnosis, helps reduce diagnostic time and prevents the unnecessary replacement of good parts.

Note. Test numbers refer to numbers on diagnostic chart.

1. This tests MIL operation. With ignition on and engine off, light should be on.
2. This step will isolate if customer complaint is a MIL or driveability problem.
3. Although the control module is powered up, a symptom could exist because of a system fault.
4. Use Tech-1 to aid in diagnosis (to check if serial data is available). If a PROM error is present, the PCM may have been able to flash DTC 12 or 51, but not enable serial data.
5. Although the control module is powered up, a "CRANKS BUT WILL NOT RUN" symptom could exist because of a PCM or system fault.
6. This step will isolate if customer complaint is a MIL or driveability problem with no MIL displayed. See PCM/TCM TROUBLE CODE DEFINITIONS table under «TROUBLE CODE DEFINITIONS»(/gmc/suburban/i-1992-1999/remont/automatic-trans/#automatic-transmission-diagnosis-4l80-e__trouble-code-definitions) , to determine if code is valid. An invalid DTC may be the result of a faulty scan tester, PROM or PCM.
7. Comparison of actual control system data with typical scan tester data values is a quick check to determine if any parameter is not within limits. A base engine problem (i.e. advanced cam timing) may substantially alter sensor values.

Scheme 133

Scheme 134

The On-Board Diagnostic (OBD) system check is an organized approach to identifying a problem created by a control module system malfunction. This is the starting point for any driveability complaint diagnosis, as it directs technician to the next logical step in diagnosis, helps reduce diagnostic time and prevents the unnecessary replacement of good parts.

Note. Test numbers refer to numbers on diagnostic chart.

1. This tests MIL operation. With ignition on and engine off, light should be on.
2. This step will isolate if customer complaint is a MIL or driveability problem.
3. Although the control module is powered up, a symptom could exist because of a system fault.
4. Use Tech-1 to aid in diagnosis (to check if serial data is available). If a PROM error is present, the TCM may have been able to flash DTC 12 or 51, but not enable serial data.
5. This step will isolate if customer complaint is a MIL or driveability problem with no MIL displayed. See PCM/TCM TROUBLE CODE DEFINITIONS table under «TROUBLE CODE DEFINITIONS»(/gmc/suburban/i-1992-1999/remont/automatic-trans/#automatic-transmission-diagnosis-4l80-e__trouble-code-definitions) , to determine if code is valid. An invalid DTC may be the result of a faulty scan tester, PROM or TCM.
6. Comparison of actual control system data with typical scan tester data values is a quick check to determine if any parameter is not within limits. A base engine problem (i.e. advanced cam timing) may substantially alter sensor values.

Scheme 135

CHART A-1, NO MIL (SERVICE ENGINE SOON LIGHT) - MODELS W/PCM

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The PCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit.

Note. Test numbers refer to numbers on diagnostic chart.

1. If fusible link or ECM-B fuse is blown, see «WIRING DIAGRAMS»(/gmc/suburban/i-1992-1999/remont/automatic-trans/#automatic-transmission-diagnosis-4l80-e) for complete circuit.
2. Using a test light connected to 12 volts, probe each of the system ground circuits to ensure a good ground is present.

Scheme 136

If engine functions properly, check for a burned out bulb, blown GAUGES or ALT fuse, or an open in the MIL (SERVICE ENGINE SOON) control circuit. This will result in no brake warning light, oil or generator lights, seat belt reminder, etc.

CHART A-1, NO MIL (SERVICE ENGINE SOON LIGHT) - "G" & "P" SERIES WITH TCM

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The PCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit.

Note. Test numbers refer to numbers on diagnostic chart.

1. If ECM-BAT fuse is blown, see «WIRING DIAGRAMS»(/gmc/suburban/i-1992-1999/remont/automatic-trans/#automatic-transmission-diagnosis-4l80-e) for complete circuit.
2. Using a test light connected to 12 volts, probe each of the system ground circuits to ensure a good ground is present.

Scheme 137

If engine functions properly, check for a burned out bulb, blown ALT fuse, or an open in the MIL (SERVICE ENGINE SOON) control circuit. This will result in no brake warning light, oil or generator lights, seat belt reminder, etc.

If engine cranks but will not operate, check for continuous battery ECM-BAT fuse blown, TCM ignition in-line fuse blown, battery feed circuit to TCM open, ignition feed circuit to TCM open, or poor connection at TCM.

CHART A-2, NO SCAN DATA, MIL (SERVICE ENGINE SOON) ON AT ALL TIMES OR WILL NOT FLASH MIL - MODELS WITH PCM

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The PCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit. With diagnostic terminal grounded, MIL should flash DTC 12, followed by any other DTC stored in memory. A steady light on indicates a short to ground in the MIL (SERVICE ENGINE SOON) control circuit or an open in the diagnostic request circuit.

Note. Test numbers refer to numbers on diagnostic chart.

1. If there is a problem with PCM that causes scan tester not to read serial data, PCM should not flash DTC 12. If DTC 12 does flash, ensure scan tester is functioning properly (on another vehicle). If scan tester is functioning properly, and serial data circuit is okay, PROM, PCM or SIR DERM may be at fault for the NO DLC symptom.
2. If light goes off when PCM connector is disconnected, MIL (SERVICE ENGINE SOON) control circuit is not shorted to ground.
3. This step checks for an open in the diagnostic request circuit.
4. At this point, MIL wiring is okay. Problem is a faulty PROM. If DTC 12 does not flash, PCM should be replaced using original PROM. Replace PROM only after trying a PCM. A defective PROM usually is an unlikely cause of problem.

Scheme 138

CHART A-2, NO DATA LINK CONNECTOR (DLC) DATA, WILL NOT DISPLAY DTC 12 OR MIL ON STEADY - "G" & "P" SERIES WITH TCM

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The TCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit. With diagnostic terminal grounded, MIL should flash DTC 12, followed by any other DTC stored in memory. A steady light on indicates a short to ground in the MIL (SERVICE ENGINE SOON) control circuit or an open in the diagnostic request circuit.

Note. Test numbers refer to numbers on diagnostic chart.

1. If there is a problem with TCM that causes scan tester not to read serial data, TCM should not flash DTC 12. If DTC 12 does flash, ensure scan tester is functioning properly (on another vehicle). If scan tester is functioning properly, and serial data circuit is okay, PROM or TCM may be at fault for the NO DLC symptom.
2. If light goes off when TCM connector is disconnected, MIL (SERVICE ENGINE SOON) control circuit is not shorted to ground.
3. This step checks for an open in the diagnostic request circuit.
4. At this point, MIL wiring is okay. Problem is a faulty PROM. If DTC 12 does not flash, TCM should be replaced using original PROM. Replace PROM only after trying a TCM. A defective PROM usually is an unlikely cause of problem.

Scheme 139

Scheme 140

Scheme 141

Scheme 142

This chart assumes battery condition and engine cranking speed are okay, there is adequate fuel in tank, and glow plug system is operating okay.

Note. Test numbers refer to numbers on diagnostic chart.

1. A MIL on is a basic test to determine if there is a 12 volts supply and ignition 12 volts to PCM. No DLC may be due to a PCM problem and CHART A-2 will diagnose PCM.
2. This step will check to see if there is an inject command coming from PCM.
3. This step will check ground circuit.

Scheme 143

If no problem is found in fuel pump circuit or ignition system and cause of a "ENGINE CRANKS BUT WILL NOT RUN" has not been found, check for: water or foreign material in fuel system, and/or a basic engine problem.

Note. If crankshaft position sensor and optical sensor are disconnected or inoperable at the same time this will cause an "ENGINE CRANKS BUT WILL NOT RUN" condition.

Scheme 144

Scheme 145

Scheme 146

Engine Coolant Temperature (ECT) sensor is a thermistor that controls signal voltage to PCM. When engine is cold, sensor resistance is high, and PCM will sense a high signal voltage. As engine warms, sensor resistance becomes less and voltage drops. DTC 14 will set when engine coolant temperature is greater than 304°F (151°C) for 2 seconds.

PCM will default to a engine coolant temperature of 171°F (77°C).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 14 is the result of a hard failure or an intermittent condition.
2. This step determines if ECT signal circuit is shorted to ground.

Scheme 147

Check wiring harness routing for a potential short circuit to ground. After starting engine, coolant temperature should rise to about 185°F (85°C). PCM default value will flash on data screen intermittently.

Test engine coolant temperature sensor at various temperature levels to evaluate possibility of a skewed (out-of-range) sensor. See TEMPERATURE VS. RESISTANCE VALUES table. A skewed sensor could result in poor driveability complaints.

DTC 15, ENGINE COOLANT TEMPERATURE (ECT) SENSOR CIRCUIT HIGH (LOW TEMPERATURE INDICATED)

Engine Coolant Temperature (ECT) sensor is a thermistor that controls signal voltage to PCM. When engine is cold, sensor resistance is high, and PCM will sense a high signal voltage. As engine warms, sensor resistance becomes less and voltage drops. DTC 15 will set when engine coolant temperature is less than -33°F (-36°C) and engine has been running for at least 8 minutes.

PCM will default to a engine coolant temperature of 64°F (18°C) and increase idle speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 15 is the result of a hard failure or an intermittent condition.
2. This step determines if ECT signal circuit is open or for a faulty PCM.

Scheme 148

Check wiring harness routing for a potential short circuit to ground. After starting engine, coolant temperature should rise to about 185°F (85°C). PCM default value will flash on data screen intermittently.

Test engine coolant temperature sensor at various temperature levels to evaluate possibility of a skewed (out-of-range) sensor. See TEMPERATURE VS. RESISTANCE VALUES table. A skewed sensor could result in poor driveability complaints.

Scheme 149

Scheme 150

Scheme 151

Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction type sensor, VSS buffer module, 4WD low switch (if equipped), and wiring. Gear teeth pressed onto output shaft induces an alternating voltage into sensor. This signal is transmitted to VSS buffer module. VSS buffer module compensates for various final drive ratios. VSS buffer module will also convert AC VSS signal into a 40 pulse per revolution DC signal on trans output speed circuit to indicate transmission output speed. On 4WD vehicles, 4WD low signal will also be used for adjustment of 40 pulse per revolution signal to PCM.

DTC 24 will set when

1. Gear selector is not in Park or Neutral.
2. Trans output speed circuit voltage is constant.
3. Engine speed is more than 3000 RPM.
4. Output speed is less than 250 RPM.
5. MAP is less than 100 kPa.
6. Throttle position is 10-100 percent.
7. All conditions are met for 3 seconds.
8. DTCs 21, 22, 28, 33 or 34 are not set.

PCM will default to 2nd gear only at maximum line pressure.

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

1. This test checks for voltage to VSS buffer module.
2. This test checks ground circuit to VSS buffer module.
3. This test checks VSS circuit at VSS buffer module.
4. This test checks for an output speed signal from VSS buffer module.

DTC 24 will set when no vehicle speed is detected at vehicle start off. DTC 72 will set when vehicle speed has been detected and is lost.

Scheme 152

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 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. When transmission electrical connector is disconnected, ground potential for the 3 signals to PCM will be removed, and with ignition on, D2 will be indicated. DTC 28 will set when range signals "A" and "C" are both zero volts (on) for 2 seconds.

PCM will default to harsh shifts, D4 shift control. TCC will be inhibited, and if in hot mode, 4th gear will not be available. DTC 28 will be stored in PCM memory, but will not turn on MIL.

Note. Test numbers refer to numbers on diagnostic chart.

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

DTC 28 will set if PCM detects one of 2 illegal combinations. See TRANSMISSION RANGE PRESSURE SWITCH COMBINATIONS table. Check all wiring connectors for proper terminal tension. For current DTC 28, range signal will default to D4 while A/B/C range values continue to read actual trans. range pressure switch output for each range selection.

TRANSMISSION RANGE PRESSURE SWITCH COMBINATIONS (1) (2)

Scheme 153

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

DTC 39 will set when

1. DTCs 28, 71 or 74 are not set.
2. TCC is commanded on.
3. TCC slip speed is more than 65 RPM.
4. Trans. range in D3 or D4.
5. 2nd or 3rd gear.
6. All conditions are met for 2 seconds.

PCM will default to no 4th gear in hot mode.

Note. Test number refers to number on diagnostic chart.

1. This checks mechanical and hydraulic operation of TCC, while commanded on by PCM.

Snapshot mode will record 5 data parameters per second.

Scheme 154

Transmission Fluid Temperature (TFT) sensor is a thermistor within transmission range pressure switch assembly, that controls signal voltage to PCM. PCM applies voltage on 5V TFT signal circuit to sensor. When transmission fluid is cold, sensor resistance is high and PCM will sense high signal voltage.

As transmission fluid temperature warms to normal transmission operating temperature of 212°F (100°C), sensor resistance becomes less and voltage decreases to 1.5-2.0 volts. If DTC 79 is also set, check transmission cooling system. DTC 58 will set when signal voltage indicates TFT greater than 306°F (151°C) for one second.

PCM will default to when DTC 58 is set, transmission will use a warm value for operation, but scan tester will display actual fluid temperature. TCC solenoid will be on in 3rd and 4th gears. Shifts will occur early. DTC 58 will be stored in PCM memory but will not turn on MIL.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks for a short to ground or a skewed sensor.
2. This test checks for an internal fault within transmission by creating an open.

Check harness routing for a potential short to ground in 5V TFT signal circuit. Scan tester TFT display should rise steadily to about 212°F (100°C) then stabilize.

Test transmission fluid temperature sensor at various temperature levels to evaluate the possibility of a skewed (out-of-range) sensor. See TEMPERATURE VS. RESISTANCE VALUES table. A skewed sensor could result in delayed garage shifts or TCC complaints.

TEMPERATURE VS. RESISTANCE VALUES

DTC 59, TRANS. FLUID TEMP. SENSOR CIRCUIT HIGH (LOW TEMPERATURE INDICATED)

Transmission Fluid Temperature (TFT) sensor is a thermistor within transmission range pressure switch assembly that controls signal voltage to the PCM. PCM applies voltage on 5-volt TFT signal circuit to sensor. When transmission fluid is cold, sensor resistance is high and PCM will sense high signal voltage.

As transmission fluid temperature warms to normal transmission operating temperature of 212°F (100°C), sensor resistance becomes less and voltage decreases to 1.5-2.0 volts. DTC 59 will set when signal voltage indicates TFT less than 40°F (40°C) for one second.

PCM will default to when DTC 59 is set, transmission will use a warm value for operation, but scan tester will display actual fluid temperature. TCC may not apply properly. DTC 59 will be stored in PCM memory but will not turn on MIL.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks for a skewed sensor.
2. This test simulates a DTC 58. If PCM recognizes low signal voltage (high temperature), and scan tester displays 295°F (146°C) or greater, PCM and wiring are okay.
3. This test checks if 5-volt TFT signal circuit is open. Five volts should be present at sensor connector when measure using DVOM.

Scan tester displays transmission fluid temperature in degrees. After transmission is operating, temperature should rise steadily to about 212°F (100°C) then stabilize. A faulty connection or open in sensor ground circuit or 5-volt TFT signal circuit may cause DTC 59 to set.

Test transmission fluid temperature sensor at various temperature levels to evaluate possibility of a skewed (out-of-range) sensor. See TEMPERATURE VS. RESISTANCE VALUES table. A skewed sensor could result in firm shifts or TCC complaints.

TEMPERATURE VS. RESISTANCE VALUES

PCM monitors difference in engine speed and input speed. With transmission in Drive and TCC locked, scan tester should display engine speed closely matching input speed. DTC 68 will set when

1. DTCs 28, 71 or 74 are not set.
2. TCC slip speed more than 200 RPM.
3. 4th gear is indicated.
4. TCC is locked.
5. Not in Park/Neutral.
6. All conditions are met for 2 seconds.

PCM will default to inhibit TCC operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks the indicated range signal to actual selected range. A faulty switch could set DTC 68.
2. This test checks torque converter for slippage while in commanded lock-up state.

Check for deformed connectors at pass-thru connector. DTC 68 will set when going to default (2nd gear). An intermittent incorrect engine speed signal will set a DTC 68 if incorrect signal lasts for more than 2 seconds. A mechanical failure in 1-2 shift solenoid (stuck off) or 2-3 shift solenoid (stuck on) could set DTC 68.

DTC 69, TORQUE CONVERTER CLUTCH STUCK ON

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

DTC 69 will set when

1. DTCs 21, 22 , 28, 71 or 74 are not set.
2. TCC slip speed indicates -5 to 10 RPM.
3. TCC solenoid is commanded off.
4. TP sensor signal is more than 25 percent.
5. Trans. range switch indicates D3 or D4.
6. Commanded gear indicates 2nd or 3rd gear.
7. All conditions are met for 2 seconds. NOTE: Test numbers refer to numbers on diagnostic chart. This test checks for proper throttle position sensor operation. This test checks mechanical state of TCC. When PCM commands TCC solenoid off, TCC slip should increase.

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

Scan TP signal while depressing accelerator pedal with engine off and ignition on. Display should vary from less than .85 volt when throttle is closed to more than 4 volts when throttle is held at WOT. Incorrect TP sensor values may affect TCC operation.

DTC 72, VEHICLE SPEED SENSOR CIRCUIT LOSS (TRANSMISSION OUTPUT SIGNAL)

Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction type sensor, VSS buffer module, 4WD low switch (if equipped) and wiring. Gear teeth pressed onto output shaft induces an alternating voltage into sensor. This signal is transmitted to VSS buffer module. VSS buffer module compensates for various final drive ratios. VSS buffer module will also convert AC VSS signal into a 40 pulse per revolution DC signal on trans. output speed circuit to indicate transmission output speed. On 4WD vehicles, 4WD low signal will also be used for adjustment of the 40 pulse per revolution signal to PCM.

DTC 72 will set when

Gear selector is not in Park or Neutral.

1. Trans. output speed change is greater than 1000 RPM.
2. Engine speed is greater than 200 RPM.
3. All conditions are met for 2 seconds.
4. DTC 28 is not set.

Or

Gear selector is in Park or Neutral.

1. Trans. output speed change is greater than 2050 RPM.
2. Engine speed is more than 200 RPM.
3. All conditions are met for 2 seconds.
4. DTC 28 is not set.

PCM will default to a soft (delayed) downshift to 2nd gear and 2nd gear starts.

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

1. This test checks for voltage to VSS buffer module.
2. This test checks ground circuit to VSS buffer module.
3. This test checks VSS circuit at VSS buffer module.
4. This test checks for an output speed signal from VSS buffer module.

DTC 24 will set when no vehicle speed is detected at vehicle start off. DTC 72 will set when vehicle speed has been detected and is lost.

Scheme 155

Sensor consists of a permanent magnet surrounded by a coil of wire. As forward clutch housing rotates, an AC voltage is induced in circuit. Signal voltage and frequency vary directly with forward clutch rotational speed. DTC 74 will set when

1. DTC 24, 28 or 71 is not set.
2. Trans. range not in Park or Neutral.
3. Engine speed more than 300 RPM.
4. Trans. output speed more than 200 RPM.
5. Trans. input speed less than 50 RPM.
6. All conditions are met for 2 seconds.

PCM will default to no TCC operation.

Note. Test numbers refer to numbers on diagnostic chart.

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

Check for poor connections in PCM related circuits.

Transmission Fluid Temperature (TFT) sensor is a thermistor within transmission range pressure switch assembly, that controls signal voltage to PCM. PCM applies voltage on 5-volt TFT signal circuit to sensor. When transmission fluid is cold, sensor resistance is high and PCM will sense high signal voltage.

As transmission fluid temperature warms to normal transmission operating temperature of 212°F (100°C), sensor resistance becomes less and voltage decreases to 1.5-2.0 volts. DTC 79 will set when signal voltage indicates TFT greater than 295°F (146°C) for 30 seconds, and DTC 58 or 59 is not set.

DTC 79 will be stored in PCM memory but will not turn on MIL.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks for a shorted circuit or a skewed sensor.
2. This test simulates a DTC 59.

Check harness routing for a potential short to ground in 5-volt TFT signal circuit. Scan tester TFT display should rise steadily to about 212°F (100°C) then stabilize.

Test transmission fluid temperature sensor at various temperature levels to evaluate possibility of a skewed (out-of-range) sensor. See TEMPERATURE VS. RESISTANCE VALUES table. A skewed sensor could result in delayed shifts or TCC complaints.

TEMPERATURE VS. RESISTANCE VALUES

Scheme 156

Ignition voltage is supplied directly to 2-3 shift solenoid. PCM controls solenoid by providing ground path through 2-3 shift solenoid control circuit. DTC 81 will set when PCM commands solenoid on and voltage remains high for 2 seconds, or PCM commands solenoid off and voltage remains low for 2 seconds.

If solenoid is shorted off, 3rd gear only will occur. If solenoid is shorted on, 2nd gear only will occur. DTC 81 will be stored in PCM memory but will not turn on MIL.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks the function of 2-3 shift solenoid and internal transmission wiring.
2. This test checks for power to 2-3 shift solenoid from ignition, through fuse.

Scheme 157

Check all connections at transmission. An open in ignition feed circuit can cause multiple DTCs to set.

Scheme 158

Ignition voltage is supplied directly to 1-2 shift solenoid. PCM controls solenoid by providing ground path through 1-2 shift solenoid control circuit. DTC 82 will set when PCM commands solenoid on and voltage remains high for 2 seconds, or PCM commands solenoid off and voltage remains low for 2 seconds.

PCM will only allow 2nd and 3rd gear, or 1st and 4th gear. DTC 82 will be stored in PCM memory but will not turn on MIL.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks the function of 1-2 shift solenoid and internal transmission wiring.
2. This test checks for power to 1-2 shift solenoid from ignition through fuse.

Scheme 159

Check all connections at transmission. An open in ignition feed circuit can cause multiple DTCs to set.

Scheme 160

TCC PWM is used in combination with TCC solenoid to regulate fluid to torque converter and attached to transmission control valve body. PCM supplies a ground, allowing current to flow through solenoid coil according to duty cycle (percentage of on and off time). This current flow through solenoid coil creates a magnetic field that magnetizes the solenoid core. The magnetized core attracts the check ball to seat against spring pressure. This blocks the exhaust for the TCC signal fluid and allows 2-3 drive fluid to feed TCC signal circuit. TCC signal fluid pressure acts on TCC regulator valve to regulate line pressure and to apply fluid pressure to torque converter clutch shift valve. When TCC shift valve is in apply position, regulated fluid pressure is directed through TCC valve to apply torque converter clutch.

DTC 83 will set when

1. PCM commands solenoid on and voltage remains high (B+).

Or

1. PCM commands solenoid off and voltage remains low (zero volt).
2. All conditions are met for 2 seconds.

PCM will default to inhibit TCC operation and inhibit 4th gear operation if in hot mode.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks if PCM is commanding TCC solenoid on.
2. This test checks for voltage to solenoid.

Scheme 161

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

Scheme 162

PCM calculates ratio based on transmission input speed and output speed sensor readings. PCM compares known transmission ratio to calculated ratio, for particular gear range selected. DTC 85 will set when

1. DTCs 21, 22, 24, 28, 71, 72 and 87 are not set.
2. TP is more than 25 percent.
3. Not in Park/Neutral or 4th gear.
4. Engine speed is more than 300 RPM.
5. Vehicle speed is more than 7 MPH.
6. All conditions are met for 2 seconds.

PCM will default to maximum line pressure and inhibits TCC operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-range transmission range pressure switch could falsely indicate the actual transmission range.
2. This test checks calculated ratio to determine if ratio is within parameters.

DTC 85 will set when an unknown gear ratio is detected for any gear but 4th. Note commanded gear and incorrect ratio. Check transmission fluid level. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.

GEAR RATIO

PCM calculates ratio based on transmission input speed and output speed sensor readings. PCM compares known transmission ratio to calculated ratio, for particular gear range selected. DTC 86 will set when

1. DTCs 21, 22, 24, 28, 71, 72 and 74 are not set.
2. Not in Park/Neutral or Reverse.
3. TP is more than 25 percent.
4. Engine speed is more than 300 RPM.
5. Vehicle speed is more than 7 MPH.
6. Trans. gear ratio is less than 1.06 in 1st or 2nd gear.
7. All conditions are met for 2 seconds.

PCM will default to 2nd gear, line pressure set to maximum and inhibits TCC operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-range transmission range pressure switch could falsely indicate actual transmission range.
2. This test compares known ratio for a commanded gear to calculated ratio displayed on scan tester.

DTC 86 will set when trans. commanded gear is 1st or 2nd and trans. is mechanically in 3rd or 4th gear. DTC 81 is used to detect a 2-3 shift solenoid circuit malfunction.

PCM calculates ratio based on transmission input speed and output speed sensor readings. PCM compares known transmission ratio to calculated ratio, for particular gear range selected. DTC 87 will set when

1. DTCs 21, 22, 24, 28, 71, 72 and 74 are not set.
2. Not in Park/Neutral or Reverse.
3. TP is more than 25 percent.
4. Engine speed is more than 300 RPM.
5. Vehicle speed is more than 7 MPH.
6. Transmission temperature is more than 68°F (20°C).
7. Trans. gear ratio is more than 1.42 in 3rd or 4th gear.
8. All conditions are met for 2 seconds.

PCM will default to 2nd gear, line pressure set to maximum and inhibits TCC operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-range transmission range pressure switch could falsely indicate actual transmission range.
2. This test compares known ratio for a commanded gear to calculated ratio displayed on scan tester.

DTC 87 will set when trans. commanded gear is 3rd or 4th and trans. is mechanically in 1st or 2nd gear. DTC 81 is used to detect a 2-3 shift solenoid circuit malfunction.

COMPONENT & WIRING HARNESS RESISTANCE CHECK

1. Install Jumper Harness (J-39775) to transmission 20-pin connector. Using an ohmmeter, measure resistance between specified terminals for each component. (Scheme 163) Compare resistance reading to known values. See «TRANSMISSION COMPONENT RESISTANCE SPECIFICATIONS»(/gmc/suburban/i-1992-1999/remont/automatic-trans/#automatic-transmission-diagnosis-4l80-e) table.
2. If resistance reading is okay, go to next step. If resistance reading is not okay, disconnect wiring harness at component and measure component resistance. Replace component if resistance is not as specified. If resistance is as specified, repair wiring harness between component and 20-pin connector.
3. Measure resistance between ground and each terminal at transmission 20-pin connector. (Scheme 163) If resistance is within specification, problem is intermittent. If resistance is low, disconnect wiring harness at component.
4. Measure resistance between component terminals and ground. If resistance is low, replace component. If resistance is high, inspect wiring harness for short to ground. Repair as necessary.

PRESSURE SWITCH ASSEMBLY

Install Jumper Harness (J39775) to transmission 20-pin connector. Compare resistance of pressure switches with gear selector in each position to known values. (Scheme 164) If resistance values are the same as chart, problem is intermittent. If resistance values are not the same, complete chart until problem is found.

TRANSMISSION COMPONENT RESISTANCE SPECIFICATIONS

Scheme 163

Scheme 164

TECHNICAL SERVICE BULLETINS (TSB)

Note. Review TSBs before using diagnostic charts to ensure problems are not addressed by these TSBs. Powertrain Control Module (PCM) may also be referred to as Vehicle Control Module (VCM) in TSBs.

If replacement of PCM is required, see PCM APPLICATION table to identify correct PCM for specified year.

PCM APPLICATION

1994-95 "C", "G" & "K" 4WD Vehicles (ATRA TSB No. 343)

Vehicles may have one or all of the following conditions

1. Lack of power, especially when carrying loads or during heavy acceleration.
2. Early upshifts regardless of throttle position.
3. Late shifts in 4WD-Low position only.
4. Erratic shifting including shifts which are early during one upshift sequence and normal during next sequence, or some of the upshifts during shift sequence are early while others are normal.
5. Incorrect forced downshift shift points or no forced downshifts at normal road speeds.

These conditions may occur on either 2WD or 4WD vehicles, but are most common on 4WD vehicles. These conditions may be intermittent and could occur without setting trouble codes. If any of these conditions occur, perform following test to assist in diagnosis.

Early, Late Or Erratic Shifts (All Models)

1. Check for trouble codes which may relate to condition. If no codes are present, go to next step. If codes are present, correct affected systems, then check for correct shift timing. If condition still exists, go to next step.
2. Operate vehicle through 2/4 high position shifts. If shifts are early and/or erratic, go to next step. If shifts are late and/or erratic, go to step 5). If shifts are correct, problem is intermittent. Operate vehicle until condition occurs, then repeat step 1). If condition does not occur, check for stuck 4WD switch, loose connections at PCM and 4WD switch, or for circuits which may be shorted to ground. Repair as necessary.
3. Using a scan tool, monitor 4WD-low input. If 4WD-low input indicates low position even though vehicle is in high position, go to next step. If 4WD-low input indicates truck is not in low position, scan tool value for 4WD-low reading will be NO at all times. If condition exists, check TP sensor, MAP sensor or VSS sensor for proper operation. Repair or replace as necessary.
4. Scan tool value for 4WD-low position will be YES at all times if 4WD-low input indicates low position even though vehicle is in high position. A short to ground in 4WD-low position circuit, 4WD switch or related circuit may exist. Perform 4WD-LOW DIAGNOSTIC CHECK.
5. Check TP sensor, MAP sensor or VSS sensor for proper operation. Repair or replace as necessary. If sensors are okay, check tire size and axle ratio. If tire size and axle ratio has been modified from stock applications, buffer (DRAC) must be recalibrated or replaced with correct calibration.

4WD-Low Diagnostic Check ("K" Series With Manually Or Electronically Shifted Transfer Case In High Position)

1) Disconnect transfer case switch connector at transfer case. Turn ignition switch to ON position. Using a scan tool, monitor 4WD-low position. If scan tool indicates YES for 4WD-low position, go to next step. If scan tool indicates NO for 4WD-low position, check for internal short to ground in transfer case switch. Replace switch as necessary.

2) A short to ground in Dark Blue wire (Gray/Black wire on vehicles with VCM) between transfer case switch connector and PCM terminal F8 (terminal No. 23 on vehicles with VCM), or a faulty PCM/VCM is indicated.

3) Disconnect connector located behind distributor at center of torque converter housing. If scan tool indicates YES for 4WD-low position, go to next step. If scan tool indicates NO for 4WD-low position, check for short to ground in Dark Blue wire (Gray/Black wire on vehicles with VCM) between transfer case switch and connector located behind distributor. Repair wire as necessary.

4) Turn ignition off. Disconnect PCM Blue connector (Black connector on vehicles with VCM). Using an ohmmeter, measure resistance between ground and PCM Blue connector terminal F8 (terminal No. 23 on vehicles with VCM). If resistance is less than 100 ohms, repair short to ground in Dark Blue wire (Gray/Black wire on vehicles with VCM).

Wiring Diagram ("C" & "K" Series - 5.0L, 5.7L & 7.4L). Scheme 165

Wiring Diagram ("C" & "K" Series - 6.5L). Scheme 166

Wiring Diagram ("G" Series - 4.3L, 5.0L, 5.7L & 7.4L). Scheme 167

Wiring Diagram ("G" Series - 6.5L With PCM). Scheme 168

Wiring Diagram ("G" Series - 6.5L With TCM). Scheme 169

Wiring Diagram ("P" Series - 4.3L). Scheme 170

Wiring Diagram ("P" Series - 5.7L & 7.4L). Scheme 171

Wiring Diagram ("P" Series - 6.5L With PCM). Scheme 172

Wiring Diagram ("P" Series - 6.5L With TCM). Scheme 173

See also:
• CLUTCH AND BAND APPLICATION
• SERIES CODE DESIGNATIONS
• CLEARING TROUBLE CODES
• TROUBLE CODE DEFINITIONS