GMC Suburban K2500 1992 - AUTOMATIC TRANSMISSIONS 4L80-E Electronic Controls

GMC Suburban K2500 1992 - INTRODUCTION

On gasoline vehicles with the 4L80-E transmission, transmission is controlled by the Powertrain Control Module (PCM). PCM controls other vehicle functions as well as the transmission. Electronic transmission is controlled by the Transmission Control Module (TCM) on diesel vehicles. 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 force motor to regulate TCC engagement, 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, covered in BASIC DIAGNOSTIC PROCEDURES article. Failure to do so may result in lost diagnostic time. On diesel vehicles, DIAGNOSTIC CIRCUIT CHECK is covered in G - DIESEL - TESTS W/ CODES article.

If no faults were found while performing checks and inspections in BASIC TESTING article, 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 article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES article.

GMC Suburban K2500 1992 - SELF-DIAGNOSTIC SYSTEM

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, see THEORY/OPERATION and G - GAS - TESTS W/ CODES article.

Diesel vehicles are equipped with a Transmission Control Module (TCM). TCM controls only the electronic transmission. Unless specifically stated otherwise, references to PCM also apply to TCM-equipped vehicles.

Control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate the SERVICE ENGINE SOON light located on instrument panel. 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 in this article. Malfunctions are recorded as either hard failures or intermittent failures.

GMC Suburban K2500 1992 - DIAGNOSTIC PROCEDURE 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.

GMC Suburban K2500 1992 - 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. On gasoline vehicles, see DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. On diesel vehicles, DIAGNOSTIC CIRCUIT CHECK is covered in G - DIESEL - TEST W/ CODES article. The diagnostic circuit check determines if the 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 SERVICE ENGINE SOON light to come on.
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 SERVICE ENGINE SOON light will go out 10 seconds after fault goes away. For intermittent diagnosis procedures, see TESTS W/O CODES article.

GMC Suburban K2500 1992 - ENTERING & EXITING DIAGNOSTIC MODE

1. With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). See Fig. 1. This places the control unit into diagnostic mode.
2. In this mode, the control unit will display Code 12 by flashing the SERVICE ENGINE SOON (SES) light. Code 12 is identified by the SES light flashing 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 the diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

Fig. 1: GMC Suburban K2500 1992 - Component Locations - Identifying ALDL Connector Terminals

GMC Suburban K2500 1992 - 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 G - TESTS W/ CODES article. See TROUBLE CODE IDENTIFICATION table to determine if code is transmission or engine related.

1. The system codes indicate failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of the control unit. Code 51 indicates PROM/MEM-CAL is installed improperly or has failed.
2. Code 52 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.
   • An incorrect PCM/TCM or PROM/MEM-CAL application may cause a malfunction, which may or may not set a code.
   • The PCM/TCM connector may be the problem. Connector terminals may have to be removed from the connector to be properly checked.
   • PROM/MEM-CAL failure. Although the PROM/MEM-CAL rarely fails, it could be the cause of the problem.
   • Replacement PCM/TCM may be faulty.
   • Intermittent problem. Make a careful physical inspection of affected sensor/circuit.
   • A shorted solenoid, coil relay, or harness may be the cause of PCM/TCM failure. Use Short Circuit Tester (J-34636) to check for short circuits.

GMC Suburban K2500 1992 TROUBLE CODE IDENTIFICATION

Code	Probable Cause
12 (1)	No Engine Speed Sensor Reference Pulse To PCM/TCM
13 (1)	Open Oxygen Sensor Circuit (Gasoline)
14 (1)	CTS Voltage Low (Sensor Or Signal Line Grounded)
15 (1)	CTS Voltage High (Sensor, Connections, Or Wires Open)
21 (1)	TPS Voltage High (Open Circuit Or Misadjusted TPS)
22 (1)	TPS Voltage Low (Circuit Grounded)
23 (1)	TPS Not Calibrated (Diesel - .25-1.3 Volts At Curb Idle)
23 (1)	Intake Air Temperature Sensor Voltage High (Gasoline)
24 (1)	Vehicle Speed Sensor Circuit Open Or Grounded
25 (1)	Intake Air Temperature Sensor Voltage Low (Gasoline)
28	Pressure Switch Manifold Range Circuit Open Or Shorted
31 (1)	MAP Voltage Low (Diesel)
31 (1)	Turbocharger Wastegate Overboost (Gasoline)
32 (1)	EGR Error (Improper Vacuum Signal)
33 (1)	MAP Voltage High
34 (1)	MAP Voltage Low (Gasoline)
35 (1)	IAC System Fault (Gasoline)
39	TCC Stuck Off (Faulty TCC Solenoid)
42 (1)	EST Circuit Fault (Gasoline)
43 (1)	ESC Fault (Gasoline)
44 (1)	Lean Exhaust Indicated (Gasoline)
45 (1)	Rich Exhaust Indicated (Gasoline)
51 (1)	Improperly Installed/Faulty PROM/MEM-CAL
52 (1)	PCM/TCM Fault
53	System Voltage High (Gasoline - Charging Problem)
53	Voltage Reference Overload (Diesel)
54 (1)	Fuel Pump Circuit Voltage Low (Gasoline)
55 (1)	Faulty ECM (Gasoline)
58	TTS High Temperature (Sensor Or Signal Line Grounded)
59	TTS Low Temperature (Sensor, Connections, Or Wires Open)
68	Overdrive Ratio Error (Engine RPM Greater Than Input Speed)
73	Force Motor Commanded Amperage Differs From Return
75	System Voltage Low (Charging System Problem)
81	QDM Solenoid "B" Monitored Voltage Differs From Commanded
82	QDM Solenoid "A" Monitored Voltage Differs From Commanded
83	QDM TCC Monitored Voltage Differs From Commanded
85	Undefined Gear Ratio (Input Or Output Sensor Failure)
86	Shift Solenoid "B" Stuck On (Commanded Gear Not Engaged)
87	Shift Solenoid "B" Stuck Off (Commanded Gear Not Engaged)
(1) Code is engine related. For information on engine-related trouble codes, see appropriate G - TESTS W/ CODES article.

GMC Suburban K2500 1992 - CLEARING TROUBLE CODES

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, remove PCM/TCM memory battery voltage for 30 seconds.

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

GMC Suburban K2500 1992 - PCM/TCM LOCATION

The ECM is located under the passenger's seat riser ("G" Series Van), or behind right side of dash (all other models).

GMC Suburban K2500 1992 - SPECIAL TOOLS (DIAGNOSTIC)

Special scan testers are plugged into the ALDL 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 manual included with tester.

GMC Suburban K2500 1992 - 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.

GMC Suburban K2500 1992 - 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 ALDL test terminal "B" with ignition on and engine off.

GMC Suburban K2500 1992 - COMPONENT CHECK CHARTS CODE 28, PRESSURE SWITCH MANIFOLD FAULT

The Pressure Switch Manifold (PSM) is actually 5 pressure switches combined into one unit and mounted on the transmission valve body. The PCM/TCM supplies battery voltage to the PSM on 3 separate wires. By grounding one or more of these circuits through various combinations of the pressure switches inside the pressure switch manifold, the PCM/TCM detects what gear range has been selected by the vehicle operator.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test compares the indicated range to the range actually selected.
2. This test checks for correct voltage from the PCM/TCM to the transmission pass-thru connector.
3. This test will detect a short to ground in any one of the 3 PSM range circuits.

GMC Suburban K2500 1992 - Diagnostic Aids

Code 28 will set if PCM/TCM detects one of 2 "illegal" PSM combinations. See VALID PSM COMBINATION for various combinations. Check pass-thru connector for good contact.

GMC Suburban K2500 1992 VALID PSM COMBINATION CHART

Application	A	B	C
Park	12	0	12
Reverse	0	0	12
Neutral	12	0	12
4th	12	0	0
3rd	12	12	0
2nd	12	12	12
1st	0	12	12
Illegal	0	12	0
Illegal	0	0	0

Fig. 2: GMC Suburban K2500 1992 - Component Locations - CODE 28, Schematic, Pressure Switch Manifold Fault

Fig. 3: GMC Suburban K2500 1992 - Component Locations - CODE 28, Flow Chart, Pressure Switch Manifold Fault

GMC Suburban K2500 1992 - CODE 39, TORQUE CONVERTER CLUTCH STUCK "OFF"

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

Fused battery ignition voltage is supplied to the TCC solenoid, which is used inside the valve body to shift a spool valve to modulate pressure to the TCC. This modulated pressure normally allows some slight slippage of TCC.

The PCM/TCM will engage TCC by grounding circuit No. 422, energizing the solenoid.

Code 39 will set under the following conditions.

• TCC is engaged.
• TCC "slip" is greater than 65 RPM for 2 seconds.
• 2nd or 3rd gear is selected or indicated.

  NOTE:

  Test numbers refer to test numbers on diagnostic chart.

  1. This test determines if transmission is receiving a TCC command from the pass-thru connector. If bidirectional scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine off.
  2. This test checks for power to transmission.
  3. This test determines if PCM/TCM is commanding TCC to be on.

GMC Suburban K2500 1992 - Diagnostic Aids

Clear codes and re-check for Code 39. If code resets, problem could be internal to the transmission. Code 39 will only set in 3rd gear. TCC slip in 4th gear will set Code 68.

Fig. 4: GMC Suburban K2500 1992 - Component Locations - CODE 39, Schematic, Torque Converter Clutch Stuck "OFF"

Fig. 5: GMC Suburban K2500 1992 - Component Locations - CODE 39, Flow Chart, Torque Converter Clutch Stuck "OFF"

GMC Suburban K2500 1992 - CODE 53, SYSTEM VOLTAGE HIGH

Code 53 will set when ignition is on and PCM terminal D1 voltage is greater than 19.5 volts for about 2 seconds. During the time the failure is present, the force motor is turned off, transmission immediately shifts to 2nd gear, and TCC operation is inhibited. The setting of additional codes may result.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Normal battery voltage is 10-17.0 volts.
2. This test checks if high voltage reading is due to the alternator, circuit No. 440 or PCM. With engine running, check voltage at battery. If voltage is greater than 19.5 volts, the PCM is okay.
3. This test checks if alternator is faulty under load condition. If voltage is greater than 19.5 volts, check alternator.

GMC Suburban K2500 1992 - NOTES ON INTERMITTENTS

Jump-starting engine, and charging battery with a battery charger may set Code 53. If code is set when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link.

Fig. 6: GMC Suburban K2500 1992 - Component Locations - CODE 53, Schematic, System Voltage High

Fig. 7: GMC Suburban K2500 1992 - Component Locations - CODE 53, Flow Chart, System Voltage High

GMC Suburban K2500 1992 - CODE 58, TRANSMISSION TEMP. SENSOR CKT (HIGH TEMP. INDICATED)

The transmission temperature sensor is a thermistor which controls the signal voltage to the PCM/TCM. The PCM/TCM applies and monitors voltage on circuit No. 1227 to the sensor. When transmission is cold, sensor resistance is high; therefore, the PCM/TCM will see high signal voltage. As the transmission warms up, sensor resistance and voltage will drop. At normal transmission operating temperature of 212?F (100?C), voltage will be about 1.5-2.0 volts.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission temperature greater than 309?F (154?C) for one second.
2. This test determines if circuit No. 1227 is shorted to ground, which will result in conditions for Code 58.

GMC Suburban K2500 1992 - Diagnostic Aids

Check harness routing for a potential short to ground in circuit No. 1227. Scan tester displays transmission 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. See TRANSMISSION SENSOR - TEMPERATURE TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts, or TCC enabled complaint.

Fig. 8: GMC Suburban K2500 1992 - Component Locations - CODE 58, Schematic, Trans. Temp. Sensor Circuit High

Fig. 9: GMC Suburban K2500 1992 - Component Locations - CODE 58, Flow Chart, Trans. Temp. Sensor Circuit High

GMC Suburban K2500 1992 - CODE 59, TRANSMISSION TEMP. SENSOR CKT (LOW TEMP. INDICATED)

The transmission temperature sensor is a thermistor which controls signal voltage to PCM/TCM. The PCM/TCM applies and monitors 5 volts to the sensor on circuit No. 1227. When transmission is cold, sensor resistance is high; therefore, the PCM/TCM will see high signal voltage. As transmission temperature warms up, sensor resistance and voltage drop. At normal transmission operating temperature of 212?F (100?C), voltage will be about 1.5-2.0 volts.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 59 will set if signal voltage indicates a transmission temperature less than -54?F (-48?C) for one second.
2. This test simulates Code 58. If PCM/TCM recognizes the low signal voltage (high temperature) and scan tester reads 304?F (151?C) or greater, the PCM/TCM and wiring are okay.
3. This test determines if circuit No. 1227 is open. There should be 5 volts present at the sensor connector if measuring with a DVOM.

GMC Suburban K2500 1992 - Diagnostic Aids

Scan tester displays transmission temperature in degrees Centigrade. After transmission is running, the displayed temperature should rise steadily to about 212?F (100?C) then stabilize. A faulty connection or an open in circuit No. 455/452 or circuit No. 1227 will result in a Code 59. Test the coolant sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMPERATURE TO RESISTANCE chart. An out-of-calibration sensor could result in firm shifts, or TCC enabled complaint.

Fig. 10: GMC Suburban K2500 1992 - Component Locations - CODE 59, Schematic, Trans. Temp. Sensor Circuit Low

Fig. 11: GMC Suburban K2500 1992 - Component Locations - CODE 59, Flow Chart, Trans. Temp Sensor Circuit Low

GMC Suburban K2500 1992 - 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 test numbers on diagnostic chart.

1. Code 68 will set under the following conditions: engine speed is 200 RPM greater than input speed, 4th gear is indicated, TCC is enabled, or if all conditions are met for 2 seconds.
2. This test monitors signal sent to the PCM/TCM from transmission.
3. This test confirms engine RPM signal to PCM/TCM.

GMC Suburban K2500 1992 - Diagnostic Aids

Check for spread connectors at pass-thru connector. Code 68 will set when going to default (2nd gear). - TESTS W/O CODES article.

Fig. 12: GMC Suburban K2500 1992 - Component Locations - CODE 68, Schematic, Overdrive Ratio Error

Fig. 13: GMC Suburban K2500 1992 - Component Locations - CODE 68, Flow Chart, Overdrive Ratio Error

GMC Suburban K2500 1992 - CODE 73, FORCE MOTOR CURRENT (CURRENT ERROR)

NOTE:

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

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

The PCM/TCM then monitors amperage at the return line. If the return amperage varies more than .16 amp from the commanded amperage for the duration of at least one second, Code 73 will set. Once Code 73 is set, the force motor is disabled and full line pressure will be applied until the next time the ignition switch is cycled. Code 73 will remain stored, but the force motor will resume normal function until the conditions for Code 73 re-occur.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Checks the PCM and TCM's ability to command the force motor.
2. Checks for voltage at PCM/TCM.
3. Checks internal transmission harness and force motor for low resistance.
4. Checks for short to ground.
5. Checks internal transmission harness and force motor for high resistance.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 14: GMC Suburban K2500 1992 - Component Locations - CODE 73, Schematic, Force Motor Current (Current Error)

Fig. 15: GMC Suburban K2500 1992 - Component Locations - CODE 73, Flow Chart, Force Motor Current (Current Error)

GMC Suburban K2500 1992 - CODE 75, SYSTEM VOLTAGE LOW

Code 75 will set when ignition is on and PCM terminal D1 voltage is less than 8.6 volts for about 4 seconds. During the time the failure is present, the force motor is turned off, maintaining only 2nd gear, and inhibiting 4th gear and TCC operation.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Normal battery voltage is 10-17.0 volts.
2. This test checks if the low voltage reading is due to the alternator, circuit No. 440 or PCM/TCM. With engine running, check voltage at the battery. If voltage is less than 8.6 volts, PCM/TCM is okay.

GMC Suburban K2500 1992 - 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 will change with temperature. Minimum voltage at -40?F (-40?C) is 6.7 volts. Minimum voltage at 304?F (150?C) is 10.5 volts, and minimum voltage at 194?F (90?C) is 8.6 volts.

Fig. 16: GMC Suburban K2500 1992 - Component Locations - CODE 75, Schematic, System Voltage Low

Fig. 17: GMC Suburban K2500 1992 - Component Locations - CODE 75, Flow Chart, System Voltage Low

GMC Suburban K2500 1992 - CODE 81, QDM FAULT (SOLENOID "B")

The PCM/TCM continually monitors voltage on each circuit connected to the quad driver, looking for either low or high voltage, depending on the commanded state of the devices connected to it. Code 81 will set if a fault has been detected on the shift solenoid "B" circuit. For example, if shift solenoid "B" is commanded on by the PCM/TCM, voltage on that circuit should drop when solenoid is grounded. If voltage stays up for at least .5 second, Code 81 will set. The opposite is also true. If shift solenoid "B" is off, voltage on the circuit should remain high. If voltage drops for more than .5 second, Code 81 will set.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This procedure checks shift solenoid "B" and internal transmission wiring for shorts.
2. This test checks for power, from the ignition through transmission fuse, to the shift solenoid "B".
3. This test ensures circuit No. 1223 is not shorted to ground.
4. This test checks PCM/TCM's ability to ground or control shift solenoid "B". If bidirectional scan tester is not available, solenoid may be activated by grounding ALDL test terminal "B" with ignition on and engine off.
5. This test ensures circuit No. 1223 is not shorted to ground.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 18: GMC Suburban K2500 1992 - Component Locations - CODE 81, Schematic, QDM Fault (Solenoid "B")

Fig. 19: GMC Suburban K2500 1992 - Component Locations - CODE 81, Flow Chart, QDM Fault (Solenoid "B")

GMC Suburban K2500 1992 - CODE 82, QDM FAULT (SOLENOID "A")

The PCM/TCM continually monitors 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 82 will set if there is a fault detected on the shift solenoid "A" circuit. For example, if shift solenoid "A" is commanded on by the PCM/TCM, then voltage on circuit should drop as soon as solenoid is grounded.

However, if voltage remains high for 2 seconds after the "on" command is given, Code 82 will set. The opposite is also true. If shift solenoid "A" is off, voltage on the circuit should be high. If voltage drops for .5 second or longer, Code 82 will set.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This procedure checks shift solenoid "A" and internal transmission wiring harness for shorts.
2. This test checks for power, from the ignition through the fuse, to the shift solenoid.
3. This test checks circuit No. 1222 for short to ground.
4. This test checks PCM/TCM's ability to ground or control shift solenoid "A". If bidirectional scan tester is not available, solenoid may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 20: GMC Suburban K2500 1992 - Component Locations - CODE 82, Schematic, QDM Fault (Solenoid "A")

Fig. 21: GMC Suburban K2500 1992 - Component Locations - CODE 82, Flow Chart, QDM Fault (Solenoid "A")

GMC Suburban K2500 1992 - CODE 83, TCC QDM 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 test numbers on diagnostic chart.

1. This test checks for low resistance in the solenoid or internal transmission harness. This test also determines which circuit triggered the fault. If bidirectional scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 22: GMC Suburban K2500 1992 - Component Locations - CODE 83, Schematic, TCC QDM Fault

Fig. 23: GMC Suburban K2500 1992 - Component Locations - CODE 83, Flow Chart, TCC QDM Fault

GMC Suburban K2500 1992 - CODE 85, UNDEFINED RATIO

The PCM/TCM calculates the actual gear ratio from input and output speed readings while in each gear, then compares these to what the gear ratio should be, taking into consideration the selected gear range. This monitor includes Reverse gear, but does not include Overdrive gear.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test verifies the proper function of the input sensor.
2. This test verifies the proper function of the output sensor.

GMC Suburban K2500 1992 - Diagnostic Aids

The PCM/TCM relies on the Power Switch Module (PSM) to indicate what gear range has been selected. PSM must be functioning properly or Code 85 may be set. Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.

GMC Suburban K2500 1992 GEAR RATIO

Gear	Less Than	More Than
1st	2.38	2.63
2nd	1.43	1.58
3rd	.95	1.05
Reverse	1.97	2.17

Fig. 24: GMC Suburban K2500 1992 - Component Locations - CODE 85, Schematic, Undefined Ratio

Fig. 25: GMC Suburban K2500 1992 - Component Locations - CODE 85, Flow Chart, Undefined Ratio

GMC Suburban K2500 1992 - CODE 86, SHIFT SOLENOID "B" STUCK "ON"

The shift solenoids are used inside the valve body to control spool valves, which determine the transmission gear. Fused ignition power is supplied to solenoid "B". The PCM/TCM will engage solenoid "B" by grounding circuit No. 1223 to energize the solenoid.

Code 86 will set under the following conditions.

• Vehicle speed is greater than 7 MPH.
• TPS is more than 25 percent.
• PCM/TCM commands 1st or 2nd gear.
• Transmission ratio indicates transmission is in 3rd or 4th gear.
• Conditions are met for 6 seconds.

  NOTE:

  Test numbers refer to test numbers on diagnostic chart.

1. This test determines if transmission is receiving a solenoid "B" on command.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 26: GMC Suburban K2500 1992 - Component Locations - CODE 86, Schematic, Shift Solenoid "B" Stuck "ON"

Fig. 27: GMC Suburban K2500 1992 - Component Locations - CODE 86, Flow Chart, Shift Solenoid "B" Stuck "ON"

GMC Suburban K2500 1992 - CODE 87, SHIFT SOLENOID "B" STUCK "OFF"

The shift solenoids are used inside the valve body to control spool valve position, which determines the transmission gear. Fused ignition power is supplied to solenoid "B." The PCM/TCM will engage solenoid "B" by grounding circuit No. 1223 to energize the solenoid.

Code 87 will set under the following conditions.

• Vehicle speed is greater than 7 MPH.
• TPS is more than 25 percent.
• PCM/TCM commands 3rd or 4th gear.
• Transmission ratio indicates transmission is in 1st or 2nd gear.
• Conditions are met for 6 seconds.

  NOTE:

  Test numbers refer to test numbers on diagnostic chart.

1. This test checks PCM/TCM's ability to ground or control solenoid "B". If bidirectional scan tester is not available, solenoid may be activated by grounding ALDL test terminal "B" with ignition on and engine off.
2. This test checks the power supply and circuit No. 1223.
3. This test checks the internal transmission harness and solenoid "B".

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 28: GMC Suburban K2500 1992 - Component Locations - CODE 87, Schematic, Shift Solenoid "B" Stuck "OFF"

Fig. 29: GMC Suburban K2500 1992 - Component Locations - CODE 87, Flow Chart, Shift Solenoid "B" Stuck "OFF"

GMC Suburban K2500 1992 - OUTPUT SPEED SENSOR CIRCUIT CHECK (2WD) "C/K/G & P" SERIES

The output sensor circuit consists of a magnetic induction-type sensor, digital ratio adapter, and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor.

This current is transmitted to a digital ratio adapter, where it is passed on to the PCM/TCM. The digital ratio adapter compensates for various axle ratios, and converts the signal to a square wave for use by the speedometer, cruise control and anti-lock brake system. If the input sensor is not operational at start up, it can cause the output sensor to read zero.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks the speed signal on circuit No. 437 at the PCM/TCM from the digital ratio adapter.
2. This test checks for a speed signal from the output speed sensor to the digital ratio adapter.
3. This test checks the output speed sensor.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 30: GMC Suburban K2500 1992 - Component Locations - Output Speed Sens Ckt Check, Schematic, C & K Series (2WD)

Fig. 31: GMC Suburban K2500 1992 - Component Locations - Output Speed Sens Ckt Check, Schematic, G & P Series (2WD)

Fig. 32: GMC Suburban K2500 1992 - Component Locations - Output Speed Sens Ckt Check, Flow Chart, C, K, G, & P (2WD)

GMC Suburban K2500 1992 - OUTPUT SPEED SENSOR CIRCUIT CHECK (4WD)

The output sensor is a magnetic induction type. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor. Since vehicle speed is taken from the transfer case on 4WD drive vehicles, the transmission output sensor signal on these units goes directly to the PCM/TCM. If input sensor is not operational at start up, it can cause the output sensor to read zero.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a speed signal at the PCM/TCM.
2. This test checks the output sensor directly.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 33: GMC Suburban K2500 1992 - Component Locations - Output Speed Sensor Circuit Check, Schematic, (4WD)

Fig. 34: GMC Suburban K2500 1992 - Component Locations - Output Speed Sensor Circuit Check, Flow Chart, (4WD)

GMC Suburban K2500 1992 - INPUT SPEED SENSOR CIRCUIT CHECK

The input sensor is of the magnetic induction type and is located on the left side of the transmission, forward of center. Serrations in the forward clutch housing induce a small A/C current as they pass by the input sensor. While there is no specific code for an input sensor problem, the PCM/TCM uses input sensor readings to calculate gear ratio, turbine speed, TCC slip, and determine if the engine is running.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks the input sensor circuit up to the PCM/TCM.
2. This test checks the sensor output.

GMC Suburban K2500 1992 - Diagnostic Aids

Check all connectors. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.

Fig. 35: GMC Suburban K2500 1992 - Component Locations - Input Speed Sensor Circuit Check, Schematic

Fig. 36: GMC Suburban K2500 1992 - Component Locations - Input Speed Sensor Circuit Check, Flow Chart

GMC Suburban K2500 1992 - BRAKE SIGNAL CIRCUIT CHECK

The normally closed brake switch supplies a 12-volt signal on circuit No. 420 to the PCM/TCM. The signal voltage is removed when brakes are applied. An incorrect brake signal may affect TCC operation.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks for voltage at brake switch.
2. This test simulates closed brake switch (brakes off).
3. This test checks circuit No. 420 from brake switch to PCM/TCM.
4. This test opens circuit No. 420 and simulates condition of brakes being applied.

GMC Suburban K2500 1992 - Diagnostic Aids

If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article. Check customer driving habits and/or unusual traffic conditions (i.e. stop and go expressway traffic).

Fig. 37: GMC Suburban K2500 1992 - Component Locations - Brake Signal Circuit Check, Schematic

Fig. 38: GMC Suburban K2500 1992 - Component Locations - Brake Signal Circuit Check, Flow Chart

GMC Suburban K2500 1992 - TORQUE CONVERTER CLUTCH (TCC) CIRCUIT CHECK

The automatic transmission TCC eliminates power loss of torque converter stage when vehicle is in a cruise condition. This allows the convenience of the automatic transmission, and fuel economy of a manual transmission. Fused battery ignition is supplied to the TCC solenoid, located inside valve body, to shift a spool valve in order to modulate pressure to the TCC. The PCM/TCM will engage TCC by grounding circuit No. 422 to energize the solenoid.

TCC will engage under the following conditions.

• Vehicle speed is greater than 30 MPH (48 km/h).
• Engine at normal operating temperature greater than 149?F (65?C).
• Transmission at normal operating temperature of 195?F (91?C).
• Throttle position sensor output is not changing, indicating a steady road speed.
• Brake switch is closed.
• 4th gear is indicated.
• No codes are stored.

  NOTE:

  Test numbers refer to test numbers on diagnostic chart.

1. This step checks for a shorted internal transmission harness or TCC solenoid.
2. This step verifies power supply to the TCC solenoid.
3. This step checks circuit No. 422 for a short to ground.
4. This step checks PCM/TCM's ability to control the TCC solenoid. If bidirectional scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

GMC Suburban K2500 1992 - Diagnostic Aids

Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article. The TCC solenoid is pulse width modulated, and is designed to keep the TCC right at the point of engagement. Therefore, some slight slippage is normal.

Fig. 39: GMC Suburban K2500 1992 - Component Locations - Torque Converter Clutch Circuit, Schematic

Fig. 40: GMC Suburban K2500 1992 - Component Locations - Torque Converter Clutch Circuit, Flow Chart

GMC Suburban K2500 1992 - SHIFT SOLENOID "A" CIRCUIT CHECK

Shift solenoid "A" is attached to the valve body and is a normally open exhaust valve. The PCM/TCM activates the solenoid by grounding it through an internal quad driver. Solenoid "A" (Blue) is on in 1st and 4th gears, but is off in 2nd and 3rd gears. When solenoid is on, it redirects fluid to act on the shift valves.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks shift solenoid "A" and the internal transmission wiring harness for shorts.
2. This test checks for power, from the ignition switch through the fuse, to the shift solenoid.
3. This test checks circuit No. 1222 for a short to ground.
4. This test checks PCM/TCM's ability to ground or control the shift solenoid "A". If bidirectional scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 41: GMC Suburban K2500 1992 - Component Locations - Shift Solenoid "A" Circuit Check, Schematic

Fig. 42: GMC Suburban K2500 1992 - Component Locations - Shift Solenoid "A" Circuit Check, Flow Chart

GMC Suburban K2500 1992 - SHIFT SOLENOID "B" CIRCUIT CHECK

The shift solenoids are used inside the valve body to control the spool valves, which determine the transmission gear. Fused ignition is supplied to solenoid "B". The PCM/TCM will engage solenoid "B" by grounding circuit No. 1223 to energize the solenoid.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks shift solenoid "B" and the internal transmission wiring for shorts.
2. This test checks for power, from the ignition through the transmission fuse, to the shift solenoid "B".
3. This test checks PCM/TCM's ability to ground or control the shift solenoid "B".
4. This test ensures circuit No. 1223 is not shorted to ground.
5. This test checks PCM/TCM's ability to ground or control the shift solenoid "B". If bidirectional scan tester is not available, TCC may be activated by grounding ALDL test terminal "B" with ignition on and engine off.
6. This test ensures circuit No. 1223 is not shorted to ground.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 43: GMC Suburban K2500 1992 - Component Locations - Shift Solenoid "B" Circuit Check, Schematic

Fig. 44: GMC Suburban K2500 1992 - Component Locations - Shift Solenoid "B" Circuit Check, Flow Chart

GMC Suburban K2500 1992 - FORCE MOTOR CIRCUIT CHECK

NOTE:

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

The force motor is a PCM/TCM-controlled device used to regulate transmission line pressure. The PCM/TCM looks at TPS voltage, engine RPM, and other inputs to determine the appropriate line pressure for a given load, then regulates the pressure by applying a variable amperage to the force motor. The applied amperage varies from .1-1.1 amps.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Checks the PCM and TCM's ability to command the force motor.
2. Checks for voltage at PCM/TCM.
3. Checks internal transmission harness and force motor for low resistance.
4. Checks for short to ground.
5. Checks internal transmission harness and force motor for high resistance.

GMC Suburban K2500 1992 - Diagnostic Aids

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

Fig. 45: GMC Suburban K2500 1992 - Component Locations - Force Motor Circuit Check, Schematic

Fig. 46: GMC Suburban K2500 1992 - Component Locations - Force Motor Circuit Check, Flow Chart