GMC Forward Control P3500 1993 - 1993 ENGINE PERFORMANCE Self-Diagnostics - Electronic Transmission

GMC Forward Control P3500 1993 - INTRODUCTION

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

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

GMC Forward Control P3500 1993 TRANSMISSION APPLICATIONS

Application	Transmission
Astro & Safari	4L60-E
Commercial Van	4L80-E
Sierra, Suburban, Yukon, Van & "C" & "K" Series Blazer & Pickup	4L60-E & 4L80-E

GMC Forward Control P3500 1993 - 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 appropriate TESTS W/CODES article in this sections.

Diesel vehicles with 4L80-E transmission 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. Malfunctions are recorded as either hard failures or intermittent failures.

GMC Forward Control P3500 1993 - 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 Forward Control P3500 1993 - 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 in this section. On diesel vehicles, DIAGNOSTIC CIRCUIT CHECK is covered in G - DIESEL TESTS W/ CODES article in this section. The diagnostic circuit check determines if computer-controlled system is working properly. If trouble codes are displayed (other than Code 12), determine if codes are hard or intermittent trouble codes.
3. A hard code is one which is present while working on vehicle, and problem still exists. Hard codes will cause the 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 in this section.

GMC Forward Control P3500 1993 - ENTERING & EXITING DIAGNOSTIC MODE

NOTE:

The Assembly Line Data Link (ALDL) may also be referred to as the Data Link Connector (DLC) in flow charts.

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

Fig. 1: GMC Forward Control P3500 1993 - Component Locations - Identifying ALDL Connector Terminals

GMC Forward Control P3500 1993 - TROUBLE CODE IDENTIFICATION

NOTE:

Trouble codes retrieved from PCM/TCM may be related to either engine or transmission. Transmission-related codes are covered in this article. For engine-related codes, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section. See 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 module. Code 51 indicates PROM/MEM-CAL is installed improperly or has failed.
2. Code 55 indicates PCM/TCM has failed (except on 3.8L) 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, relay, or harness may be the cause of PCM/TCM failure. Use Short Circuit Tester (J-34636) to check for short circuits.

GMC Forward Control P3500 1993 TROUBLE CODE IDENTIFICATION

Code	Probable Cause
12	(1) No Engine Speed Sensor Reference Pulse To PCM/TCM
13	(1) Open Oxygen Sensor Circuit
14	(1) CTS Voltage Low (Sensor Or Signal Line Grounded)
15	(1) CTS Voltage High (Sensor Or Signal Line Open)
16	(1) System Voltage High/Low (3.8L) (2) Transmission Speed Error
17	(1) RPM Signal Problem (3.8L)
18	(1) Cam/Crank Error (3.8L)
21	(1) TPS Voltage High (Open Circuit Or Misadjusted TPS)
22	(1) TPS Voltage Low (Circuit Grounded)
23	(1) Intake Air Temperature Sensor Voltage High
24	(3) Vehicle Speed Sensor Circuit Open Or Grounded
25	(1) Intake Air Temperature Sensor Voltage Low
26	(1) Quad-Driver "A" Fault (3.8L) Quad-Driver 1 Fault (4.3L M/T "S" & "T" Pickup)
27	(1) Quad-Driver 2 Fault (4.3L M/T "S" & "T" Pickup)
28	(4) Pressure Switch Manifold Fault
31	(1) Park/Neutral Switch Circuit (3.8L)
31	(1) Turbocharger Wastegate Overboost (4.3L Turbo)
31	(1) Engine Speed Control Governor (5.7L "G" Van)
31	(5) MAP Voltage Low (Diesel)
31	(1) Turbocharger Wastegate Overboost (4.3L Turbo)
32	(1) EGR Error (Improper Vacuum Signal)
33	(1) MAP Voltage High (Circuit Open Or Short To Voltage)
34	(1) MAP Voltage Low (Circuit Open Or Short To Ground)
34	(1) MAF Sensor Signal Voltage Low (3.8L)
35	(1) IAC System Fault
36	(1) Shift Control Problem (3.8L)
37	(4) Brake Switch Stuck On (4L60-E)
38	(1) Brake Switch Circuit (3.8L) Knock Sensor Circuit Open (4.3L M/T "S" & "T" Pickup) (4) Brake Switch Stuck Off (4L60-E)
39	TCC Stuck Off ((1) 3.8L & (4) 4L80-E)
39	(1) Knock Sensor Circuit Shorted (4.3L M/T "S" & "T" Pickup)
41	(1) Cam Sensor Circuit (3.8L)
42	(1) EST Circuit Fault
43	(1) ESC Fault
44	(1) Lean Exhaust Indicated
45	(1) Rich Exhaust Indicated
51	(1) Improperly Installed/Faulty PROM/MEM-CAL
52	(4) Long System Voltage High (4L60-E) (1) Fuel CALPAC Missing
53	(3) System Voltage High (Charging System Problem)
53, 54 & 55	(1) EGR Fault (3.8L)
54	(1) Fuel Pump Circuit Voltage Low
55	(1) Faulty ECM/PCM
56	(1) Quad-Driver "B" Fault (3.8L)
58	(4) TTS High Temperature (Sensor Or Signal Line Grounded)
59	(4) TTS Low Temperature (Sensor, Connections Or Wires Open)
61	(1) Cruise Vent Solenoid (3.8L)
62	(1) Cruise Vacuum Solenoid (3.8L)
63	(1) Cruise System Problem (3.8L)
65	(1) Cruise Servo Position (3.8L)
66	(1) Low A/C Refrigerant Charge (3.8L)
66	(4) 3-2 Control Solenoid Fault (4L60-E)
67	(4) TCC Solenoid Circuit Fault (4L60-E) (1) Cruise Engage Switches (3.8L)
68	(4) Overdrive Ratio Error
68	(Engine RPM Greater Than Input Speed)
68	(1) Cruise System Problem (3.8L)
69	(4) Torque Converter Stuck On (4L60-E) (1) A/C Head Pressure Switch (3.8L)
72	(4) VSS Signal Loss (4L60-E)
73	(4) Force Motor (Pressure Control Solenoid) Error
75	(4) System Voltage Low (Charging System Problem)
79	(4) Transmission Fluid Temperature High (4L60-E)
81	(4) Solenoid "B" (2nd-3rd) Circuit Fault
82	(4) Solenoid "A" (1st-2nd) Circuit Fault
83	(4) TCC Solenoid Circuit Fault (4L80-E)
85	(4) Undefined Gear Ratio (4L80-E)
86	(4) Low Gear Ratio (4L80-E)
87	(4) High Gear Ratio (4L80-E)
(1) Gasoline engine code. See appropriate TESTS W/CODES article in this section. (2) Models equipped with 4L60-E transmission, and 4.3L "S" and "T" series utility vehicles with man. trans. (3) Common gasoline engine and transmission code. See appropriate TESTS W/CODES article in this section. (4) Transmission code. Covered in this article. (5) Diesel engine code. For diesel code diagnosis, see G - DIESEL TESTS W/ CODES article in this section.

GMC Forward Control P3500 1993 - 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, disconnect PCM/TCM power feed for 30 seconds.

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

GMC Forward Control P3500 1993 - PCM/TCM LOCATION

The PCM/TCM is located under the driver's seat riser ("G" Series Van and "P" Series Commercial Van) or behind right side of dash (all other models). On "G" Series Van Motorhome, PCM/TCM has been relocated to behind left side "B" pillar.

GMC Forward Control P3500 1993 - SPECIAL TOOLS (DIAGNOSTIC)

Special scan testers are plugged into the ALDL to read trouble codes and check system voltages on serial data line. These testers can save a great deal of time. For additional information, see manual included with tester.

GMC Forward Control P3500 1993 - 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 Forward Control P3500 1993 - 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 Forward Control P3500 1993 - TROUBLE CODE CHARTS (4L60-E) CODE 28, FLUID PRESSURE SWITCH ASSEMBLY FAULT

The fluid pressure switch assembly consists of a fluid temperature sensor and 5 pressure switches combined into one unit and mounted on the transmission valve body. The PCM supplies battery voltage to the fluid pressure switch on 3 separate wires. By grounding one or more of these circuits through various combinations of the pressure switches inside the fluid pressure switch assembly, the PCM detects what gear range has been selected by the vehicle operator while idling.

With engine off, PARK/NEUTRAL will be indicated. If range signals "A" and "C" are zero volts, or all 3 signals are zero volts for 1.5 seconds on diesel engine or 2 seconds on gasoline engine, Code 28 will set. When Code 28 is set, harsh shifts, no TCC, and possibly no 4th gear (if in hot mode) will occur.

NOTE:

Test numbers refer to 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 to the transmission pass-thru connector.
3. This test will detect a short to ground in any one of the 3 fluid pressure switch assembly range circuits.

GMC Forward Control P3500 1993 - Diagnostic Aids

Code 28 will set if PCM detects one of 2 "illegal" fluid pressure switch assembly combinations. See VALID FLUID PRESSURE SWITCH ASSEMBLY COMBINATION table below for various combinations. Check pass-thru connector for good contact.

GMC Forward Control P3500 1993 VALID FLUID PRESSURE SWITCH ASSEMBLY COMBINATION

Value	A	B	C
Park	Off	On	Off
Reverse	On	On	Off
Neutral	OFF	ON	OFF
4th	Off	On	On
3rd	Off	Off	On
2nd	Off	Off	Off
1st	On	Off	Off
Illegal	On	Off	On
Illegal	On	On	On

Fig. 2: GMC Forward Control P3500 1993 - Component Locations - CODE 28, Schematic, FPS Assembly Fault (Astro, Safari & C & K Series)

Fig. 3: GMC Forward Control P3500 1993 - Component Locations - CODE 28, Schematic, FPS Assembly Fault ("G" Series)

Fig. 4: GMC Forward Control P3500 1993 - Component Locations - CODE 28, Flowchart, Fluid Pressure Switch Assembly Fault

GMC Forward Control P3500 1993 - CODE 37, BRAKE SWITCH STUCK ON

The normally closed brake switch supplies a 12-volt signal on circuit No. 420 to the PCM. The signal voltage is removed when brakes are applied. An incorrect brake signal may affect TCC and 4th gear operation in hot mode.

Code 37 will set when circuit No. 420 is open, vehicle speed less than 5 MPH for 6 seconds, then between 5 MPH and 20 MPH for 6 seconds, then greater than 20 MPH for 6 seconds, for a complete total of 7 times.

NOTE:

Test numbers refer to numbers on diagnostic chart. Display may vary depending upon manufacturer of scan tester. Display may read APPLIED, RELEASED, ON or OFF.

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.
4. This test opens circuit No. 420 and simulates brakes being applied.

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 5: GMC Forward Control P3500 1993 - Component Locations - CODE 37, Schematic, Brake Switch Stuck On

Fig. 6: GMC Forward Control P3500 1993 - Component Locations - CODE 37, Flowchart, Brake Switch Stuck On

GMC Forward Control P3500 1993 - CODE 38, BRAKE SWITCH STUCK OFF

The normally closed brake switch supplies a 12-volt signal on circuit No. 420 to the PCM. The signal voltage is removed when brakes are applied. An incorrect brake signal may affect TCC and 4th gear operation.

Code 38 will set when circuit No. 420 has voltage, vehicle speed is greater than 20 MPH for 6 seconds, then between 5 MPH and 20 MPH for 6 seconds, for a complete total of 7 times.

NOTE:

Test numbers refer to numbers on diagnostic chart. Display may vary depending upon manufacturer of scan tester. Display may read APPLIED, RELEASED, ON or OFF.

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.
4. This test opens circuit No. 420 and simulates brakes being applied.

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 7: GMC Forward Control P3500 1993 - Component Locations - CODE 38, Schematic, Brake Switch Stuck Off

Fig. 8: GMC Forward Control P3500 1993 - Component Locations - CODE 38, Flowchart, Brake Switch Stuck Off

GMC Forward Control P3500 1993 - CODE 52, LONG SYSTEM VOLTAGE HIGH

Code 52 will set when ignition is on and PCM battery feed terminal voltage is greater than 16 volts for 109 minutes. During the time the failure is present, the pressure control solenoid is turned off, transmission immediately shifts to 3rd gear, and TCC operation is inhibited. The setting of additional diagnostic trouble codes may result.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Normal battery voltage is 9-15 volts.
2. This test checks if high voltage reading is due to alternator, circuit No. 440 or PCM. With engine running, check voltage at battery. If voltage is greater than 15 volts, PCM is okay.
3. This test checks if alternator is faulty under load condition. If voltage is greater than 15 volts, check alternator.

GMC Forward Control P3500 1993 - Diagnostic Aids

Circuit No. 440 supplies battery voltage to the PCM. Jump-starting engine or 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. 9: GMC Forward Control P3500 1993 - Component Locations - CODE 52, Schematic, Long Sys. Voltage Hi (Astro, Safari & C & K Series)

Fig. 10: GMC Forward Control P3500 1993 - Component Locations - CODE 52, Schematic, Long Sys. Voltage Hi ("G" Series)

Fig. 11: GMC Forward Control P3500 1993 - Component Locations - CODE 52, Flowchart, Long System Voltage High

GMC Forward Control P3500 1993 - CODE 53, SYSTEM VOLTAGE HIGH

Code 53 will set when ignition is on and PCM battery feed terminal voltage is greater than 19.5 volts for 2 seconds. During the time the failure is present, the pressure control solenoid is turned off, transmission immediately shifts to 3rd gear, and TCC operation is inhibited. The setting of additional diagnostic trouble codes may result.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Normal battery voltage is 9-15 volts.
2. This test checks if the high voltage reading is due to alternator, circuit No. 440 or PCM. With engine running, check voltage at battery. If voltage is greater than 15 volts, PCM is okay.
3. This test checks if alternator is faulty under load condition. If voltage is greater than 15 volts, check alternator.

GMC Forward Control P3500 1993 - Diagnostic Aids

Circuit No. 440 supplies battery voltage to the PCM. Jump-starting engine or 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. 12: GMC Forward Control P3500 1993 - Component Locations - CODE 53, Schematic, Sys. Voltage Hi (Astro, Safari & C & K Series)

Fig. 13: GMC Forward Control P3500 1993 - Component Locations - CODE 53, Schematic, System Voltage High ("G" Series)

Fig. 14: GMC Forward Control P3500 1993 - Component Locations - CODE 53, Flowchart, System Voltage High

GMC Forward Control P3500 1993 - CODE 58, TRANS FLUID TEMP. SENS CKT (HIGH TEMP. INDICATED)

The transmission fluid temperature sensor, located within the fluid pressure switch assembly, is a thermistor which controls the signal voltage to the PCM. The PCM applies and monitors voltage on circuit No. 1227 to the sensor. When transmission fluid is cold, sensor resistance is high; therefore, the PCM will see high signal voltage.

As the 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. If Code 79 is also present, check the transmission cooling system. When Code 58 is set, transmission will use a warm value for operation, but scan tester will display the actual fluid temperature.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission fluid 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 Forward Control P3500 1993 - Diagnostic Aids

Check harness routing for a potential short to ground in circuit No. 1227. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, the temperature display should rise steadily to about 100?C then stabilize. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts, or TCC enabled complaint. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

Fig. 15: GMC Forward Control P3500 1993 - Component Locations - CODE 58, Schematic, Trans. Fluid Temp. Sensor - Hi Temp. Indicated (Astro, Safari & "C" & "K" Series)

Fig. 16: GMC Forward Control P3500 1993 - Component Locations - CODE 58, Schematic, Trans. Fluid Temp. Sensor - Hi Temp. Indicated ("G" Series)

Fig. 17: GMC Forward Control P3500 1993 - Component Locations - CODE 58, Flowchart, Trans. Fluid Temp. Sensor - Hi Temp. Indicated

GMC Forward Control P3500 1993 - CODE 59, TRANSMISSION FLUID TEMP. SENSOR CIRCUIT (LOW TEMP. INDICATED)

The transmission fluid temperature sensor, located within the fluid pressure switch assembly, is a thermistor which controls the signal voltage to the PCM. The PCM applies and monitors 5 volts to the sensor on circuit No. 1227. When transmission fluid is cold, sensor resistance is high; therefore, the PCM will see high signal voltage. As the 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. When Code 59 is set, transmission will use a warm value for operation, but scan tester will display the actual fluid temperature.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 59 will set if signal voltage indicates a transmission fluid temperature less than -27?F (-33?C) for one second.
2. This test simulates Code 58. If PCM recognizes the low signal voltage (high temperature) and scan tester reads 309?F (154?C) or greater, the PCM 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 18: GMC Forward Control P3500 1993 - Component Locations - CODE 59, Schematic, Trans. Fluid Temp. Sensor - Lo Temp Indicated (Astro, Safari & "C" & "K" Series)

Fig. 19: GMC Forward Control P3500 1993 - Component Locations - CODE 59, Schematic, Trans. Fluid Temp. Sensor - Lo Temp. Indicated("G" Series)

Fig. 20: GMC Forward Control P3500 1993 - Component Locations - CODE 59, Flowchart, Trans. Fluid Temp. Sensor - Lo Temp. Indicated

GMC Forward Control P3500 1993 - CODE 66, 3-2 CONTROL SOLENOID CIRCUIT FAULT

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

For example, if the PCM commands the solenoid on, the duty cycle on that circuit should drop when the solenoid is grounded. If voltage stays up for 4 seconds, Code 66 will set. If the 3-2 control solenoid is off, the duty cycle on the circuit should remain high. If voltage drops for 4 seconds, Code 66 will set. When code is set, a soft landing to 3rd gear will occur.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks 3-2 control solenoid and internal transmission harness for short circuits.
2. This test checks for power, from the ignition through the fuse, to the 3-2 control solenoid.
3. This test ensures circuit No. 897 is not shorted to ground.
4. This test checks ability of PCM to ground or control 3-2 control solenoid. If bidirectional scan tester is not available, solenoid may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

GMC Forward Control P3500 1993 - 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 in this section. Some slight TCC slippage is normal. 3-2 control solenoid feedback normally oscillates on/off when the duty cycle is applied.

Fig. 21: GMC Forward Control P3500 1993 - Component Locations - CODE 66, Schematic, 3-2 Control Solenoid Circuit Fault

Fig. 22: GMC Forward Control P3500 1993 - Component Locations - CODE 66, Flowchart, 3-2 Control Solenoid Circuit Fault

GMC Forward Control P3500 1993 - CODE 67, TCC SOLENOID CIRCUIT CHECK

The TCC solenoid is a normally open exhaust valve. The PCM will engage the solenoid by grounding circuit No. 422 with an internal quad-driver. The PCM continually monitors voltage on TCC circuit for either low or high voltage, depending on the commanded state of the circuit. Code 67 will set if a fault is detected.

For example, If the PCM commands the TCC solenoid on, the voltage on that circuit should drop when the solenoid is grounded. If voltage stays up for 2 seconds, Code 67 will set. If solenoid is off, the voltage on that circuit should remain higher. If voltage drops for 2 seconds, Code 67 will set. When code is set, no TCC will occur and no 4th gear in hot mode.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks TCC solenoid and the internal transmission harness for short circuits.
2. This test checks for power, from the ignition through the fuse, to the TCC solenoid.
3. This test ensures circuit No. 422 is not shorted to ground.
4. This test checks ability of PCM to ground or control TCC solenoid.

GMC Forward Control P3500 1993 - 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 in this section. Some slight TCC slippage is normal.

Fig. 23: GMC Forward Control P3500 1993 - Component Locations - CODE 67, Schematic, TCC Solenoid Circuit Check

Fig. 24: GMC Forward Control P3500 1993 - Component Locations - CODE 67, Flowchart, TCC Solenoid Circuit Check

GMC Forward Control P3500 1993 - CODE 69, TORQUE CONVERTER CLUTCH STUCK ON

The TCC solenoid is a normally open exhaust valve. The PCM will engage the solenoid by grounding circuit No. 422 with an internal quad-driver. The PCM continually monitors voltage on TCC circuit for either low or high voltage, depending on the commanded state of the circuit. Code 69 will set under the following conditions.

• TCC slip is greater than -20, but less than 20.
• Without TCC locked.
• TPS is more than 25 percent.
• Conditions are met for 4 seconds.

When code is set, TCC is on in all gears.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks TCC solenoid and the internal transmission harness for short circuits.
2. This test checks for power, from the ignition through the fuse, to the TCC solenoid.
3. This test ensures circuit No. 422 is not shorted to ground.
4. This test checks ability of PCM to ground or control TCC solenoid.

GMC Forward Control P3500 1993 - Diagnostic Aids

Check for stall with parking brake applied and in 2nd gear. Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section. Some slight TCC slippage is normal. If Code 69 is set and Code 67 is not set, problem could be internal to the transmission.

Fig. 25: GMC Forward Control P3500 1993 - Component Locations - CODE 69, Schematic, TCC Stuck On

Fig. 26: GMC Forward Control P3500 1993 - Component Locations - CODE 69, Flowchart, TCC Stuck On

GMC Forward Control P3500 1993 - CODE 72, VEHICLE SPEED SENSOR LOSS (2WD, AWD & 4WD) 2WD & AWD

The Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction-type sensor, vehicle speed sensor buffer module 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 the buffer module. The buffer module compensates for various axle ratios, and converts the signal to a square wave for use by the speedometer, cruise control, anti-lock brake system and PCM.

The buffer module sends 2 different signals to the PCM. Circuit No. 437 relays the transmission output speed which is used to control shift points, line pressure, TCC, Code 24 and Code 72. Circuit No. 1716 relays the vehicle speed which is used to control engine operating functions and Code 16.

Code 72 will set under the following conditions.

• When in gear other than Park/Neutral, transmission output speed changes greater than 1000 RPM.
• When in Park/Neutral, transmission output speed changes greater than 2050 RPM.
• Conditions are met for 2 seconds.

GMC Forward Control P3500 1993 - 4WD

The Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction-type sensor, vehicle speed sensor buffer module, 4WD low switch 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 the buffer module. The buffer module compensates for various axle ratios, and converts the signal to a square wave for use by the speedometer, cruise control, anti-lock brake system and PCM.

The buffer module sends 2 different signals to the PCM. Circuit No. 437 relays the transfer case output speed to the PCM. PCM uses both the transfer case output speed signal and the 4WD low switch to determine transmission output shaft speed. PCM uses transmission output shaft speed to control shift points, line pressure, TCC, Code 24 and Code 72. Circuit No. 1716 relays the vehicle speed which is used to control engine operating functions and Code 16.

Code 72 will set under the following conditions.

• When in gear other than Park/Neutral, transmission output speed changes greater than 1000 RPM.
• When in Park/Neutral, transmission output speed changes greater than 2050 RPM.
• Conditions are met for 2 seconds.

  NOTE:

  Test numbers refer to numbers on diagnostic chart.

1. This test checks the VSS signal to the PCM.
2. This test checks the VSS signal to the buffer module.
3. This test checks the VSS signal.

GMC Forward Control P3500 1993 - 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 in this section.

Fig. 27: GMC Forward Control P3500 1993 - Component Locations - CODE 72, Schematic, Vehicle Speed Sensor Loss (2WD & AWD)

Fig. 28: GMC Forward Control P3500 1993 - Component Locations - CODE 72, Schematic, Vehicle Speed Sensor Loss (4WD)

Fig. 29: GMC Forward Control P3500 1993 - Component Locations - CODE 72, Flowchart, Vehicle Speed Sensor Loss

GMC Forward Control P3500 1993 - CODE 73, PRESSURE CONTROL SOLENOID (CURRENT ERROR)

The pressure control solenoid is a PCM-controlled device used to regulate transmission line pressure. The PCM 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 pressure control solenoid. Applied amperage varies from .1 to 1.1 amps.

The PCM then monitors the amperage at the return line. If the return amperage varies greater than .16 amp from the commanded amperage for at least one second, Code 73 will set. When Code 73 sets, the full line pressure will be applied, causing harsh shifts until the next ignition cycle. The code will remain stored in memory, and the pressure control solenoid will resume normal operation until conditions for the code reoccur.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks ability of PCM to command pressure control solenoid.
2. This test checks for voltage at PCM.
3. This test checks internal transmission harness and pressure control solenoid for low resistance.
4. This test checks internal transmission harness and pressure control solenoid for high resistance.

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 30: GMC Forward Control P3500 1993 - Component Locations - CODE 73, Schematic, Pressure Control Solenoid (Current Error)

Fig. 31: GMC Forward Control P3500 1993 - Component Locations - CODE 73, Flowchart, Pressure Control Solenoid (Current Error)

GMC Forward Control P3500 1993 - CODE 75, SYSTEM VOLTAGE LOW

Code 75 will set when ignition is on and PCM terminal E16 voltage is less than temperature to voltage scale for about 4 seconds. During the time the failure is present, the pressure control solenoid is turned off, maintaining only 3rd gear, and inhibiting TCC operation.

Minimum voltage allowed for Code 75 to set is on a graduated scale and changes with temperature. Minimum voltage at -40?F (-40?C) is 7.3 volts. Minimum voltage at 302?F (150?C) is 11.7 volts, and minimum voltage at 194?F (90?C) is 10.3 volts.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Normal battery voltage is 9-15 volts.
2. This test checks if low voltage reading is due to alternator, circuit No. 440 or PCM. With engine running, check voltage at the battery. If voltage is less than 8.6 volts, PCM is okay.

GMC Forward Control P3500 1993 - Diagnostic Aids

Circuit No. 440 supplies battery voltage to PCM. 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.

Fig. 32: GMC Forward Control P3500 1993 - Component Locations - CODE 75, Schematic, Sys. Voltage Lo (Astro, Safari & C & K Series)

Fig. 33: GMC Forward Control P3500 1993 - Component Locations - CODE 75, Schematic, Sys. Voltage Lo ("G" Series)

Fig. 34: GMC Forward Control P3500 1993 - Component Locations - CODE 75, Flowchart, System Voltage Low

GMC Forward Control P3500 1993 - CODE 79, TRANSMISSION FLUID OVERTEMP

The transmission fluid temperature sensor, located within the fluid pressure switch assembly, is a thermistor which controls the signal voltage to the PCM. The PCM applies and monitors voltage on circuit No. 1227 to the sensor. When transmission fluid is cold, sensor resistance is high; therefore, the PCM will see high signal voltage.

As the 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. If fluid temperature becomes greater than 304?F (150?C) but less than 309?F (154?C) for 15 minutes, Code 79 will set. If fluid temperature becomes greater than 309?F (154?C), Code 58 will also set.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission fluid temperature greater than 309?F (154?C). 2) This test simulates a Code 59.

GMC Forward Control P3500 1993 - Diagnostic Aids

Check harness routing for a potential short to ground in circuit No. 1227. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, the temperature display should rise steadily to about 100?C then stabilize. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

Fig. 35: GMC Forward Control P3500 1993 - Component Locations - CODE 79, Schematic, Trans. Fluid Overtemp (Astro, Safari & "C" & "K" Series)

Fig. 36: GMC Forward Control P3500 1993 - Component Locations - CODE 79, Schematic, Trans. Fluid Overtemp ("G" Series)

Fig. 37: GMC Forward Control P3500 1993 - Component Locations - CODE 79, Flowchart, Transmission Fluid Overtemp

GMC Forward Control P3500 1993 - CODE 81, 2-3 SHIFT SOLENOID CIRCUIT FAULT

The PCM 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 2-3 shift solenoid circuit.

If 2-3 shift solenoid is commanded on by the PCM, voltage on that circuit should drop when solenoid is grounded. If 2-3 shift solenoid is off, voltage on the circuit should remain high. If solenoid is shorted off, 3rd gear only will occur. If solenoid is shorted on, 2nd gear only will occur. If voltage stays up for at least 2 seconds, Code 81 will set. If voltage drops for more than 2 seconds, Code 81 will set.

NOTE:

Test numbers refer to numbers on diagnostic chart.

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

GMC Forward Control P3500 1993 - 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 in this section. An open in the circuit can cause multiple codes to set.

Fig. 38: GMC Forward Control P3500 1993 - Component Locations - CODE 81, Schematic, 2-3 Shift Solenoid Circuit Fault

Fig. 39: GMC Forward Control P3500 1993 - Component Locations - CODE 81, Flowchart, 2-3 Shift Solenoid Circuit Fault

GMC Forward Control P3500 1993 SHIFT SOLENOID COMBINATION

Gear	1-2 Shift Solenoid	2-3 Shift Solenoid
1st	On	On
2nd	Off	On
3rd	Off	Off
4th	On	Off

GMC Forward Control P3500 1993 - CODE 82, 1-2 SHIFT SOLENOID CIRCUIT FAULT

The PCM 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 82 will set if a fault has been detected on the 1-2 shift solenoid circuit.

If 1-2 shift solenoid is commanded on by the PCM, voltage on that circuit should drop when solenoid is grounded. If Code 82 sets, the transmission will allow 2nd and 3rd gears only, or 1st and 4th gears only. If voltage stays up for at least 2 seconds after 1-2 shift solenoid is commanded on, Code 82 will set. If 1-2 shift solenoid is off, voltage on the circuit should be high. If voltage drops for more than 2 seconds, Code 82 will set.

NOTE:

Test numbers refer to numbers on diagnostic chart.

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

GMC Forward Control P3500 1993 - 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 in this section.

Fig. 40: GMC Forward Control P3500 1993 - Component Locations - CODE 82, Schematic, 1-2 Shift Solenoid Circuit Fault

Fig. 41: GMC Forward Control P3500 1993 - Component Locations - CODE 82, Flowchart, 1-2 Shift Solenoid Circuit Fault

GMC Forward Control P3500 1993 SHIFT SOLENOID COMBINATION

Gear	1-2 Shift Solenoid	2-3 Shift Solenoid
1st	On	On
2nd	off	On
3rd	Off	Off
4th	On	Off

GMC Forward Control P3500 1993 - COMPONENT CHECK CHARTS (4L60-E) VEHICLE SPEED SENSOR CHECK (2WD & AWD)

The Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction-type sensor, vehicle speed sensor buffer module and wiring. Gear teeth, pressed on the outside diameter of output carrier assembly, induce an alternating current in the sensor. This current is transmitted to the buffer module. Buffer module compensates for various axle ratios, and converts signal to a square wave for use by speedometer, cruise control, anti-lock brake system and PCM. 5 Buffer module sends 2 different signals to PCM. Circuit No. 437 relays transmission output speed, which is used to control shift points, line pressure, TCC, Code 24 and Code 72. Circuit No. 1716 relays vehicle speed, which is used to control engine operating functions and Code 16.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks the VSS signal to the PCM.
2. This test checks the VSS signal to the buffer module.
3. This test checks the VSS signal.

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 42: GMC Forward Control P3500 1993 - Component Locations - Schematic, Vehicle Speed Sensor

Fig. 43: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Vehicle Speed Sensor

GMC Forward Control P3500 1993 - VEHICLE SPEED SENSOR CHECK (4WD)

The Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction-type sensor, vehicle speed sensor buffer module, 4WD low switch and wiring. Gear teeth, pressed on outside diameter of output carrier assembly, induce an alternating current in the sensor. This current is transmitted to the buffer module. Buffer module compensates for various axle ratios, and converts signal to a square wave for use by speedometer, cruise control, anti-lock brake system and PCM.

Buffer module sends 2 different signals to PCM. Circuit No. 437 relays transfer case output speed to PCM. PCM uses transfer case output speed signal and 4WD low switch to determine transmission output shaft speed. PCM uses transmission output shaft speed to control shift points, line pressure, TCC, Code 24 and Code 72. Circuit No. 1716 relays vehicle speed, which is used to control engine operating functions and Code 16.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks VSS signal to PCM.
2. This test checks VSS signal to the buffer module.
3. This test checks VSS signal.

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 44: GMC Forward Control P3500 1993 - Component Locations - Schematic, Vehicle Speed Sensor - 4WD

Fig. 45: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Vehicle Speed Sensor - 4WD

GMC Forward Control P3500 1993 - BRAKE SIGNAL CIRCUIT CHECK

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

NOTE:

Test numbers refer to numbers on diagnostic chart. Display may vary depending upon manufacturer of scan tester. Display may read APPLIED, RELEASED, ON or OFF.

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.
4. This test opens circuit No. 420 and simulates condition of brakes being applied.

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 46: GMC Forward Control P3500 1993 - Component Locations - Schematic, Brake Signal

Fig. 47: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Brake Signal

GMC Forward Control P3500 1993 - TORQUE CONVERTER CLUTCH (TCC) SOLENOID CIRCUIT CHECK

TCC solenoid is a normally open exhaust valve. PCM engages solenoid by grounding circuit No. 422 with an internal quad-driver. PCM continually monitors voltage on TCC circuit for either low or high voltage, depending on commanded state of circuit. A code will set if a fault is detected.

For example, if PCM commands TCC solenoid on, voltage on that circuit should drop when solenoid is grounded. If voltage stays up for 2 seconds, a code will set. If solenoid is off, voltage on that circuit should remain high. If voltage drops for 2 seconds, a code will set.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test check TCC solenoid and internal transmission harness for short circuits.
2. This test checks for power, from the ignition through the fuse, to the TCC solenoid.
3. This test ensures circuit No. 422 is not shorted to ground.
4. This test checks ability of PCM to ground or control TCC solenoid.

GMC Forward Control P3500 1993 - 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 in this section. Some slight TCC slippage is normal.

Fig. 48: GMC Forward Control P3500 1993 - Component Locations - Schematic, TCC Solenoid

Fig. 49: GMC Forward Control P3500 1993 - Component Locations - Flowchart, TCC Solenoid

GMC Forward Control P3500 1993 - 3-2 CONTROL SOLENOID CIRCUIT CHECK

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

For example, if PCM commands solenoid on, duty cycle on that circuit should drop when solenoid is grounded. If voltage stays up for 4 seconds, a code will set. If 3-2 control solenoid is off, duty cycle on circuit should remain high. If voltage drops for 4 seconds, a code will set. When code is set, a soft landing to 3rd gear will occur.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test check 3-2 control solenoid and internal transmission harness for short circuits.
2. This test checks for power, from the ignition through the fuse, to the 3-2 control solenoid.
3. This test ensures circuit No. 897 is not shorted to ground.
4. This test checks ability of PCM to ground or control 3-2 control solenoid. If bidirectional scan tester is not available, solenoid may be activated by grounding ALDL test terminal "B" with ignition on and engine off.

GMC Forward Control P3500 1993 - 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 in this section.

Fig. 50: GMC Forward Control P3500 1993 - Component Locations - Schematic, 3-2 Control Solenoid

Fig. 51: GMC Forward Control P3500 1993 - Component Locations - Flowchart, 3-2 Control Solenoid

GMC Forward Control P3500 1993 - 1-2 SHIFT SOLENOID CIRCUIT CHECK

The 1-2 shift solenoid is attached to valve body and is a normally open exhaust valve. PCM engages solenoid by grounding circuit No. 1222 with an internal quad-driver. The 1-2 shift solenoid is on in 1st and 4th gears, and off in 2nd and 3rd gears. When on, 1-2 shift solenoid redirects fluid to act on shift valves.

NOTE:

Test numbers refer to numbers on diagnostic chart.

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

GMC Forward Control P3500 1993 - 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 in this section.

Fig. 52: GMC Forward Control P3500 1993 - Component Locations - Schematic, 1-2 Shift Solenoid

Fig. 53: GMC Forward Control P3500 1993 - Component Locations - Flowchart, 1-2 Shift Solenoid

GMC Forward Control P3500 1993 SHIFT SOLENOID COMBINATION

Gear	1-2 Shift Solenoid	2-3 Shift Solenoid
1st	On	On
2nd	Off	On
3rd	Off	Off
4th	On	Off

GMC Forward Control P3500 1993 - 2-3 SHIFT SOLENOID CIRCUIT CHECK

Shift solenoids are used inside valve body to control spool valves and determine transmission gear. The 2-3 shift solenoid is on in 1st and 2nd gears, and off in 3rd and 4th gears. Fused battery ignition voltage is supplied to 2-3 shift solenoid. PCM engages solenoid by grounding circuit No. 1223.

NOTE:

Test numbers refer to numbers on diagnostic chart.

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

GMC Forward Control P3500 1993 - 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 in this section.

GMC Forward Control P3500 1993 SHIFT SOLENOID COMBINATION

Gear	1-2 Shift Solenoid	2-3 Shift Solenoid
1st	On	On
2nd	Off	On
3rd	Off	Off
4th	On	Off

Fig. 54: GMC Forward Control P3500 1993 - Component Locations - Schematic, 2-3 Shift Solenoid

Fig. 55: GMC Forward Control P3500 1993 - Component Locations - Schematic, 2-3 Shift Solenoid

GMC Forward Control P3500 1993 - PRESSURE CONTROL SOLENOID CIRCUIT CHECK

Pressure control solenoid is a PCM-controlled device used to regulate transmission line pressure. PCM looks at TPS voltage, engine RPM, and other inputs to determine appropriate line pressure for a given load, then regulates pressure by applying a variable amperage to pressure control solenoid. Applied amperage varies from .1 to 1.1 amps.

PCM then monitors amperage at return line. If return amperage varies greater than .16 amp from commanded amperage for at least one second, Code 73 will set. When Code 73 sets, full line pressure will be applied, causing harsh shifts until next ignition cycle. Code will remain stored in memory, and pressure control solenoid will resume normal operation until conditions for code reoccur.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks ability of PCM to command pressure control solenoid.
2. This test checks for voltage at PCM.
3. This test checks internal transmission harness and pressure control solenoid for low resistance.
4. This test checks internal transmission harness and pressure control solenoid for high resistance.

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 56: GMC Forward Control P3500 1993 - Component Locations - Schematic, Pressure Control Solenoid

Fig. 57: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Pressure Control Solenoid

GMC Forward Control P3500 1993 - FLUID PRESSURE SWITCH ASSEMBLY CIRCUIT CHECK

Fluid pressure switch assembly consists of a fluid temperature sensor and 5 pressure switches combined into one unit and mounted on the transmission valve body. The PCM supplies battery voltage to the fluid pressure switch on 3 separate wires. By grounding one or more of these circuits through various combinations of switches inside the fluid pressure switch assembly, the PCM detects what gear range has been selected by the vehicle operator.

If range signals "A" and "C" are zero volts, or all 3 signals are zero volts for 1.5 seconds on diesel engine or 2 seconds on gasoline engine, Code 28 will set. When Code 28 is set, harsh shifts, no TCC, and no 4th gear (if in hot mode) may occur.

NOTE:

Test numbers refer to 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 to the transmission pass-thru connector.
3. This test will detect a short to ground in any one of the 3 fluid pressure switch assembly range circuits.

GMC Forward Control P3500 1993 - Diagnostic Aids

Code 28 will set if PCM detects one of 2 "illegal" fluid pressure switch assembly combinations. See VALID FLUID PRESSURE SWITCH ASSEMBLY COMBINATION table below for various combinations. Check pass-thru connector for good contact.

Fig. 58: GMC Forward Control P3500 1993 - Component Locations - Schematic, Fluid Pressure Switch(Astro,Safari & C & K Series)

Fig. 59: GMC Forward Control P3500 1993 - Component Locations - Schematic, Fluid Pressure Switch ("G" Series)

Fig. 60: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Fluid Pressure Switch

GMC Forward Control P3500 1993 VALID FLUID PRESSURE SWITCH ASSEMBLY COMBINATION

Value	A	B	C
Park	Off	On	Off
Reverse	On	On	Off
Neutral	Off	On	Off
4th	Off	On	On
3rd	Off	Off	On
2nd	Off	Off	Off
1st	On	Off	Off
Illegal	On	Off	On
Illegal	On	On	On

GMC Forward Control P3500 1993 - TROUBLE CODE CHARTS (4L80E) CODE 28, PRESSURE SWITCH MANIFOLD FAULT

The Pressure Switch Manifold (PSM), also referred to as fluid pressure switch assembly, 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 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 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 61: GMC Forward Control P3500 1993 - Component Locations - CODE 28, Schematic, Pressure Switch Manifold Fault

Fig. 62: GMC Forward Control P3500 1993 - Component Locations - CODE 28, Flowchart, Pressure Switch Manifold Fault

GMC Forward Control P3500 1993 VALID PSM COMBINATION

Value	A	B	C
Park	Off	On	Off
Reverse	On	On	Off
Neutral	Off	On	Off
4th	Off	On	On
3rd	Off	Off	On
2nd	Off	Off	Off
1st	On	Off	Off
Illegal	On	Off	On
Illegal	On	On	On

GMC Forward Control P3500 1993 - 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 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 Forward Control P3500 1993 - 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. 63: GMC Forward Control P3500 1993 - Component Locations - CODE 39, Schematic, TCC Stuck Off ("C", "K" & "P" Series)

Fig. 64: GMC Forward Control P3500 1993 - Component Locations - CODE 39, Schematic, TCC Stuck Off ("G" Series)

Fig. 65: GMC Forward Control P3500 1993 - Component Locations - CODE 39, Flowchart, TCC Stuck Off

GMC Forward Control P3500 1993 - CODE 53, SYSTEM VOLTAGE HIGH

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

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Normal battery voltage is 9-15 volts.
2. This test checks if high voltage reading is due to the alternator, circuit No. 440 or PCM/TCM. With engine running, check voltage at battery. If voltage is greater than 15 volts, the PCM/TCM is okay.
3. This test checks if alternator is faulty under load condition. If voltage is greater than 15 volts, check alternator.

GMC Forward Control P3500 1993 - NOTES ON INTERMITTENTS

Jump-starting engine or 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. 66: GMC Forward Control P3500 1993 - Component Locations - CODE 53, Schematic, System Voltage High

Fig. 67: GMC Forward Control P3500 1993 - Component Locations - CODE 53, Flowchart, System Voltage High

GMC Forward Control P3500 1993 - CODE 58, TRANS FLUID TEMP. SENS CKT (HIGH TEMP. INDICATED)

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

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission fluid temperature greater than 305?F (151?C) for one second.
2. This test determines if circuit No. 1227 is shorted to ground, which will result in conditions for Code 58.

GMC Forward Control P3500 1993 - Diagnostic Aids

Check harness routing for a potential short to ground in circuit No. 1227. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, the temperature display should rise steadily to about 100?C then stabilize. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

Fig. 68: GMC Forward Control P3500 1993 - Component Locations - CODE 58, Schematic, Trans. Fluid Temp. Sensor - Hi Temp. Indicated

Fig. 69: GMC Forward Control P3500 1993 - Component Locations - CODE 58, Flowchart, Trans. Fluid Temp. Sensor - Hi Temp. Indicated

GMC Forward Control P3500 1993 - CODE 59, TRANSMISSION FLUID TEMP. SENSOR CIRCUIT (LOW TEMP. INDICATED)

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

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 59 will set if signal voltage indicates a transmission fluid temperature less than -34?F (-37?C) for one second.
2. This test simulates Code 58. If PCM/TCM recognizes low signal voltage (high temperature) and scan tester reads 305?F (151?C) or greater, PCM/TCM and wiring are okay.
3. This test determines if circuit No. 1227 is open. There should be 5 volts present at the sensor connector if measuring with a DVOM.

GMC Forward Control P3500 1993 - Diagnostic Aids

Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, the displayed temperature should rise steadily to about 100?C then stabilize. A faulty connection or an open in circuit No. 455/452 or circuit No. 1227 will result in a Code 59. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in firm shifts or TCC enabled complaint. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

Fig. 70: GMC Forward Control P3500 1993 - Component Locations - CODE 59, Schematic, Trans. Fluid Temp. Sensor - Low Temp. Indicated

Fig. 71: GMC Forward Control P3500 1993 - Component Locations - CODE 59, Flowchart, Trans. Fluid Temp. Sensor - Low Temp. Indicated

GMC Forward Control P3500 1993 - CODE 68, OVERDRIVE RATIO ERROR

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

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 68 will set under the following conditions: engine speed is 200 RPM greater than input speed, 4th gear is indicated, TCC is enabled, and 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 72: GMC Forward Control P3500 1993 - Component Locations - CODE 68, Schematic, Overdrive Ratio Error

Fig. 73: GMC Forward Control P3500 1993 - Component Locations - CODE 68, Flowchart, Overdrive Ratio Error

GMC Forward Control P3500 1993 - CODE 73, FORCE MOTOR CURRENT (CURRENT ERROR)

NOTE:

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

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

The PCM/TCM then monitors amperage at the return line. 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 numbers on diagnostic chart.

1. Checks the ability of the PCM/TCM 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 74: GMC Forward Control P3500 1993 - Component Locations - CODE 73, Schematic, Force Motor Current (Current Error)

Fig. 75: GMC Forward Control P3500 1993 - Component Locations - CODE 73, Flowchart, Force Motor Current (Current Error)

GMC Forward Control P3500 1993 - CODE 75, SYSTEM VOLTAGE LOW

Code 75 will set when ignition is on and PCM/TCM 12-volt battery feed voltage is less than 8.6 volts for about 4 seconds. During the time failure is present, force motor, also referred to as pressure control solenoid, is turned off, maintaining only 2nd gear, and inhibiting TCC operation.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Normal battery voltage is 9-15 volts.
2. This test checks if low voltage reading is due to alternator, circuit No. 440 or PCM/TCM. With engine running, check voltage at battery. If voltage is less than 8.6 volts, PCM/TCM is okay.

GMC Forward Control P3500 1993 - NOTE ON INTERMITTENTS

If code sets when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link. Minimum voltage allowed for Code 75 to set is on a graduated scale and changes with temperature. Minimum voltage at -40?F (-40?C) is 6.7 volts. Minimum voltage at 304?F (150?C) is 10.5 volts, and minimum voltage at 194?F (90?C) is 8.6 volts.

Fig. 76: GMC Forward Control P3500 1993 - Component Locations - CODE 75, Schematic, System Voltage Low

Fig. 77: GMC Forward Control P3500 1993 - Component Locations - CODE 75, Flowchart, System Voltage Low

GMC Forward Control P3500 1993 - CODE 81, SOLENOID "B" CIRCUIT FAULT

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, also referred to as 2-3 shift solenoid. 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 2 seconds, 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 2 seconds, Code 81 will set.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks shift solenoid "B" and internal transmission wiring for short circuits.
2. This test checks for power, from the ignition through the fuse, to the shift solenoid.
3. This test ensures circuit No. 1223 is not shorted to ground.
4. This test checks ability of PCM/TCM 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 78: GMC Forward Control P3500 1993 - Component Locations - CODE 81, Schematic, Solenoid "B" Circuit Fault

Fig. 79: GMC Forward Control P3500 1993 - Component Locations - CODE 81, Flowchart, Solenoid "B" Circuit Fault

GMC Forward Control P3500 1993 - CODE 82, SOLENOID "A" CIRCUIT FAULT

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, also referred to as 1-2 shift solenoid. 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 numbers on diagnostic chart.

1. This test checks shift solenoid "A" and internal transmission wiring harness for short circuits.
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 ability of PCM/TCM 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 80: GMC Forward Control P3500 1993 - Component Locations - CODE 82, Schematic, Solenoid "A" Circuit Fault

Fig. 81: GMC Forward Control P3500 1993 - Component Locations - CODE 82, Flowchart, Solenoid "A" Circuit Fault

GMC Forward Control P3500 1993 - CODE 83, TCC SOLENOID CIRCUIT FAULT

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

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks for low resistance in the solenoid or internal transmission harness. Test also determines which circuit triggered fault. 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 voltage in the solenoid.
3. This test checks circuit No. 422 for short to ground.
4. This test checks if PCM/TCM is commanding TCC solenoid on.

GMC Forward Control P3500 1993 - 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 in this section.

Fig. 82: GMC Forward Control P3500 1993 - Component Locations - CODE 83, Schematic, TCC Solenoid Circuit Fault (C, K & P Series)

Fig. 83: GMC Forward Control P3500 1993 - Component Locations - CODE 83, Schematic, TCC Solenoid Circuit Fault ("G" Series)

Fig. 84: GMC Forward Control P3500 1993 - Component Locations - CODE 83, Flowchart, TCC Solenoid Circuit Flowchart

GMC Forward Control P3500 1993 - CODE 85, UNDEFINED GEAR RATIO

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

Fig. 85: GMC Forward Control P3500 1993 - Component Locations - CODE 85, Schematic, Undefined Gear Ratio

NOTE:

Test numbers refer to 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 (vehicle speed sensor).

GMC Forward Control P3500 1993 - Diagnostic Aids

The PCM/TCM relies on Vehicle Speed Sensor Buffer Module (DRAC) to indicate what gear range has been selected. DRAC 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 in this section. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.

Fig. 86: GMC Forward Control P3500 1993 - Component Locations - CODE 85, Schematic, Undefined Gear Ratio

Fig. 87: GMC Forward Control P3500 1993 - Component Locations - CODE 85, Flowchart, Undefined Gear Ratio

GMC Forward Control P3500 1993 GEAR RATIO

Gear	Less Than	More Than
1st	12.38	22.63
2nd	11.43	21.58
3rd	1.95	21.05
Reverse	11.97	22.17

GMC Forward Control P3500 1993 - CODE 86, LOW GEAR RATIO

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", also referred to as 2-3 shift solenoid. PCM/TCM engages solenoid "B" by grounding circuit No. 1223.

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 numbers on diagnostic chart.

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

GMC Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 88: GMC Forward Control P3500 1993 - Component Locations - CODE 86, Schematic, Low Gear Ratio

Fig. 89: GMC Forward Control P3500 1993 - Component Locations - CODE 86, Flowchart, Low Gear Fault

GMC Forward Control P3500 1993 - CODE 87, HIGH GEAR RATIO

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", also referred to as 2-3 shift solenoid. PCM/TCM engages solenoid "B" by grounding circuit No. 1223.

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 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 90: GMC Forward Control P3500 1993 - Component Locations - CODE 87, Schematic, High Gear Ratio

Fig. 91: GMC Forward Control P3500 1993 - Component Locations - CODE 87, Flowchart, High Gear Ratio

GMC Forward Control P3500 1993 - COMPONENT CHECK CHARTS (4L80-E) VEHICLE SPEED SENSOR CHECK

The Vehicle Speed Sensor (VSS) circuit consists of a magnetic induction-type sensor, VSS buffer module (also referred to as the DRAC) and wiring. Gear teeth, pressed on outside diameter of output carrier assembly, induce an alternating current in sensor. This current is transmitted to the buffer module, where it is passed on to the PCM/TCM. The buffer module 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.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks the VSS signal to the PCM/TCM.
2. This test checks the VSS signal to the buffer module.
3. This test checks the VSS signal.

GMC Forward Control P3500 1993 - Diagnostic Aids

Check all connections, especially at transmission pass-thru connector. If the input sensor is not operational at start up, it can cause the VSS to read zero. While Code 24 is set, scan tester will display RPM derived from input speed.

Fig. 92: GMC Forward Control P3500 1993 - Component Locations - Schematic, Vehicle Speed Sensor ("C" & "K" Series)

Fig. 93: GMC Forward Control P3500 1993 - Component Locations - Schematic, Vehicle Speed Sensor ("G" Series)

Fig. 94: GMC Forward Control P3500 1993 - Component Locations - Schematic, Vehicle Speed Sensor ("P" Series)

Fig. 95: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Vehicle Speed Sensor

GMC Forward Control P3500 1993 - 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 and TCC slip, and determine if engine is running.

NOTE:

Test numbers refer to 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 96: GMC Forward Control P3500 1993 - Component Locations - Schematic, Input Speed Sensor

Fig. 97: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Input Speed Sensor

GMC Forward Control P3500 1993 - 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 numbers on diagnostic chart. Display may vary depending upon manufacturer of scan tester. Display may read APPLIED, RELEASED, ON or OFF.

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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 98: GMC Forward Control P3500 1993 - Component Locations - Schematic, Brake Signal

Fig. 99: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Brake Signal

GMC Forward Control P3500 1993 - PRESSURE SWITCH MANIFOLD CIRCUIT CHECK

A fault in the Pressure Switch Manifold (PSM) circuit, also referred to as fluid pressure switch assembly circuit, should result in the setting of a related trouble code. If a problem is suspected with this circuit and a code is not set, go to CODE 28, PRESSURE SWITCH MANIFOLD FAULT chart under TROUBLE CODE CHARTS (4L80-E) for diagnosis.

GMC Forward Control P3500 1993 - 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 an automatic transmission and fuel economy of a manual transmission.

Fused battery ignition is supplied to TCC solenoid, located inside valve body, to shift a spool valve in order to modulate pressure to TCC.

PCM/TCM engages TCC by grounding circuit No. 422 to energize solenoid.

TCC will engage under the following conditions.

• Vehicle speed is greater than 30 MPH.
• 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 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 Forward Control P3500 1993 - 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 in this section. 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. 100: GMC Forward Control P3500 1993 - Component Locations - Schematic, Torque Converter Clutch ("C", "K" & "P" Series)

Fig. 101: GMC Forward Control P3500 1993 - Component Locations - Schematic, Torque Converter Clutch ("G" Series)

Fig. 102: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Torque Converter Clutch

GMC Forward Control P3500 1993 - SHIFT SOLENOID "A" CIRCUIT CHECK

Shift solenoid "A", also referred to as 1-2 shift solenoid, 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" (Gray) 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 numbers on diagnostic chart.

1. This test checks shift solenoid "A" and the internal transmission wiring harness for short circuits.
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 Forward Control P3500 1993 - 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 in this section.

Fig. 103: GMC Forward Control P3500 1993 - Component Locations - Schematic, Shift Solenoid "A"

Fig. 104: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Shift Solenoid "A"

GMC Forward Control P3500 1993 - SHIFT SOLENOID "B" CIRCUIT CHECK

Shift solenoid "B", also referred to as 2-3 shift solenoid, is attached to the valve body and is a normally open exhaust valve, which determine the transmission gear. Solenoid "B" (Green) is on in 3rd and 4th gears, but is off in 1st and 2nd gears. Fused ignition is supplied to solenoid "B". PCM/TCM engages solenoid "B" by grounding circuit No. 1223.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test checks shift solenoid "B" and the internal transmission wiring for short circuits.
2. This test checks for power, from the ignition through the fuse, to the shift solenoid.
3. This test ensures circuit No. 1223 is not shorted to ground.
4. This test checks ability of PCM/TCM 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.
5. This test ensures circuit No. 1223 is not shorted to ground.

GMC Forward Control P3500 1993 - 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 in this section.

Fig. 105: GMC Forward Control P3500 1993 - Component Locations - Schematic, Shift Solenoid "B"

Fig. 106: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Shift Solenoid "B"

GMC Forward Control P3500 1993 - FORCE MOTOR CIRCUIT CHECK

NOTE:

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

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

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Checks the ability of the PCM/TCM 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 Forward Control P3500 1993 - Diagnostic Aids

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

Fig. 107: GMC Forward Control P3500 1993 - Component Locations - Schematic, Force Motor Circuit

Fig. 108: GMC Forward Control P3500 1993 - Component Locations - Flowchart, Force Motor Circuit