Automatic Transmission - Diagnosis - 4L80-E: Other GMC Pickup C3500

VEHICLE CODE DESIGNATION

1991 SERIES CODE DESIGNATIONS

1992 SERIES CODE DESIGNATIONS

PCM/TCM

Note. Models with gasoline engines use a Powertrain Control Module (PCM) computer. Models with diesel engines use a Transmission Control Module (TCM) computer. These computers control TCC, force motor (hydraulic pressure) and shifting solenoids "A" and "B".

The PCM/TCM is located behind glove box on all vehicles except "G" Series. On "G" Series, PCM/TCM is under driver's seat. PCM/TCM controls TCC, force motor (hydraulic pressure) and shifting solenoids "A" and "B". In addition, PCM also controls ignition, fuel and emission devices related to the engine.

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

SENSORS & SWITCHES

The PCM/TCM controls upshifts and downshifts based on coolant temperature, transmission temperature, system voltage, throttle position, transmission oil pressure switches, and transmission output and input speed sensors. (Scheme 490) The system includes several other switches and sensors that are used for engine control (gasoline engines). For additional information and testing of engine components, see appropriate article in ENGINE PERFORMANCE.

Shift Solenoids "A" & "B"

Transmission is shifted up or down by 2 electric solenoids. Both solenoids are located on valve body. (Scheme 490) Ignition power is supplied to each solenoid by the transmission fuse. Solenoid "A" controls hydraulic pressure to 1-2 and 3-4 shift valves. Solenoid "B" controls hydraulic pressure to 2-3 shift valve.

Force Motor

Force motor solenoid has a spool valve and operates pressure regulator valve. (Scheme 490) The computer sends a frequency signal to the force motor to regulate hydraulic line pressure. The frequency signal (duty cycle) is measured with a dwell meter or lab scope. When the duty cycle is zero, line pressure is at maximum, and force motor draws zero amp. When the duty cycle is 40%, line pressure is at minimum, and force motor draws 1.1 amps at 4-5 volts.

TCC Solenoid

This solenoid is used to control TCC apply valve. The computer sends a frequency signal to the TCC solenoid to gradually apply or release the TCC. (Scheme 490)

Scheme 490

RETRIEVING CODES (WITH "SCAN" TOOL)

Note. To read trouble codes and check system voltages on serial data line, plug "Scan" tool into ALDL.

The "Scan" tool is a specialized tester which, when plugged into ALDL, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to backprobe sensors and connectors.

"Scan" tools may also furnish information on status of output devices (solenoids and relays). However, status parameters are only an indication that output signals have been sent to devices by the control module. It does not indicate if devices have responded properly to that signal. This will need to be checked at output device using a voltmeter or test light.

Note. Code 12 should always exist when ALDL is grounded with key on and engine off, but may not be indicated by all makes of "Scan" tools.

If trouble code is not present, this is not an indication that is no problem. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time.

Out-of-specification sensors WILL NOT set trouble code, but WILL cause driveability problems. Using "Scan" tool is the easiest method of checking sensor specifications and other data parameters. Intermittent wiring problems may be identified by wiggling wiring harnesses and connections (key on, engine off) while observing "Scan" tool.

Note. If erroneous voltage signals are suspected, it will be necessary to verify tester information using digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off, remove tester, turn ignition on, and ground ALDL test terminal "B". The same codes should be retrieved whether "Scan" tool or TRANS or SERVICE ENGINE SOON light is used.

CODE 15 - COOLANT TEMPERATURE SENSOR LOW TEMPERATURE INDICATED (SIGNAL VOLTAGE HIGH)

see scheme 5for Component Diagram.

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

1. This checks if conditions for Code 15 still exist. Code 15 indicates control module has sensed high resistance in coolant sensor circuit. This could be due to high resistance (low temperature) or high voltage at coolant sensor terminal at control module for one second.
2. This test simulates conditions for Code 14. If control module recognizes low voltage signal, "Scan" tool will display temperature greater than 266°F (130°C), indicating control module and wiring are not at fault.
3. This test determines if coolant sensor ground or signal circuit is open. There should be 5 volts present at sensor connector.

CODE 22 - THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW

see scheme 5for Component Diagram.

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

1. This test checks if code is result of a hard failure or an intermittent failure.
2. This test simulates conditions for Code 21. If control module recognizes state change, control module and wiring are okay.
3. This simulates high signal voltage to check for open in TPS signal line to control module. "Scan" tool should recognize signal and display high TPS voltage.

Intermittents

Check for poor connections or damaged harness. Also, check for intermittent condition by starting engine and wiggling connection while monitoring battery voltage on "Scan" tool. If voltage status changes abruptly or engine stalls, check for loose connections.

Scheme 491

Scheme 492

The quad driver (inside PCM/TCM) has a fault detect circuit, which is monitored by PCM/TCM. PCM/TCM compares voltage values of fault detect circuit with acceptable values in PCM/TCM memory. If PCM/TCM senses voltage is not within acceptable range, Code 81 will be set.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks shift solenoid "B" and circuits inside transmission for shorts.
2. Battery voltage should be present at shift solenoid "B".
3. This step checks circuit 1223 for short to ground.
4. This step checks if PCM/TCM can control solenoid "B".
5. This determines whether circuit 1223 is open or PCM/TCM is faulty.