SERVICE ENGINE SOON LIGHT POSSIBLE FALSE CODE 32

SUBJECT: "SERVICE ENGINE SOON" LIGHT - POSSIBLE FALSE CODE 32

MODELS: 1987-91 LIGHT DUTY TRUCKS EQUIPPED WITH 4L60 AUTOMATIC TRANSMISSION

Some 1987-91 light duty trucks equipped with 4L60 transmissions may experience a possible false Code 32 - EGR System Malfunction. The customer may not experience a change in performance or driveability with a Code 32 set.

If normal EGR diagnostics do not reveal the cause for Code 32, the following steps should be taken to correct the condition:

Check the TCC (Torque Converter Clutch) engagement solenoid/wiring harness for a short-circuit.

1. Disconnect the TCC harness connector from the left side of the transmission.

2. Measure resistance between TCC transmission connector terminals "A" and "D" with a Digital Volt Ohmmeter (DVOM). Solenoid resistance must measure 20 - 25 ohms at 20 degrees C (68 degrees F). Resistance will be slightly higher if the transmission is at operating temperature. If the resistance is less than 20 ohms the TCC solenoid diode or wiring is shorted and the solenoid must be replaced.

3. With key on, using a DVOM check for voltage at pin "D" of the TCC wiring harness. Voltage greater than one volt should not be found. If voltage exceeding one volt is present, circuit 422 is shorted to voltage and the short must be repaired. Refer to the appropriate service manual for circuit 422 diagnosis and repair.

Test operation of the TCC through the use of a scan tool connected to the vehicle ALDL connector. The following conditions must be satisfied when testing TCC operation.

- Clear stored trouble codes.

- Engine at normal operating temperature (above 65 degrees C, 149 degrees F) and in closed loop.

- Constant throttle position. TPS voltage = 1.3 V (minimum = 1.1 V, maximum = 1.5 V).

- Vehicle speed 45 mph (minimum = 35 mph, maximum 55 mph).

- Brake switch closed (brake pedal not applied).

- 3rd or 4th gear.

Confirmation

The scan tool will display the ECM command for TCC engagement when the above conditions are satisfied. When the TCC engages there should be a decrease in engine speed by a minimum of 100 RPM. If TCC operation is normal it is not necessary to replace the solenoid assembly. After verification of TCC operation, check for stored trouble codes.

If TCC fails to engage consult Section 10, Transmission Controls located in Fuel and Emissions Manual and section 7A of the appropriate service manual.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.