Powertrain Control Module (Pcm): Diagnosis

NOTE: To prevent internal PCM damage, the ignition must be "OFF" when disconnecting or reconnecting power to PCM (for example, battery positive cable, PCM connectors, PCM fuse, jumper cables, etc.).

Since the PCM can have a failure which may affect only one circuit, the PFI SYSTEM CHECK in this article can reliably tell when a failure has occurred in the PCM.

Poor engine performance, idle or part throttle surges, loss of cruise control, a flashing "Engine Control System Fault" telltale, no starts, and/or hard starting may be experienced by a poorly seated MEM-CAL unit. If the MEM-CAL unit is missing, a Code B334 will set and the car will not start.

Service of the PCM should normally consist of either replacement of the PCM or a MEM-CAL change. If the diagnostic procedures call for the PCM to be replaced, the Memory-Calibration unit (MEM-CAL) and PCM should be checked first to see if they are the correct parts.

Also, perform PFI CHART C-1: PCM REPLACEMENT CHECK before replacing a PCM to assure that the replacement PCM will not be damaged. The PROM can easily be checked using parameter ED99. If the part numbers are correct, remove the MEM-CAL from the faulty PCM and install it in the new service PCM. THE SERVICE PCM WILL NOT CONTAIN A MEM-CAL, so care should be taken when removing the MEM-CAL from a defective PCM which is being replaced.

Trouble Code B334 (No PCM Data indicates the MEM-CAL is installed improperly or has malfunctioned. If Code B334 is current, check to see if the MEM-CAL is fully seated. If not fully seated, press firmly on the ends of the MEM-CAL and make sure the retaining clips have locked. If the MEM-CAL is installed correctly and Code "B334" still shows, diagnose using the procedure outlined in BODY COMPUTER MODULE article (BCM DTC B334). After replacement of a PCM, clear stored codes, and check the vehicle for proper operation.