Powertrain Control Module (Pcm): Notes

CAUTION: To prevent internal PCM damage, the ignition must be "OFF" when disconnecting or reconnecting power to the PCM, as when the battery cables, PCM connectors, or fuses are connected or disconnected.

The Powertrain Control Module (PCM), is located under the instrument panel on the passenger side. The PCM controls fueling, idle speed, exhaust emissions, transaxle shifts, ignition timing, cooling fans and cruise control functions. The PCM can also recognize operational problems, alert the driver through the "SERVICE ENGINE SOON" MIL and store codes which identify problem areas for the technician.

In order to perform all of these functions, the PCM contains two processors, each with its own outputs, inputs, memory and program. The two processors function together by sharing input data, but they also function independently by controlling only certain outputs. The function of each processor can be loosely categorized as engine controls or transaxle controls. The only functions performed by both processors is the diagnosis of the serial data line, battery power circuits, and the PCM PROM. Because each processor diagnoses these on their own, each sets its own code in the case of a failure. For example, each time the battery is disconnected, two codes will set - P052 and P109.

The PCM is connected to the engine wiring harness by three 32 pin connectors. Each connector is color coded red, white, or blue and each row of pins in these three connectors has been given a unique alphabetical designation: A or B (red connector), C or D (white connector) or E or F (blue connector) see PCM CONNECTOR & VOLTAGE IDENTIFICATION CHARTS . The PCM consists of two separately serviceable parts: the PCM and the PCM Prom . The PROM plugs into the PCM and is located under the PROM cover.

The Northstar powertrain has very complex components with equally complex operating characteristics. Therefore, it is essential that the powertrain diagnostic procedures given in this and other service articles be used for diagnosis before any repairs are made. While the PCM and PROM may appear on the surface to be the source of a problem, they may, in fact, be operating properly based on incorrect inputs they receive. Powertrain diagnostic procedures are designed to arrive at the CORRECT repair as QUICKLY as possible. DO NOT, under any circumstances, replace a PCM or PROM unless directed to do so by diagnostic procedures in the service article, other service information sources, or through technical assistance.

Service of the PCM should normally consist of either replacement of the PCM or PROM, not both. If a diagnostic procedure calls for the PCM or PCM PROM to be replaced, the PROM and PCM should be checked first to see if they are the correct parts. The PROM can easily be checked using parameters PD97 and PD99. If the part numbers are correct, remove and separate the PCM and the PROM, replace one or the other (whichever is faulty) and reconnect the two.

NOTE: The service PCM will not contain a PROM. The original PROM must be retained to reinstall in the service PCM.

Care should be taken when separating the PROM from the PCM so that neither is damaged. After replacement of a PCM or PROM, clear PCM codes and check the vehicle for proper operation. Poor engine performance, surging, loss of cruise control, improper "SERVICE ENGINE SOON" MIL operation, no starts, and/or hard starts may be caused by an improperly seated PROM and if the PROM is missing, Code B334 will set and the vehicle will not start.