Pcm Prom

To allow one PCM to be used for many different cars, a device called a Programmable Read Only Memory unit, or PROM is used. See Figure . The PROM which is located inside PCM, contains calibration information used by PCM to control fueling, idle speed, cruise control, ignition timing, transaxle shifts and cooling fans. This calibration information is based on various aspects of vehicle, such as engine size, vehicle weight, transaxle, axle ratio and numerous others. While PCM contains two separate processors to control various functions, PROM contains two separate PROM'S. Each PROM provides specific information to one of processors of PCM. However, PROM'S are physically connected and form one larger PROM, making them serviceable only as a unit.

While one PCM can be used for several different vehicles, PROM is very specific and can only be used for certain applications. Therefore, PCM is serviced separately from PROM, even though PROM is housed inside PCM. It is very important to check latest parts book and serviceman bulletin information for correct part number when replacing PROM. Parameters PD97 and PD99 can be used to verify proper PROM I.D.

Replacement PCM's used for service come without a PROM. Therefore, PROM from old PCM must be carefully removed and installed in new PCM. If PROM is missing, it will result in a no start, no run condition, and PCM will not send data to BCM. A current code B334 and a "SYSTEM PROBLEM" message on DIC will result.