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. The PROM, which is located inside the PCM, contains the calibration information used by the PCM to control fueling, idle speed, cruise control, ignition timing, transaxle shifts and cooling fans. This calibration information is based on various aspects of the vehicle, such as engine size, vehicle weight, transaxle, axle ratio and numerous others.

While the PCM contains two separate processors to control various functions, the PROM contains two separate PROM'S. Each PROM provides specific information to one of the processors of the PCM. However, the 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, the PROM is very specific and can only be used for certain applications. Therefore, the PCM is serviced separately from the PROM, even though the PROM is housed inside the PCM.

It is very important to check the latest parts information and technical service bulletins for the correct part number when replacing the PROM. Parameters PD97 and PD99 can be used to verify the proper PROM ID.

Replacement PCM's used for service come without a PROM. Therefore, the PROM from the old PCM must be carefully removed and installed in the new PCM. See POWERTRAIN CONTROL MODULE (PCM) & PROM in REMOVAL, OVERHAUL & INSTALLATION article. If the PROM is missing, it will result in a no start, no run condition, and the PCM will not send data to the BCM. A current Code B334 and a "SYSTEM PROBLEM" message on the DIC will result. See BODY COMPUTER MODULE -- ALLANTE article.