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Powertrain Control Module (Pcm): Memories

PCM uses 5 types of memories: Read Only Memory (ROM), Random Access Memory (RAM), Programmable Read Only Memory (PROM), fuel system Calibration Package (CAL-PAC) and Memory Calibration unit (MEM-CAL).

ROM is programmed information that can only be read by ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.

RAM is the scratch pad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from PCM, all information stored in RAM is lost.

PROM is factory programmed engine calibration data which "tailors" PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio applications. The PROM can be removed from PCM. If battery voltage is removed, PROM information will be retained. An Electronically Erasable Programmable Read Only Memory (EEPROM) is used on some models. This is the same as a PROM except it can be electronically reprogrammed by the manufacturer using special equipment.

Some models use a PROM and a device called a CAL-PAC. The CAL-PAC provides fuel delivery back-up so engine will run in case of a PROM or PCM failure. Anytime PCM is replaced, PROM and CAL-PAC must both be installed into replacement PCM. If battery voltage is removed, CAL-PAC information will be retained.

Models may also use another type of PCM containing a Memory Calibration unit (MEM-CAL). This assembly contains functions of PROM and CAL-PAC and, on some models, the ESC control module. If power to PCM is removed, MEM-CAL information will be retained.

Some models may use an Electronically Erasable Programmable Read Only Memory (EEPROM). This is the same as a PROM except it can be electronically reprogrammed by the manufacturer using special equipment.

NOTE: Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the control unit. The second category covers OUTPUT SIGNALS, which are components controlled by the control unit.
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