Electronic Control Module (ECM): Memories

ECM use 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).

Read Only Memory (ROM) 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.
Random Access Memory (RAM) 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 ECM, all information stored in RAM is lost.
Programmable Read Only Memory (PROM) PROM is factory programmed engine calibration data which "tailors" ECM for specific transmission, engine, emission, vehicle weight and rear axle ratio applications. The PROM can be removed from ECM. If battery voltage is removed, PROM information will be retained.
Calibration Package (CAL-PAC) 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 ECM failure. Any time ECM is replaced, PROM and CAL-PAC must both be installed into replacement ECM. If battery voltage is removed, CAL-PAC information will be retained.
Memory Calibration Unit (MEM-CAL) Models may also use another type of ECM 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 ECM is removed, MEM-CAL information will be retained.

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.