Electronic Control Module (ECM): Memories

ECM uses 5 types of memory:

Read Only Memory (ROM) ROM is programmed information which only ECM can read. The ROM program cannot be changed. If battery voltage is removed, ROM information is 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 application. The PROM can be removed from ECM. If battery voltage is removed, PROM information is retained.
Calibration Package (CALPAC) Some models use a PROM and a CALPAC. CALPAC provides fuel delivery back-up so engine runs in case of PROM or ECM failure. Any time ECM is replaced, PROM and CALPAC must both be installed into replacement ECM. If battery voltage is removed, CALPAC information is retained.
Memory Calibration (MEM-CAL) Some vehicles may use a ECM containing a MEM-CAL unit. This assembly contains functions of PROM and CALPAC. If power to ECM is removed, MEM-CAL information is retained. MEM-CAL also contains an internal ESC module on models equipped with ESC.

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