Electronic Control Module (ECM): Memories

The 5 types of memories used in ECMs are: Read Only Memory (ROM), Random Access Memory (RAM), Programmable Read Only Memory (PROM), fuel system CALPAC 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 application. The PROM can be removed from ECM. If battery voltage is removed, PROM information will be retained.
• CALPAC  Some fuel injected models use a PROM and a device called a CALPAC. The CALPAC provides fuel delivery back-up so engine will run in case of a 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 will be retained.
• MEM-CAL  Vehicles with fuel injection may also use another type of ECM containing a Memory Calibration unit (MEM-CAL). This assembly contains functions of PROM and CALPAC and, on some models, the ESC control module. If power to ECM is removed, MEM-CAL information will be retained.