Electronic Control Module (ECM): Memories

The 4 types of memories in the ECM are: Read Only Memory (ROM), Random Access Memory (RAM), Programmable Read Only Memory (PROM) and the fuel system CALPAC.

• Read Only Memory (ROM)  - The ROM is programmed information that can only be read by the ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.
• Random Access Memory (RAM)  - This memory is the decision making center for the CPU. It works like a calculator. Data sensor input, diagnostic codes and results of calculations are temporarily stored in the RAM memory. If battery voltage is removed from the ECM, all information stored in the memory is lost.
• Programmable Read Only Memory (PROM)  - This memory is factory programmed information, including engine calibration data, transmission, vehicle weight and rear axle ratio application. The PROM can be removed from the ECM. If battery voltage is removed, PROM information will be retained.
• CALPAC  - Fuel Injected models use a new type of ECM. This ECM uses a PROM and a device called a CALPAC. The CALPAC allows fuel delivery so the engine will run in case of a PROM or ECM failure. Any time the ECM is replaced, the PROM and the CALPAC must both be installed into the replacement ECM. If battery voltage is removed, CALPAC information will be retained.