Electronic Control Module (ECM): Memories

ECM uses 3 types of memory:

• READ ONLY MEMORY (ROM)  ROM is programmed information which only ECM can read. 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. PROM can be removed from ECM. If battery voltage is removed, PROM information is retained.