Computerized Engine Controls (Diesel): Memories

The 3 types of memories used in ECMs are: Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM).

• 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.