Electronic Control Module (ECM) Operation: Memories

The 3 types of memories in the ECM 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 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 RAM. If battery voltage is removed from the ECM, all information stored in this 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.