Electronic Control Module (ECM) Operation: Memories

Three types of memory are used by the ECM. These are: Read Only Memory, Random Access Memory, and Programmable Read Only Memory.

• 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 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 contains factory programmed information including engine calibration data, transmission type, vehicle weight and rear axle ratio. The PROM can be removed from the ECM. If battery voltage is removed, PROM information will be retained.