Electronic Control Module (ECM): Memories

Three types of memories are used in the ECM. They are the Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM). Function of each memory is as follows:

• Read Only Memory (ROM) 

  The ROM is programmed information that can only be read by ECM. 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. Information can be read into or out of RAM memory; similar to a calculator. Data sensor information, diagnostic codes and results of calculations are temporarily stored in RAM. If battery voltage is removed, all information stored here is lost.

• Programmable Read Only Memory (PROM) 

  This memory contains factory-programmed information concerning engine calibration data for each engine, transmission, body and rear axle ratio combination. The PROM is easily removed from ECM. If battery voltage is removed, PROM information will be retained.