Powertrain Control Module (Pcm): Memories

PCM uses 3 types of memory:

• Read Only Memory (ROM)
  
  ROM is programmed information which only control module can read. The 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, all information stored in RAM is lost.
• Programmable Read Only Memory (PROM)
  
  PROM is factory programmed engine calibration data which "tailors" control module for specific transmission, engine, emission, vehicle weight and rear axle ratio application. The PROM can be removed from control module. If battery voltage is removed, PROM information is retained.