Powertrain Control Module: Memories

PCM uses 3 types of memory:

• Programmable Read Only Memory (PROM)
  
  PROM is factory programmed engine calibration data which "tailors" PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. PROM can be removed from PCM. If battery voltage is removed, PROM 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 from PCM, all information stored in RAM is lost.
• Read Only Memory (ROM)
  
  ROM is programmed information which only PCM can read. ROM program cannot be changed. If battery voltage is removed, ROM information is retained.