Powertrain Control Module: Memories

Control module may use one or more of 5 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.
• Calibration Package (CALPAC)
  
  Some models use a PROM and a CALPAC. CALPAC provides fuel delivery back-up so engine runs in case of PROM or control module failure. Any time control module is replaced, PROM and CALPAC must both be installed into replacement unit. If battery voltage is removed, CALPAC information is retained.
• Memory Calibration (MEM-CAL)
  
  Some vehicles may use a control module containing a MEM-CAL unit. This assembly contains functions of PROM and CALPAC. If power to control module is removed, MEM-CAL information is retained. MEM-CAL also contains an internal ESC module on models equipped with ESC.