Powertrain Control Module (Pcm)/Vehicle Control Module (VCM): Memories

PCM/VCM may use one or more of 5 types of memory:

• Read Only Memory (ROM)
  
  ROM is programmed information which only PCM/VCM 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" PCM/VCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. The PROM can be removed from PCM/VCM. 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 PCM/VCM failure. Any time PCM/VCM 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 PCM/VCM containing a MEM-CAL unit. This assembly contains functions of PROM and CALPAC. If power to PCM/VCM is removed, MEM-CAL information is retained. MEM-CAL also contains an internal ESC module on models equipped with ESC.