Engine Control Module: Description and Operation

The Electronic Control Module (ECM) or Powertrain Control Module (PCM) is located in the passenger compartment and is the control center of the Computer Command Control system. The ECM/PCM constantly receives information from various sensors. The computer performs the diagnostic functions of the system. The computer can recognize operational problems and alert the driver through the "SERVICE ENGINE SOON" light. The computer stores a trouble code which identifies the problem area to diagnose and repair. The computer is designed to process the various input information, and send the necessary electrical response to control fuel delivery, spark control, and other emission control systems. The input information has an interrelation between sensor output. If one of the input devices failed, such as the oxygen sensor, this could affect more than one of the systems controlled by the computer.

The computer has a learning ability which allows it to make corrections for minor variations in the fuel system to improve driveability. If the battery is disconnected to clear trouble codes or for repair, the learning process has to begin all over again, and a change may be noticed in the driving performance of the vehicle. To reset the vehicles learning ability, make sure the engine is at operating temperature and operate the vehicle at part throttle, moderate acceleration, and idle conditions, until normal performance returns.

A vehicle equipped with a 4-cylinder engine has an ECM referred to as GMP4 with two parts for service. A controller which is the ECM itself, without a MEM-CAL, and a memory calibration unit called a MEM-CAL.

Vehicles with a V-6 or V-8 cylinder engine have an ECM referred to as GMCM with three serviceable parts. A controller called a ECM, without a PROM, a PROM with specific programming information, and a CAL- PAK with specific engine specifications.

Vehicles with a hydro-matic 4L80-E transmission have a Powertrain Control Module PCM or GMP6 which uses a memory and calibration unit (MEM-CAL).