Engine Control Module: Description and Operation

Powertrain Control Module (PCM) And Knock Sensor Module:

Purpose:
The PCM constantly receives information from various sensors. The PCM performs the diagnostic functions of the system and can recognize operational problems and alert the driver through the "Malfunction Indicator Light" (MIL).

The PCM stores trouble codes which identify the problem area to diagnose or repair. The PCM 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 PCM operates most output devices by completing the circuit to ground through Quad Driver Modules (QDM) within the PCM. When the PCM is commanding a device or a component "ON," the PCM completes the ground circuit and the voltage potential of the circuit goes "LOW" or near zero volts. When the PCM is commanding a device or component "OFF," the ground circuit is opened and the voltage potential of the circuit will be "HIGH," or near 12 volts (or referenced voltage).

Operation:
Inputs have complex interrelations with outputs. If one of the input devices fails, 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 and intake systems, 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.

Location:
The Powertrain Control Module (PCM) is located at the left rear of the engine compartment and is the control center of the Computer Command Control system.