Engine Control Module: Description and Operation

Powertrain Control Module (PCM):

COMPUTER INPUTS/OUTPUTS

The Control Module (referred to as ECM in previous years - "Electronic Control Module" or "Engine Control Module", now called the Powertrain Control Module - PCM) is located in the passenger compartment and is the control center of the Computer Command Control (C3) system. The 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 trouble codes which identify the problem areas 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 computer can control these devices through the use of Quad Driver Modules (QDM). Most output devices are operated by completing their circuit to ground. When the PCM is commanding a device or a component "ON," the driver for that device completes the circuit to ground and voltage potential at the output terminal is "LOW" or near zero volts. When the PCM is commanding a device or component "OFF," the driver opens the ground circuit and voltage potential of the circuit will be "HIGH," or near 12 volts.

Inputs are related to more than one output. If an input device fails, such as the oxygen sensor, this could affect more than one of the systems controlled by the computer.

ADAPTABILITY

The system has a learning ability which allows it to make corrections for minor variations in the fuel system to improve driveability. The computer stores actual sensor signal values in its memory during normal operation and uses them as the default values when the engine is started again. This ensures good performance and driveability each time the vehicle is driven, even if conditions are changing. 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.


The PCM has several serviceable parts. A controller, called a PCM, and one of the following combinations of programming memory; a PROM (Programmable Read-Only Memory), a PROM and a CAL-PAK (Calibration Package), or a MEM-CAL (Memory Calibration unit), which also contains the fail-safe programming to operate the vehicle if the PCM or other vital system fails.