Engine Control Module: Description and Operation

Closed Loop Operation:

ECM And PROM Access Cover:

The Electronic Control Module (ECM) is located in the passenger compartment and is the control center of the Computer Command Control (C3) system. The ECM 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 computer used in these vehicles is a type called GMP4. The computer can control these devices through the use of Quad Driver Modules (QDM). When the ECM is commanding a device or a component "ON," the voltage potential of the output is "LOW" or near zero volts. When the ECM is commanding a device or component "OFF," the voltage potential of the circuit will be "HIGH," or near 12 volts. The primary function of the QDM is to supply the ground for the component being controlled. 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.

The ECM used in these vehicles uses an ECM with two serviceable parts. A controller called a ECM, without a PROM, and a PROM with specific programming information.

NOTE: When a production ECM is replaced with a service ECM, it is important to transfer the broadcast code and production number from the original ECM to the service ECM. This allows for positive identification of ECM parts throughout the service life of the vehicle. DO NOT affix the information to the ECM cover.

For expanded coverage of the C-3 system, see COMPUTERIZED ENGINE CONTROLS.