Engine Control Module: Description and Operation

FUEL INJECTOR CONTROL
The electronic control module is located under the carpet in the passenger footwell. The ECM/PCM contains information to calculate basic injector duration based upon engine speed and manifold pressure. After the basic duration information is read from memory, the signal is modified by the ECM/PCM in reference to the information provided from the other engine sensors. Once the signal is processed, it is passed on to the fuel injectors which respond by opening for the appropriate amount of time.

ELECTRONIC IDLE CONTROL
The intake manifold mounted idle air control valve (IAC) is used by the ECM/PCM to modify idle speed to compensate for engine load or idle instability caused from cold operation, transmission in gear (A/T), alternator charging or air conditioning compressor operation.

IGNITION TIMING CONTROL
Timing control is controlled much the same as injector duration. The ECM/PCM contains a basic program for calculating engine timing based on engine speed and manifold pressure. The base signal is then modified by the various other sensor information before the signal is passed on to the igniter unit to fire the spark plugs.

ECU FAIL-SAFE FUNCTION
If a signal from a sensor is determined to be inaccurate, the ECM/PCM ignores the signal and substitutes a preset value allowing the engine to remain running until repairs can be made. The ECM/PCM then illuminates the check engine light to alert the driver that a malfunction exists.

BACK-UP FUNCTION
In the event of an ECM failure, the fuel injectors are controlled by a back-up system, independent of the ECM/PCM. This permits minimal driving until the vehicle can be repaired.

SELF-DIAGNOSIS
If a sensor begins producing inaccurate information, the ECM/PCM illuminates the "Check Engine" warning lamp on the dashboard. Once a failure is noticed, the ECM/PCM also stores a fault code in eraseable memory. The fault code is indicated by the flashing Malfunction Inductor Light, when the service check connector is jumped.