Engine Control Module: Description and Operation

NOTE: The ECU constantly monitors input devices for malfunctions. However, it only checks for output device faults during a three second period after the ignition is switched "ON" and test connector (green, 1 pin) is grounded.

The Engine Control Unit (ECU) is the central processor (computer) for all engine management functions. The system maintains an air/fuel mixture as close as possible to the ideal stoichiometric ratio, to maintain good driveability and reduce emissions for all operating conditions. The ECU monitors various sensors mounted on the engine, including an exhaust gas oxygen sensor, then uses this information to modify fuel injector duration ("ON" time), regulate ignition timing, and operate certain emission control devices. Fuel is supplied to the injectors at high pressure and the computer energizes the injectors (either all at once or in pairs) for varying lengths of time, depending on the amount of fuel required for any given engine speed and load.

To provide a margin of safety by maintaining driveability in the event of certain system failures, the ECU has a fail-safe mode of preprogrammed values. The output of the ECU will automatically switch to fail-safe mode if a fault is detected in any of these components and/or systems:

^ Airflow meter
^ Atmospheric pressure sensor
^ Feedback system
^ Intake air thermosensor (in airflow meter)
^ Intake air thermosensor (in dynamic chamber)
^ Oxygen sensor
^ Throttle sensor
^ Water thermosensor

The ECU incorporates self-diagnostic capability monitored via a dash mounted malfunction indicator lamp (MIL). When the system detects a fault in a monitored component and/or circuit, the lamp will light and stay on (while the engine is running). To extract codes, the technician can either utilize a "Scan" tool or cause the MIL to display codes by grounding the test connector (green, 1 pin) in the engine compartment.