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 ECU is a computer that judges air/fuel ratio as rich or lean according to a voltage signal generated by an oxygen sensor. Based on oxygen sensor and other input data, the ECU generates a controlling signal to the fuel injectors that regulates "ON" time. Because fuel pressure remains constant, the ECU controls air/fuel ratio adjustments by varying injector "ON" time.

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
^ EGR position sensor
^ Feedback system
^ Intake air thermosensor
^ Knock sensor/knock control unit
^ Oxygen sensor
^ Throttle sensor
^ Water thermosensor

This turbocharged system utilizes a knock sensor and knock control unit. When engine knock is detected by the sensor, it signals the control unit which in turn signals the ECU. The ECU regulates boost through its control of the wastegate solenoid valve. When knocking occurs, boost is reduced to prevent engine damage.

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.