Engine Control Module: Description and Operation

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
Feedback system
Intake air thermosensor
Oxygen sensor
Throttle sensor (A/T)
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 ("CHECK") 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 diagnosis connector "TEN" terminal in the engine compartment.

Dechoke Control System:

The Dechoke Control System functions to facilitate starting if the engine becomes flooded during cold weather. When specified conditions are met, the ECU prevents injectors from opening during cranking so that spark plugs can dry and excess fuel will be purged from cylinders.

The system operates under these conditions:

1. Coolant temperature below 0°C (32° F).
2. Throttle valve fully open.
3. Ignition switch in "START" position, engine cranking at less than 500 rpm.

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 diagnosis connector "TEN" terminal is grounded.