Engine Control Module: Description and Operation

PURPOSE
The Engine Control Unit (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.

FAIL-SAFE MODE
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. It automatically switches 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

ON-BOARD DIAGNOSTICS
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 use a "Scan" tool or cause the MIL to display codes by grounding the diagnosis connector "TEN" terminal in the engine compartment.

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.

DECHOKE CONTROL SYSTEM

Dechoke Control System:

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. Fuel injection is cut when the throttle is held wide open during cranking and coolant temperature is 32°F (0°C) or below.