General System

System Description:

The Engine Control Unit (ECU) is the central component of this computerized fuel, ignition and emission control system. To maintain fuel economy, good driveability and reduced vehicle emissions, the ECU monitors data from various sensors and controls functions such as the fuel injection system (air/fuel ratio), emission control devices and on-board diagnostics.

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 system 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), alerting the driver to seek service. The technician can then extract trouble codes from the ECU to aid in the performance of efficient troubleshooting and repair.

Components can be grouped as ECU input and output devices. Input devices control or produce voltage signals monitored by the ECU. Output devices are components controlled by the ECU. Not all of these devices are monitored by the on-board diagnostic system.

INPUT DEVICES
^ A/C switch
^ Airflow meter (code 08)
^ Atmospheric pressure sensor (code 14)
^ Diagnostic connector (TEN terminal)
^ Distributor "Ne" signal (code 02)
^ DRL control unit (Canadian models)
^ EC-AT control unit (timing retard signal)
^ Electrical load signal
^ Feedback system (code 17)
^ Ignition switch ("START" and "ON" positions)
^ Inhibitor switch (A/T)
^ Intake air thermosensor (code 10)
^ Main relay
^ Neutral and clutch switches (M/T)
^ Oxygen sensor (code 15)
^ Power steering pressure switch
^ Stop light switch
^ Throttle sensor (A/T, code 12)
^ Water thermosensor (code 09)

OUTPUT DEVICES
^ A/C relay/cut-off control
^ Air bypass valve
^ Circuit opening relay
^ EC-AT control unit (lockup control)
^ Fuel injectors/injection timing
^ Idle speed control (ISC) valve (code 34)
^ Igniter (timing control)
^ Malfunction indicator lamp (MIL)
^ Pressure regulator solenoid valve (code 25)
^ Purge control solenoid valve (code 26)
^ Switch monitor function