Computers and Control Systems: Description and Operation

System Diagram:

PURPOSE
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), ignition system, emission control devices, turbocharger system, 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.

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. ECU output will automatically switch to fail-safe mode if a fault is detected in these monitored components and/or systems:

^ Crank Angle Sensor (NE Signal)
^ Crank Angle Sensor (G Signal)
^ Knock Sensor
^ Speedometer Sensor
^ Water Thermosensor
^ Intake Air Thermosensor
^ Throttle Sensor (Full Range)
^ Throttle Sensor (Narrow Range)
^ Pressure Sensor
^ Atmospheric Pressure Sensor
^ Oxygen Sensor
^ EGR Switch (California Only)
^ Feedback System
^ Metering Oil Pump
^ Fuel Thermosensor

ON-BOARD DIAGNOSTICS
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 ("CHECK") 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. Not all of these devices are monitored by the on-board diagnostic system.

INPUT DEVICES
Input devices control or produce voltage signals monitored by the ECU.

^ Main Relay
^ Ignition Switch
^ A/C Switch
^ Diagnosis Connector
^ Speedometer Sensor (Code 6)
^ Atmospheric Pressure Sensor (Code 14)
^ P/S Pressure Switch
^ Pressure Sensor (Code 13)
^ Clutch Switch (M/T)
^ EC-AT Control Unit (A/T)
^ Stoplight Switch
^ Fuel Thermosensor (Code 23)
^ Heat Hazard Sensor
^ 1-2 Switch (M/T)
^ EC-AT CU (A/T)
^ Metering Oil Pump Position Sensor (Code 20 & 27)
^ E/L Unit
^ Oxygen Sensor (Code 15)
^ Feedback System (Code 17)
^ Cooling Fan Relay
^ Water Thermosensor (Code 9)
^ Throttle Sensor (Narrow Range) (Code 18)
^ Throttle Sensor (Full Range) (Code 12)
^ EGR Switch (California Only) (Code 16)
^ DRL Relay (Canada Only)
^ Intake Air Thermosensor (Code 11)
^ Knock Sensor (Code 5)
^ Crank Angle Sensor
-Ne Signal (Code 2)
-G Signal (Code 3)

OUTPUT DEVICES
Output devices are components controlled by the ECU.

^ Self-Diagnosis Checker
^ Igniter (Trailing)
^ Igniter (Leading)
^ Fuel Pump Relay (Code 51)
^ A/C Relay
^ Air Pump Relay (Code 54)
^ Circuit Opening Relay
^ Diagnosis Connector
^ EC-AT Control Unit (A/T)
^ A/P Relay
^ Cooling Fan Relay
^ Solenoid Valve (Purge Control) (Code 40)
^ Solenoid Valve (Port Air Bypass) (Code 33)
^ Solenoid Valve (Double Throttle Control) (Code 50)
^ Solenoid Valve (Relief 1) (Code 31)
^ Solenoid Valve (Relief 2) (Code 39)
^ Solenoid Valve (Split Air Bypass) (Code 30)
^ Solenoid Valve (Pressure Regulator Control) (Code 25)
^ Solenoid Valve (Switching) (Code 32)
^ Solenoid Valve (EGR) (Code 28)
^ Solenoid Valve (AWS) (Code 38)
^ Solenoid Valve (ISC) (Code 34)
^ Solenoid Valve (Turbo Control) (Code 44)
^ Solenoid Valve (Charge Relief) (Code 46)
^ Solenoid Valve (Charge Control) (Code 45)
^ Solenoid Valve (Wastegate Control) (Code 43)
^ Solenoid Valve (Turbo Pre-Control) (Code 42)
^ Front Secondary Injector (Code 71)
^ Rear Secondary Injector (Code 73)
^ Torque Converter Lockup "OFF" Signal (Code 76)
^ Torque Reduction Signal (Code 77)
^ Metering Oil Pump Stepping Motor (Code 26 & 37)