Computers and Control Systems: Description and Operation

System Diagram:

PURPOSE
The Powertrain Control Module (PCME) is the central component of this computerized fuel, ignition and emission control system. To maintain fuel economy, good driveability and reduced vehicle emissions, the PCME 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 PCME is a computer that judges air/fuel ratio as rich or lean according to a voltage signal generated by two oxygen sensors (one on each side). Based on oxygen sensor and other input data, the PCME generates a controlling signal to the fuel injectors that regulates "ON" time. Because fuel pressure remains constant, the PCME 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 PCME has a fail-safe mode of preprogrammed values. PCME output will automatically switch to fail-safe mode if a fault is detected in any of these components and/or systems:

^ Crankshaft Position Sensor
^ Knock Sensor
^ EGR Function Sensor
^ Airflow Sensor
^ Engine Coolant Temperature Sensor (CIS)
^ Engine Coolant Temperature Sensor (Fan)
^ Intake Air Temperature Sensor
^ Throttle Position Sensor
^ Barometric Atmospheric Presure Sensor
^ Heated Oxygen Sensor
^ Closed Loop System (Oxygen Sensor Output)

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

Components can be grouped as PCME input and output devices. Not all devices are monitored by the on-board diagnostic system.

INPUT DEVICES
Input devices control or produce PCME monitored voltage signals.

^ Main Relay
^ Ignition Switch
^ Data Link Connector
^ Daytime Running Lights Relay (Canada)
^ Rear Window Defroster Relay
^ Vehicle Speed Sensor
^ Knock Sensor (Code 5)
^ Stoplight Switch
^ Steering Pressure Sensor
^ A/C Sensor
^ A/C Pressure Switch
^ Blower Control Switch
^ Headlight Switch
^ Park/Neutral and Clutch Switches (M/T)
^ Reference Voltage
^ Engine Coolant Temperature Sensor (Fan) (Code 69)
^ Engine Coolant Temperature Sensor (CIS) (Code 9)
^ EGR Function Sensor (Code 16)
^ Intake Air Temperature Sensor (Code 10)
^ Throttle Position Sensor (TVO) (Code 12)
^ Throttle Position Sensor (Idle Switch)
^ Heated Oxygen Sensor (Left) (Codes 15 & 17)
^ Heated Oxygen Sensor (Right) (Codes 23 & 24)
^ Airflow Sensor (Code 8)
^ Crankshaft Position Sensor 1 (Codes 3 & 4)
^ Crankshaft Position Sensor 2 (Code 2)
^ PCMT (Powertrain Control Module-Transmission)

OUTPUT DEVICES
Output devices are PCME controlled components.

^ Data Link Connector
^ Malfunction Indicator Lamp (MIL)
^ Fuel Pump Relay
^ A/C relay
^ Coolant Fan Relay 1 (Code 67)
^ Coolant Fan Relays 2 & 3
^ Igniter
^ Barometric Absolute Pressure Sensor (Code 14)
^ Throttle Position Sensor/Idle Switch
^ Solenoid Valve (EGR Vent) (Code 29)
^ Solenoid Valve (EGR Vacuum) (Code 28)
^ Solenoid Valve (Pressure Regulator) (Code 25)
^ Solenoid Valve (Purge Control) (Code 26)
^ Solenoid Valve (VRIS 1) (Code 41)
^ Solenoid Valve (VRIS 2) (Code 46)
^ Fuel Injectors
^ Idle Air Control Valve (Code 34)