Engine Control Module: 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, PCME monitors data from various sensors and controls functions such as the fuel injection system (air/fuel ratio), ignition system, emission control devices and on-board diagnostics.

PCME 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, PCME generates a controlling signal to the fuel injectors that regulates ON time. Because fuel pressure remains constant, PCME controls air/fuel ratio adjustments by varying injector "ON" time.

FAIL-SAFE MODE
To provide a margin of safety and maintain driveability in event of certain system failures, PCME fail-safe mode substitutes a fixed "in range" signal for that of the failed sensor. This allows the vehicle to be operated until repaired, although driveability will probably be affected. PCME automatically switches to fail-safe mode if a fault is detected in any of these inputs:

^ Mass Air Flow Sensor
^ Engine Coolant Temperature Sensor
^ Throttle Position Sensor
^ Intake Air Temperature Sensor
^ Barometric Absolute Pressure Sensor
^ Heated Oxygen Sensor
^ EGR Function Sensor

0N-BOARD DIAGNOSTICS
PCME 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 PCME trouble codes to aid in troubleshooting and repair.