Engine Control Module: Description and Operation

Wiring Diagram:

DESCRIPTION
The ECM/PCM is the control center of the fuel injection system. It constantly monitors information from various sensors and controls the systems that affect vehicle performance. The ECM/PCM is serviceable only as an assembly.

SELF-DIAGNOSIS FUNCTION
The ECM/PCM also controls the diagnostic function of the fuel control system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (MIL), and store a DTC which identifies the problem areas to aid the technician in making the necessary repairs to the following circuits.

^ Oxygen Sensor (02S 1).
^ Heated Oxygen Sensor (HO2S 2).
^ Engine Coolant Temperature (ECT) Sensor.
^ Intake Air Temperature (IAT) Sensor.
^ Manifold Absolute Pressure (MAP) Sensor.
^ Ignition Signal.
^ Vehicle Speed Sensor (VSS).
^ Exhaust Gas Recirculation (EGR) System.
^ Central Processing Unit (CPU) of ECM/PCM.

The ECM/PCM and MIL operate as follows:

^ The MIL illuminates when the ignition switch is turned "ON" (but the engine is "OFF"). This is known as an Instrument Panel bulb check and is only to check the MIL and its circuit.

^ If no malfunctions in the electronic engine control system exist after the engine starts, the MIL goes out. If a malfunction exists in the electronic engine control system, the MIL may turn off, but only until the ECM/PCM detects the malfunction.

^ If a malfunction is detected in the electronic engine control system while the engine is running, the ECM/PCM lights the MIL to alert the driver. At the same time, the ECM/PCM stores a DTC in the ECM/PCM memory.

^ The DTC is stored in the ECM/PCM memory even if the malfunction was temporary and is no longer present. The DTC remains stored in the ECM/PCM memory until the power to the ECM/PCM is interrupted for 10 seconds or longer.

FAIL-SAFE FUNCTION

When a malfunction occurs in the electronic engine control system and is detected by the ECM/PCM, the injectors, Idle Air Control (IAC) valve and other major components are supported or compensated for, by signals from a program stored in the ECM/PCM to sustain minimum driveability until repairs to the electronic engine control system can be performed.

This function is called the "fail-safe function." This function provides a minimum level of engine performance which is available even when failure in the electronic engine control system occurs.