Engine Control Module: Description and Operation

DESCRIPTION
The 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.

SELF-DIAGNOSIS FUNCTION
The 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 Diagnostic Trouble Code (DTC) which identifies the problem areas to aid the technician in making the necessary repairs to the following circuits:

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


PCM AND MIL OPERATION
The PCM and MIL operate as follows:

The MIL illuminates when the ignition switch is turned "ON" (but the engine is stopped) with the diagnostic request terminals ungrounded regardless of the condition of the electronic engine control system. 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 PCM detects the malfunction.

^ If a malfunction is detected in the electronic engine control system while the engine is running, the PCM lights the MIL to alert the driver. At the same time, the PCM stores a DTC in the PCM memory. The DTC is stored in the PCM memory even if the malfunction was temporary and is no longer present. The DTC remains stored in the PCM memory until the power to the PCM is interrupted for 10 seconds or longer.

^ The PCM also indicates DTCs in its memory by means of flashing the MIL at the time of inspection (i.e., when diagnostic request terminals are grounded and the ignition switch is in the "ON" position).

FAIL-SAFE FUNCTION
When a malfunction occurs in the electronic engine control system and is detected by the 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 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.