Engine Control Module: Description and Operation

Component Location:

ENGINE CONTROL MODULE

DESCRIPTION
The Engine Control Module (ECM) is a precision unit consisting of a one chip microcomputer, Analog/Digital (A/D) converter and an Input/Output (I/O) unit. It is an essential part of the electronic control system, for its functions include not only such major functions as to control the fuel injector, Idle Air Control (IAC) valve, fuel pump relay, etc., but also a self-diagnosis function and a fail-safe function as described in the following section. The ECM is installed below the instrument panel left of the steering column.

SELF-DIAGNOSIS FUNCTION
The Engine Control Module diagnoses troubles which may occur in the system when the ignition switch is in the "ON" position with the engine running. The ECM indicates a malfunction by lighting the Malfunction Indicator Lamp (MIL) when a fault occurs in any of the following systems:

^ Heated Oxygen Sensor (HO2S).
^ Engine Coolant Temperature (ECT) sensor.
^ Throttle Position (TP) sensor.
^ Vehicle Speed Sensor (VSS).
^ Intake Air Temperature (IAT) sensor.
^ Mass Air Flow (MAF) sensor.
^ Ignition fail-safe signal.
^ Camshaft Position (CMP) sensor.
^ Idle switch circuit.
^ Exhaust Gas Recirculation (EGR) system including EGR temperature sensor.
^ Central Processing Unit (CPU) of ECM.

OPERATION
The ECM and the MIL operate as follows:

MIL Circuit Check
The MIL lights when the ignition switch is turned to the "ON" position (engine not running) with the diagnostic request terminal not grounded, regardless of the condition of the Sequential Multiport Fuel Injection (SFI) system. This is only to check the MIL circuit.

Fault Detection
Once the engine is started and no faults are detected by the ECM, the MIL goes out. When the ECM detects a malfunction in one of the above areas, it lights the MIL to warn the driver of the occurrence of a fault. At the same time it stores a Diagnostic Trouble Code (DTC) in the ECM memory. If the cause of the malfunction is no longer detected by the ECM, the MIL will go out, but the corresponding DTC will be stored in the ECM memory.

MIL Flashes DTCs
The ECM will indicate DTCs that are stored in its memory by means of flashing the MIL at the time of inspection (i.e., when diagnostic request terminal is grounded and ignition switch is turned to the "ON" position) or through the Tech 1 scan tool.

Stored DTCs
Any DTCs that occur with the engine running are stored in the ECM memory even if the fault was only temporary and disappeared immediately. The ECM memory is not erased unless the power to the ECM is shut "OFF" for 20 seconds or longer.

NOTICE: If a malfunction occurs in the EGR system, while the engine is running, the MIL remains lit. That is, once an EGR malfunction is detected, the MIL will remain lit with the engine running regardless if the trouble is present or not, until the necessary repairs have been made. Only the ignition signal circuit (DTC 41) is not stored in backup memory of the ECM. (In other words, even if the ECM has detected trouble in the ignition signal circuit, once the ignition switch is turned to "LOCK," DTC 41 will not be indicated when the diagnostic request terminal is grounded and ignition switch is turned "ON.") Therefore, to check for DTCs when the engine fails to start, crank the engine and then ground the diagnostic request terminal with ignition switch in the "ON" position.

FAIL-SAFE FUNCTION
When a malfunction occurs within the SFI system, the ECM maintains control over the fuel injector, IAC valve etc., on the basis of the calculated signals and/or backup program prestored within the ECM.

This function is called the "Fail-Safe Function." Thus, with this function, a certain level of engine performance is available even when a failure occurs, thus avoiding complete disability in engine performance.

FAIL-SAFE SYSTEMS
The systems covered are as follows:
^ Engine Coolant Temperature sensor.
^ Throttle Position sensor.
^ Mass Air Flow sensor.
^ Central Processing Unit of ECM.