Metro L4-079 1.3L VIN 9 TBI (1995)
Engine Control Module: Description and Operation
ENGINE CONTROL MODULE
The Engine Control Module (ECM) is a precision unit consisting of a one chip microcomputer; Analog/Digital (A/D) converter, Input/Output
(I/O) unit; Read Only Memory (ROM) and Random Access Memory (RAM). It is an essential part of the electronic engine control system, for its
functions include not only such major functions as to control the fuel injector, idle Speed Control (ISC) motor, fuel pump relay, but alto a
self-diagnosis function and a fail-safe function as described below.
SELF-DIAGNOSIS FUNCTION
The ECM diagnoses troubles which may occur in the system when the ignition switch is in the "ON" position or when the engine is running. The
ECM indicates a malfunction by lighting the Malfunction Indicator Lamp (MIL) when a fault occurs in any of the following systems:
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Heated Oxygen Sensor (HO2S).
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Engine Coolant Temperature (ECT) sensor.
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Throttle Position (TP) sensor.
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Intake Air Temperature (IAT) sensor.
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Manifold Absolute Pressure (MAP) sensor.
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Ignition signal.
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Camshaft Position (CMP) sensor.
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Vehicle Speed Sensor (VSS).
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Exhaust Gas Recirculation (EGR) system.
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Central Processing Unit (CPU) of ECM.
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Idle Speed Control (ISC) motor.
ECM AND THE MIL OPERATION
The ECM and the MIL operate as follows:
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 Throttle Body Fuel Injection (TBI) system. This is only to check the MIL circuit.
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.
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.
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 ECM is shut "OFF" for 60 seconds or longer.
NOTICE: If a malfunction occurs in the Exhaust Gas Recirculation (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 back-up memory of the ECM. (In other words, even if the ECM
has detected a switch is turned to "LOCK," DTC 41 will not be indicated malfunction in the ignition signal circuit, once the ignition when the
diagnostic request terminal is grounded and ignition switch is turned "ON.") Therefore, to check to 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 Throttle Body Fuel Injection (TBI) system, the ECM maintains control over the fuel injector, ISC motor
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
malfunction occurs, thus avoiding complete disability in engine performance. The systems covered are as follows:
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Engine Coolant Temperature sensor.
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Throttle Position sensor.
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Intake Air Temperature sensor.
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Vehicle Speed Sensor.
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Manifold Absolute Pressure sensor.
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Central Processing Unit in ECM.
