Engine Control Module: Description and Operation
POWERTRAIN CONTROL MODULEThe Powertrain Control Module (PCM) 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 also a self-diagnosis function and a fail-safe function as described below.
SELF-DIAGNOSIS FUNCTION
The PCM diagnoses troubles which may occur in the system when the ignition switch is in the "ON" position or when the engine is running. The PCM indicates a malfunction by lighting the Malfunction Indicator Lamp (MIL) when a fault occurs in any of the following systems:
^ Heated Oxygen Sensor 1(HO2S)-Sensor 1.
^ Heated Oxygen Sensor 2(HO2S)-Sensor 2.
^ Engine Coolant Temperature (ECT) Sensor.
^ Throttle Position (TP) Sensor.
^ Intake Air Temperature (IAT) Sensor.
^ Manifold Absolute Pressure (MAP) Sensor.
^ Camshaft Position (CMP) Sensor.
^ Crankshaft Position CKP Sensor.
^ Vehicle Speed Sensor (VSS).
^ Fuel Level Sensor.
^ Early Fuel Evaporation EFE Heater.
^ Idle Speed Control (ISC) Motor.
^ Cooling Fan System.
^ Exhaust Gas Recirculation (EGR) system.
^ Misfire Detection.
^ Fuel Trim.
^ Catalyst Monitor.
^ Evaporative Emission EVAP System.
^ Central Processing Unit (CPU) of PCM.
PCM AND THE MIL OPERATION
The PCM and the MIL operate as follows:
The MIL lights when the ignition switch is turned to the "ON" position (engine not running) 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 PCM, the MIL goes out.
When the PCM 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 PCM memory. If the cause of the malfunction is no longer detected by the PCM, the MIL will go out after 3 consecutive Ignition Cycles, but the corresponding DTC will be stored in the PCM memory.
The PCM will indicate DTCs that are stored in its memory through a scan tool.
FAIL-SAFE FUNCTION
When a malfunction occurs within the Throttle Body Fuel Injection (TBI) system, the PCM maintains control over the fuel injector, ISC motor etc., on the basis of the calculated signals and/or backup program prestored within the PCM.
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:
^ Engine Coolant Temperature sensor.
^ Throttle Position sensor.
^ Intake Air Temperature sensor.
^ Vehicle Speed Sensor.
^ Manifold Absolute Pressure sensor.
^ HO2S Sensor 1.
^ Fuel Level Sensor.
^ ISC System.
^ Fuel Trim.
^ Misfire.
^ Central Processing Unit in PCM.