Engine Control Module: Description and Operation

Engine Control Module (ECM), Powertrain Control Module (PCM)





ECM (PCM) is installed to the underside of the instrument panel at the passenger's seat side.





ECM (PCM) is a precision unit consisting of microcomputer, A/D (Analog/Digital) converter, I/O (Input/Output) unit, etc.. It is an essential part of the electronic control system, for its functions include not only such a major function as to control fuel injector, IAC valve, fuel pump relay, etc. but also on-board diagnostic system (self-diagnosis function) and fail-safe function as described below

On-board diagnostic system (Self-diagnostic function)
ECM (PCM) performs on-board diagnosis on the system and parts that influence the vehicle emission in compliance with OBD-II regulation.
Refer to Computers and Control Systems/Testing and Inspection Procedures.
ECM (PCM) and malfunction indicator lamp ("SERVICE ENGINE SOON" light) operate as follows.
- Malfunction indicator lamp lights when the ignition switch is turned ON (but the engine at stop) regardless of the condition of Engine & emission control system (Electronic Fuel injection system). This is only to check the malfunction indicator lamp bulb and its circuit.
- If the system and parts that influence the vehicle emission is free from any malfunction after the engine start (while engine is running), malfunction indicator lamp turns OFF.
- When ECM (PCM) detects a malfunction, it makes malfunction indicator lamp turn ON or flashing while the engine is running to warm the driver of such occurrence of trouble and at the same time it stores the applicable diagnostic trouble codes in ECM (PCM) memory. Refer to Computers and Control Systems/Testing and Inspection Procedures.

Fail-safe function
Even when a trouble has occurred in such area of engine & emission control system that includes the parts described in fail-safe table of "Engine Diagnosis" section and a failure signal is sent to ECM (PCM), control over the injections, IAC valve and others is maintained on the basis of the standard signals and/or back-up program prestored in the ECM (PCM) while ignoring that failure signal and/or CPU. This function is called "fail-safe function". Thus, with this function, a certain level of engine performance is available even when some failure occurs in such area and disability in running is avoided.