Engine Control Module: Description and Operation: Overview

PURPOSE
The ECM provides precise control of fuel injection to ensure optimum performance and strict emissions compliance.

OPERATION
The ECM performs system control functions such as fuel injection and ignition timing control. In order to maintain optimum air/fuel ratio through its control of the fuel injection system, the ECM monitors exhaust gas oxygen content by way of input from an oxygen sensor. When adjustments are needed, the ECM varies injector pulse width ("ON" time) as the means of controlling air/fuel ratio. Factors such as engine temperature, intake air volume and engine speed are also factored into ECM calculations.

LIMP HOME
The system also incorporates a "limp home" mode. When the ECM detects certain sensor failures, it will substitute preprogrammed fixed values in its calculations, permitting continued vehicle operation until repairs can be made. However, the ECM does not substitute a value for engine speed provided by the crankshaft sensor. The engine will not run in the event of crankshaft sensor failure.

TRANSMISSION CONTROL
To coordinate functioning and achieve smooth operation, the engine management ECM provides the electronic transmission control ECM with engine speed and load inputs. In return, the transmission ECM provides a signal which causes ignition timing to be momentarily retarded on upshifts. This reduces transmission torque input, resulting in smoother shifts.

SELF DIAGNOSIS
When the ECM detects a fault, the "CHECK ENGINE" lamp on the left side of the instrument cluster will illuminate. It will remain lit with ignition "ON" until the fault is corrected. A record of the fault is stored in ECM memory as a numbered code. Codes can be displayed on the LCD message display by switching ignition "OFF" for at least 5 seconds, then switching to position "II" without starting the engine. Press VCM button on trip computer and fault codes will be displayed.

ECU Locations:

LOCATION
The engine control module (ECM), located behind the passenger side dash liner, contains a microprocessor and separate electronic component circuits that interface with various input and output devices of the engine management system.