Engine Control Module: Description and Operation

Electronic Control Module (ECM):

PURPOSE
The ECM is a computer that judges air/fuel ratio as rich or lean according to oxygen sensor output voltage. Based on oxygen sensor and other input data, the ECM generates a controlling signal to the fuel injectors that regulates "ON" time. Because fuel pressure remains constant, the ECM controls air/fuel ratio adjustments by varying injector "ON" time.

LOCATION
Behind L.H. kick panel.

OPERATION
When operating in "Open Loop," the ECM defaults to preprogrammed values for air/fuel ratio control. The system is in "Open Loop" before the oxygen sensor has warmed up to operating temperature. When operating in "Closed Loop," the ECM continually adjusts air/fuel ratio according to oxygen sensor input and operating information received from various other sensors.

This system incorporates self-diagnostic capability that is monitored via a dash mounted "Check Engine" light. When the system detects a fault in a monitored component and/or circuit, the light will come on and stay on (while the engine is running), alerting the driver to seek service. With intermittent problems, the light may come on and then go out, but a trouble code will be stored in ECM memory if correct parameters are met. The technician can then extract trouble codes from the ECM that will aid in the performance of efficient troubleshooting and repair.

PROM
To allow one model of ECM to be used for different vehicles, a device called a programmable read only memory (PROM) is utilized. The PROM is located in the ECM and contains information on vehicle weight, engine, transmission, axle ratio, etc. Because the PROM is vehicle specific, it must be be transferred to a new ECM if the original is being replaced.

CAL-PAK
The CAL-PAK is used to allow fuel delivery if other parts of the ECM are damaged. It has an access door in the ECM, and removal replacement procedures are the same as with the PROM. If the CAL-PAK is missing, the vehicle will not start. Because the CAL-PAK is vehicle specific, it must be be transferred to a new ECM if the original is being replaced.