Engine Control Module: Description and Operation

Electronic Control Module (ECM):

The Computer Command Control System is a computerized fuel, ignition and emission control system designed to maintain fuel economy, good driveability and reduced vehicle emissions. An Electronic Control Module (ECM) monitors data from various sensors and controls functions such as air/fuel ratio, emission control devices and on-board diagnostics.

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.

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.

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.

A device called a CALPAK is used to allow fuel delivery if other parts of the ECM are damaged. If the CALPAK is missing, it will result in a no-start and run condition. The CALPAK must be transferred to a replacement ECM.

ECM CONNECTIONS

ECM Connector Identification: