Engine Control Module: Description and Operation

Powertrain Control Module (PCM) Typical:

PROM Typical:

GENERAL DESCRIPTION

CAUTION: To prevent possible Electrostatic Discharge damage to the Powertrain Control Module (PCM), Do not touch the connector pins or soldered components on the circuit board.

The PCM, located under the instrument panel, is the control center of the fuel injection system.It controls the following:

^ Fuel metering system.
^ Transaxle shifting.
^ Cruise control operation
^ Ignition timing.

It constantly looks at the information from various sensors and controls the systems that affect vehicle performance. The PCM performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (MIL) (Service Engine Soon), and store a DTC or DTC(s) which identify the problem areas to aid the technician in making repairs. For service, the PCM has two parts: a controller (the PCM without the Programmable Read Only Memory (PROM)), and an assembly called a PROM.

CAUTION: To prevent possible Electrostatic Discharge damage to the PROM or PCM, Do Not touch the component leads, and Do Not remove integrated circuit from carrier.

Prom
This assembly contains the functions of the PROM, PRNDL decoder, Vehicle Speed Sensor (VSS) buffer, cruise control module, and ESC module used on past GM applications. It contains the calibrations needed for a specific vehicle based on axle ratio, tire side, and other vehicle characteristics.

PCM FUNCTION
The PCM supplies either 5 or 12 volts to power various sensors or switches. This is done through resistances in the PCM which are so high in value that a test light will not light when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, a 10 megohm input impedance digital voltmeter is required to assure accurate voltage readings.

The PCM controls output circuits such as the injector, Idle Air Control (IAC), cooling fan relay, etc. by controlling the ground circuit through transistors in the PCM or a device called a Quad-Driver.