Engine Control Module: Description and Operation

Powertrain Control Module (PCM):

PURPOSE
The Powertrain Control Module (PCM), located under the instrument panel, is the control center of the vehicle. 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 also 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 diagnostic trouble codes which identify the problem areas to aid the technician in making repairs. For service, the PCM consists of two parts: a controller (the PCM without a PROM) and an assembly called a Programable Read Only Memory (PROM).

PROM
This assembly contains the functions of the PROM, Transmission Position Switch (PRNDL) decoder, Vehicle speed Sensor (VSS) buffer, cruise control module, and Electronic Spark Control (ESC) module used on past GM applications. It contains the calibrations needed for a specific vehicle based on axle ratio, tire size, and other vehicle characteristics.

OPERATION
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. Tool J 39200 meets this requirement.

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