General System Description

PCM:

DESCRIPTION
The Powertrain Control Module (PCM) is located in the engine compartment. The PCM is the control center of the vehicle. It controls the following:

^ Fuel metering system.
^ Transmission shifting.
^ Ignition timing.
^ On-board diagnostics for powertrain functions.

OPERATION
The PCM constantly monitors 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. The PCM also alerts the driver through the MIL (Malfunction Indicator Lamp). When the PCM detects a malfunction, it stores a diagnostic trouble code (DTC). A DTC stored, will identify the problem areas. This will aid the technician in making repairs.

The PCM supplies either 5.0 or 12.0 volts to power various sensors or switches. This is done through resistances in the PCM. The resistance is so high in value that a test lamp will not illuminate 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 digital voltmeter (J 39200) with at least 10 megaohms input impedance is required to ensure accurate voltage readings.

The PCM controls output circuits such as the injectors, IAC, cooling fan relays, etc. by controlling the ground or the power feed circuit through transistors or a device called an Output Driver Module.

USE OF CIRCUIT TESTING TOOLS
Do not use a test lamp in order to diagnose the Powertrain electrical systems unless specifically instructed by the diagnostic procedures. Use the Connector Test Adapter Kit, J 35616 whenever diagnostic procedures call for probing any connectors.

PCM SERVICE PRECAUTIONS
The PCM is designed to withstand normal current draws associated with vehicle operations. Avoid overloading any circuit. When testing for opens or shorts, do not ground any of the PCM circuits unless instructed. When testing for opens or shorts, do not apply voltage to any of the PCM circuits unless instructed. Only test these circuits with a digital voltmeter J 39200, while the PCM connectors remain connected.