General 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. The PCM constantly monitors the information from various sensors and controls the systems that affect vehicle performance and emissions. The PCM also performs the diagnostic functions for those systems. It can recognize operational problems and will alert the driver through the MIL (Malfunction Indicator Lamp) when a malfunction has occurred. When the PCM detects a malfunction, it stores a diagnostic trouble code (DTC) which will help identify problem areas. This is done to 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 a minimum of 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.