Powertrain Control Module
The powertrain control module (PCM) is located in the engine compartment. The PCM is the control center of the vehicle and controls the following systems:
- The fuel metering system
- The transmission shifting
- The ignition timing
- The 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. The PCM can recognize operational problems and alert the driver through the malfunction indicator lamp (MIL) when a malfunction has occurred. When a malfunction is detected, the PCM stores a diagnostic trouble code (DTC) which helps to 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 and switches. This is done through resistances in the PCM. The resistance is so high in value that a test lamp does not illuminate when connected to the circuit. In some cases, even an ordinary shop voltmeter does not give an accurate reading because the voltmeters resistance is too low. Therefore, a DMM with a minimum of 10 megaohms input impedance is required to ensure accurate voltage readings.
The PCM controls output circuits such as the fuel injectors, the idle air control (IAC), and the cooling fan relays by controlling the ground or the power feed circuit through transistors or a device called an output driver module (ODM).