Powertrain Control Module: Notes

For exact location of Powertrain Control Module (PCM), see PCM LOCATION in appropriate SELF-DIAGNOSTICS article or COMPONENT LOCATIONS in appropriate SYSTEM & COMPONENT TESTING article. On some models, the PCM may be referred to as an Electronic Control Module (ECM). Although the 2 units may process different signals, the 2 terms are interchangeable.

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 looks at 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 Malfunction Indicator Lamp (MIL) when a malfunction has occurred. When the PCM detects a malfunction, it stores a DTC which will help identify problem areas. This is done to aid the technician in making repairs.

The PCM supplies either 5 or 12 volts to power various sensors and switches. This is done through transistors in the PCM. The circuits have very high impedance and typically will not illuminate a test lamp 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 in comparison with the input impedance of the circuit being probed. Therefore, a DMM with at least 10 megohms input impedance is required to ensure accurate voltage readings.

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