What is a PCM?

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Purpose:

Explain what a PCM is and its purpose.

What is a PCM?

PCM stands for Powertrain Control Module. Basically, this is the brain of your vehicle.  It's generally a combined controller consisting of the Engine Control Unit (ECU) and Transmission Control Unit (TCU). On some vehicles even, there are multiple computers for a total of three separate computers. ECU, TCU, and the Body Control Module (BCM).

For example, the PCM receives signals from sensors that measure air flow into the engine and out the exhaust, the coolant temperature, how far down the accelerator pedals is, the speed at which each wheel is turning and other parameters. Then, the PCM uses that information to make several decisions per second, such as how much fuel to inject into each cylinder, when to fire the spark plugs, and when an automatic transmission should shift to a different gear to deliver the best performance for the current conditions.

As fast and intelligent as PCMs can be, they can be dumb as bricks when things go wrong. For example, if an oxygen sensor goes out, the PCM will not be able to figure out how to adjust the air to fuel mixture going into the engine. The check engine light will probably be on, and the engine might run roughly, have slow acceleration or other issues. Also, if the PCM fails, the engine won’t start.

PCMs rely heavily on inputs from other sources. PCMs sometimes are falsely accused of causing a check engine light or engine problem. Even if the diagnostic code for a check engine light points to the PCM, mechanics test PCMs, and often check inputs to make sure it's the issue before replacing it.  

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