A PCM relay is an electrical device used to switch power on and off to the powertrain control module.
Today’s vehicles rely heavily on computer controls. Depending on the vehicle make and model, the main computer that controls engine function is the engine control module, or ECM. Some manufacturers, however, include the transmission and other powertrain components under the control of the computer. In those cases, where the engine and transmission are both monitored and controlled together, a powertrain control module, or PCM, is used.
The PCM is a rather large electronic device with, in some cases, more than a hundred wires connected to it that lead to all areas of the engine and transmission. Due to the immense amount of functions of the PCM, it uses a considerable amount of electricity. The PCM is powered on when you turn the key or press a button to start the engine, but only indirectly so. That is because all of the power required for the PCM would overload the ignition switch. Therefore, a relay - the PCM relay - switches the PCM on and off instead.
As is the case with other relays, the PCM relay is essentially a large capacity switch that is “switched” by a smaller capacity circuit. In this case, a relatively small electrical current from the ignition switch is able to turn on and off the larger PCM circuit through the relay.
A PCM relay is generally located inside of a weather-sealed fuse and relay box or “block”. Replacement of a PCM relay is a relatively simple procedure. In order to replace the device, a technician will typically need to remove any components blocking access to the fuse block, remove the water-tight cover, identify the proper relay, and remove the relay with a puller tool. The new relay is plugged into the same space in the fuse/relay block, and the cover is reinstalled. The technician will likely verify the repair by road testing the vehicle and checking for any PCM-related diagnostic trouble codes with a scan tool.
If the PCM relay is bad, the likelihood is that the vehicle’s main computer, the PCM, will not get power. When that occurs, any sensor, module, or system that relies on the PCM will be unable to function. Because of this, a bad PCM relay can lead to any number of issues, the most likely of which include:
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