What is a PCM/ECM-Power Relay?
The ECM, or electronic control module, also known as the PCM, or powertrain control module, is the computer responsible for controlling all of the engine management functions. It is the central control unit of most vehicles and controls a variety of functions, such as electrical system power distribution, emissions, ignition, and fuel systems. It is a very important, if not the most important component of a modern engine management system, and just the same as any other important electrical component it is powered through a relay.
The ECM-power relay is the relay responsible for sending power to the ECM. If the relay fails, or has any issues, it can cause the vehicle to have all sorts of problems, and may even render it undriveable. You may receive the P0685 trouble code, which means there is a malfunction with your vehicle's PCM-power relay circuit. Usually, a problem with the ECM-power relay will produce a few symptoms that can alert the driver of a problem that should be serviced.
1. No Power When Key is Inserted
One of the first symptoms of a bad PCM-power relay is no power when the key is inserted. If the PCM-power relay fails it can cut off power to the entire vehicle. A failed relay will disable the dash lights and warning chimes that normally come on when the key is inserted into the ignition, and will probably be unable to start or turn over the vehicle.
2. Engine Won’t Start
Another common symptom of a problem with the PCM-power relay is an engine that will not start or turn over. The fuel and ignition systems, as well as several other engine management functions, of many vehicles are controlled by the PCM. If the PCM or its relay fails, the entire engine management system will be left without power and will not function as a result. A bad PCM-power relay may cause the vehicle to crank, but not start, or sometimes not even crank at all.
3. Battery Drain or Dead Battery
The final bad ECM relay symptom is a battery drain or dead battery. If the relay shorts it can leave power on to the computer, even when the vehicle is turned off. This will place a parasitic drain on the battery, which will eventually cause it to go dead.
The ECM-power relay is one of the most important relays as it provides power for the vehicle’s computer system. Without it, the entire engine management system will be disabled and the vehicle will not run. For this reason, if you notice any symptoms of a bad ECM-power relay on your vehicle, have the vehicle inspected by a professional technician, such as one from YourMechanic, to determine if the relay should be replaced.
