Where Is the Glow Plug Module on a Ford F-350 Powerstroke?

The glow plug control module or relay is responsible for turning the glow plugs on and off so that you can start your diesel engine when it's cold. Unlike a gasoline engine, the diesel engine does not have spark plugs, so the fuel can't get hot enough to combust when the engine block is very cold. When sensors inside the engine show that it's warm enough, the glow plug relay or control module turns the glow plugs off.

A normal glow plug relay only controls the function of the glow plugs. A glow plug control module is a requirement of California emissions regulations. It communicates with the powertrain control module (PCM) in addition to its normal duty of controlling the glow plugs. The glow plug control module senses faults in the glow plug system or individual glow plugs, and sends the data to the PCM, where it can be read with diagnostic tools.

You can test the basic function of a glow plug relay with a test light. If you attach one test lead to each of the two large terminals, the test light should come on when you turn the key in the ignition. One of the two big terminals connects to the battery. When one test lead connects to the other big terminal and the second test lead connects to a ground, the light should once again come on when you turn on the ignition.

To replace the glow plug relay or control module, remove the wiring harness (in the case of the glow plug control module). Mark the position of the four wires attaching to the two large and two small terminals and remove the wires. Unbolt the relay or control module from the valve cover. Bolt the replacement unit into place, re-attach the wires to the terminals and plug in the wiring harness, if it is present.