(1983-1994 Ford Glow Plug Relay/Controller)
Glow Plug Controller – Theory of Operation:
The controller consists of a four post automotive starter type relay that is bolted on top of a small black box (see above). Connecting the black box and the relay is a flat metal resistor. The black box contains a small electrical circuit board that has been sprinkled with a non conductive material, and then sealed. There are a total of six wires coming out of the black box.
The controller monitors the resistance in the glow plug circuit. Assuming that all the wires in the circuit are good and not rubbed through the insulation in any spots, there are just a couple things that change the resistance of the circuit:
The first thing that changes the resistance in the circuit, and the thing that the controller was designed to look for, is the heating of the glow plugs. As the plugs heat up their resistance increases, this holds true for any wire or metal with current flowing through it. When the resistance reaches a certain level the controller stops sending power to the plugs because they are hot enough for starting.
The second thing that causes the resistance of the system to change is burnt glow plugs or good plugs with a higher resistance. If there is one or more glow plugs burned out, then the controller sees a higher resistance in the circuit. The higher resistance causes the controller to think that the plugs are hot enough to start, even though they are not, and it will short cycle or not cycle at all. Remember that this is assuming the wiring in the circuit is in good shape, because a shorted wire will also change the resistance and cause the controller to think that the plugs are hot enough to start when they are not.
The problem with the factory controller logic is that it only takes one burned out glow plug to cause the system to think that the plugs are hot enough to start. It does get some input from a temperature sensor located near the water pump, but this sensor only tells it if the truck is hot or not, it doesn’t tell it the source of the resistance in the system. Therefore, the resistance in the circuit is the controller’s only way of knowing how long to supply power to the plugs.
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