(1983-1994 Ford Glow Plug Relay)
This relay is a common point of failure. The glow plug relay is located on top of the engine right behind the fuel filter housing and slightly to the passenger side. If there are two relays located there, it will be the relay toward the back of the vehicle with large yellow and brown wires leading to and from it.
To test the relay you want to check the voltage at both large terminal posts right after turning on the truck. The glow plug relay should activate for 10 seconds or more, no matter what the temperature, longer when it is colder. You might wish to have someone turn the key to start while you have the meter on the terminals. Place the ground probe on a bare piece of metal somewhere on the engine and the source probe on each terminal.
Both terminals should read around 12 volts. If one reads that and the other reads zero, you next want to check if the control module is bad or if it is the relay itself. It will most likely be the relay.
To test directly for function of the relay and make sure the problem isn’t the control module, you will need to apply battery voltage across the two small terminals on the top of the relay. At this point, if the relay is good, you should have 12 volts across both terminals. If the relay fails both of these tests you will have to replace the glow plug relay.
You should remove the negative battery cable from the batteries before any repairs. This will prevent any accidents caused by working with hot wires.
Replacement of the glow plug relay is quite straightforward, just be certain to pay attention to which wires go to which terminals and don’t get them crossed up. Remove the four wires on top of the relay. The relay is held to the engine with two nuts. Remove the two nuts and lift out the relay. Put the replacement relay in, replace the nuts holding it in place and reattach the wires. And you’re all done.