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I have a '95 NA diesel. The truck Doug spoke about was a '97 TD, I think.
The glow plug controller is located on top of the engine, passenger side, just aft of the windshield. It is a dark colored box, about the shape of a large deck of cards. It has two large (~1/4") studs on the top.
One of the large studs is the power feed. This is a fusable link from the starter motor. The other stud is the connection to the glow plugs. I believe that the forward-most stud is the one for the glow plugs.
The glow plug controller functions basically like a relay, but with some computer control. The computer reacts to temperature and other input, and tries not to burn out the glow plugs.
First diagnostic: check for 12V at the power feed terminal. It should be present, even with the ignition off. If there is no power, jumper from the battery to this stud. Be sure to use a large gauge wire (~ #10).
If there is power, the controller is malfunctioning. Using a heavy jumper wire (~ #10), apply power to the glow plug stud for a few seconds. Try to simulate what the glow plug controller would do. Don't just connect it up and leave it. After a few seconds, disconnect the wire and start the truck.
There are several low-current connections to the glow plug controller. I cannot recall if there is one or several connectors. (Sorry) Check them out, clean them, etc. before replacing the controller.
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