Paul Gagnon said:
The glow plugs appear to be working fine, the glow plug relay shuts off I can hear it clicking.
Paul: Even though the glow plugs are getting juice to them, it is still possible that the plugs are burned out. On my 91 I used to change out all 8 every year. They not very expensive, I used to get a set of 8 for around $100. This is something I have saved on file for checking the glow plug circuit:
"Get you a 12 volt test light. Get a friend.
Now make sure your batteries post are clean. Take the clip end of the test light, and attach it to a suitable ground. Touch the positive post of the battery to make sure the test light works, and the ground is good. Go to the starter solinoid which is on the right fender. Make sure all the connections are clean and tight. Make sure the wire termanils are tight, and not corrodided. check with the test light for power. Next locate the glow plug relay. It should be on the rear of the engine, behind the air cleaner, I don't remember if it closer to the right or left head. The relay looks alot like the starter solinoid. If it has a protective cap over it remove it. There are 4 wire connections here. There are 2 big ones and 2 small ones. Make sure all the connections are clean and tight. Make sure under the wire connections, the nuts on the big postes are tight. Make sure all the wire termanels are tight on the wires. To make sure give them a little tug. Take the 12 volt test light and test for power at the big terminals. One should have power the other shouldn't. Hold your hand on the relay. Next have the friend, turn the key on. Do not start the engine. If the relay is working, you should feel it click or hear it. Take the test light and touch the other big termanil. Does the test light light? If so the relay is good. Otherwise it is not and time to replace it. Now you may have to turn the key on and off a couple of times to do these tests, depending on how fast you are. If the relay is good,take one glow plug wire off the front glow plug of each head. Place the test light point in the end of the termanils. Turn the key on again. If the test light comes on, on each glow plug wire you have removed, the circuit is good to this point. If one side lights and the other doen't, then the wiring harness to the side that didn't light, has blown it's fuseable link. Replace the harness to that side. Next step, remove all the wires to the glow plugs. I know you just change the glow plugs, but they are electrical things, and they do not like to much voltage. Take the test light clip and attach it to battery positive. Take the point of the test light and touch each glow plug at the terminal point. If the glow plug is good, the light will light, the glow plug is good. If no light, replace the glow plug. Make sure you removed the wires from the glow plugs. If you don't, all you need is one plug to be good, and it will lead you to believe all are. The glow plugs need to be tested seperatly.
Now the contoller. It does several things. Senses water tempature, Senses glow plug tempature, and retards the fuel pump timing and energizes the fast idle solinoid, and activates the relay. So the simple tests. If when you were testing the relay, and it worked. the controller is okay. If when you turn on the key and the fast idle solinoid jumps out and stays there when the throttle is depressed, the contoller is okay. If you try to drive the truck when it is cold, and it is in fast idle, and the engine feels like you have little control or it feels like you got alot of power all of a sudden, the contoller is okay, especially if the engine settles down, and returns to normal idle, the same. If you turn on the key, and the glow plugs only cycle once, or don't cycle at all, and all of the relay and glow plug tests are good the controller is not good. The last thing is the water temp sending unit. This will be in the head somewhere, I don't remember where. Actully you have 2. One is for the temp gauge on the dash. Make sure the wire connection is good to both of them. With your test light take the point and touch the termanil. With the clamp end to battery positive, with the engine cold, both should light the test light. Make sure you do this with the wire disconnected. After the engine warms up to about 140 degrees F. One tempature send unit should be open, and the test light should not light. On the other tempature sending unit, the test light will light but not be as bright, this is the sending unit to the tempature gauge. There are other tests with a volt/ohm meter. These meters cost more than the test light, but never the less the testing with them are more detailed. Along with all this a good book on the truck will lead you along the same path, and would cover some of the things I may have forgotten. I write this from memory, and it has been awhile since I have done this. Remember sometimes the simple problems can be the cheapest to fix. Good grounds and clean tight connections can eleminate alot of headache. "
How to test your glowplugs:
"If you have all the glowplugs out now is the easy time to test them, put a plug in a pair of pliers and ground one side of it to the vehicle then use a piece of # 10 or 12 Ga. wire and jump frome the hot terminal on the g/p relay to the G/P terminal and hold it there about 7 seconds and it should glow red if it does you have a good G/P they are really easy to check out of the engine.
Tip of GP to the base or to groung should be 1-2 ohms with V.O.M.
Tip of GP to positive on battery should give 12-13 volts on V.O.M.
Tip of GP to base that is open is bad!"