Those relays are famous for failing
I have a '92 F-SuperDuty w/ a mechanical 7.3 and a 5 speed. This is a computer-free truck.
There is a glow plug controller at the rear of the intake. It has a relay mounted to it that is very similar to a Ford starter relay. One of the large terminals on this relay has battery voltage at all times. The other large terminal is connected to the glow plugs, and it has 6.5 to 7 volts when the key is first turned on.
When my glow plugs were working, I would see a big drop on my dash voltmeter for a few seconds when the key was first turned on, then it would come up to normal, then cycle every few seconds if you didn't start the truck. Now, there is no drop on my volt meter, which suggests that the large glow plug load is not being applied.
This suggests to me that the problem is in this relay, since that is where the big voltage drop is. I wanted to run this by those of you who are more familiar w/ these engines than I am before I changed this relay.
I just got this truck this summer, and have no experience w/ light duty diesels.
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Those relays are famous for failing
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Steve answered the direct question but I thought I would add some tidbits since you are likely to revisit this system sooner or later.
It is an annoying system where every single connection must be in perfect shape or it fails to work.
It works by sensing the voltage drop across the big flat wavy bar that is part of it, and as the plugs heat, the current decreases and the voltage drop decreases. So if you have a loose connection from the battery to the glow plug anywhere it will think that the plugs are already hot, and cut off power to them. And the truck won't start.
It is at least better than the 6.9 system that failed in the on position, sometimes leading to burning and swelling a glow plug tip and wrecking a piston.
The glow plugs are well known for burning out too. Check them for open circuit(burnt out) with a circuit tester clipped onto the positive battery cable. If the bulb doesent light, the glowplug is toast.
I thought that the 7.3's didn't come out until 94? I had a 95 7.3 (that I wish I had kept - traded it with 325,000 miles on it) that was definetly not a completely mechanical system so this may not apply at all to Mitch504's question. I had to replace glow plugs occasionally and teseted them as mike in idaho suggested. I also had to replace the valve cover gaskets which had the wiring running through the middle of them.
As Steve mentioned, the relays are prone to failure. Often times you can swap relays with another pretty quickly to determine whether or not that is the cause. These are all methods that one who is not a mechanic (don't posess the equipment or knowledge to utilize more proffessional diagnostics) can use to help find the problem. My 95 had a glow plug relay right on top of the engine which was easy to test with test light to see if it was receiving the signal to connect, and if it did indeed make the connection.
The power stroke didn't come out until 94. There was a 7.3 IDI and a 7.3 IDI Turbo.
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The boss bought a (well) used '94 powerstroke 3/4 ton several years back. Had to change the turbo first thing, later rewire the glowplug circuit, another new turbo after that. It's parked behind the shop right now with a holed piston. One thing the 7.3 powerstrke motors will do is burn up the glow plug harness, under the valve covers. New wire harness pieces from Ford are scary expensive, fortunately an affordable repair harness can be had at the NAPA store.
Ah, thanks Austin G, I remember the "Powerstroke" distinction now.
Mitch, you have the 7.3 IDI engine as you know, not a powerstroke as others are suggesting. It does sound like the GP controller is not working properly. Even if a couple of gp's are bad, it should cycle, just for a very short period of time. As stated, the GP's can be checked with a continuity tester or ohm meter. Keep in mind when replacing glow plugs you only want to use the Motorcraft/Beru plugs. the Autolites and the Bosch one are prone to premature failure and will also swell over time causing them to become stuck in the head when you need to remove them at a later date.
Oliburners.net is the best site going for the 6.9/7.3 IDI engine. You can find all the information you could imagine over there. Good luck with yours and keep us posted on your progress.
another good idi forum, http://www.oilburners.net/forums/for...3L-IDI-Diesels glow plugs are different as are the controllers, there are some that have proven to be more reliable. I can't remember which ones they were but the guys on here have the info.
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IIRC there is a mod you can do that evolves switching to a starter relay and going to 12v GPs. I think it was the bosh ones that where the best. It s pretty common trick so you should be able to find info on one of the Powerstroke sites.
A '92 has 12v plugs already. It was the 6.9s except for the very last ones that had 6v plugs. For what it's worth I liked the 6v system better when it was working right. It heats up much faster. The 12v 7.3 system is smoother starting, though. Maybe the longer heat time makes it that way.
All good info. My 1990 with an aftermarket banks turbo is having the same problem. I have somewhere to start.
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They are not too hard to figure out if you have a voltmeter and know how to use it. A clamp on ammeter is nice, too. The trouble with them is that they are too blasted sensitive to every little thing going wrong that will take down the whole system until repaired.
My old 6.9 had only 3 glow plugs working for many years and it always started immediately unless at high elevation and very cold and then it would still start fine, just caused a wall of smoke. Under normal conditions the 3 cylinders firing would pick up the other ones so fast you couldn't even tell. If you have a 7.3 system with any glow plugs burned out, you have a no start condition because the whole system collapses due to lower current draw than it's expecting.