Glow Plugs staying on

I had a problem a year and a half ago where my glow plugs got stuck on. They stayed on for half an hour at idle while I was stuck in the snow. It ruined my batteries, alternator, melted the GPR, and just irritated me beyond belief. I had just done the UVCH a month previously with good working glow plugs for a month. The plugs were a month old and were the motorcraft ones. When I came back out to the truck (stuck in snow at the time) after half an hour the truck was off and batteries dead. I assumed the alternator went out and pulled it off to the side and connected my large battery charger to it and left it alone for the weekend. The thing was still drawing 100 amps when I came back to it two days later.

I replaced the batteries, radio (not sure how that got ruined), alternator, and glow plug relay after that. I thought I had it working well but it was still staying on after that. I disconnected the GPR and have left it like that for a year and a half. I've been driving my excursion in the winter and using ether when this thing has to start. I'm not exactly proud of that but it's what I did.

Anyway, my GPR is grounded on both terminals with the truck shut off. The GPR is a ford unit and is not case grounded. When I turn the key on, one lead goes to 12v and the other stays grounded. After I start the truck it goes to 13.5 volts and stays on for a while. One side stays at 13.5 volts and the other goes to 12v. The relay stays on. I took the first relay I changed out back to ford as defective and exchanged it with one that does the same thing.

When the relay is disconnected from the PCM the one side stays at 13.5 and the other goes to 0v, grounded. This seems proper. I spoke with someone today that recommended disconnecting the UVCH leads and seeing if it acts the same. I'll try that. I don't think it will work but I'll try it...

Does anyone else have input on what to try? I was going to replace the PCM with another one, but that seems to be the only thing left.

Thanks in advance!

You know what might be the easiest if you do not mind would be a manual switch. The PCM controls the relay by supplying the ground FYI.

Tom S said:

You know what might be the easiest if you do not mind would be a manual switch. The PCM controls the relay by supplying the ground FYI.

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I've thought about that. I'd rather not, but will if I have to.

Where does the hot lead come from? Does it come from the PCM too? What is the switched lead supposed to do when it's de-energized? Does it just go dead or to 12v?

I will try and research this for you tonight.

From what I remember the wire going to the pcm goes to an open circuit when it turns off. I would look for a chaffed wire from the relay to the pcm that is grounding out somewhere. If you do not find that I would say that output on the pcm is bad.

The hot side wire comes from fuse 22 which is the same as the fuel heater and I think PCM power. It is Red with a Light Green tracer in my book.

OK, so the hot is switched by a relay and the other is grounded and goes open right??

If so I guess I'm looking for a relay that's getting backfed through the wiring harness. Do you agree? The other wire should go to 13.5v due to the voltage from the other side of the coil when the switched lead goes open, correct?

I can not remember for certain but I think the hot wire going to the relay is hot all the time.

No the hot will be hot all the time. The PCM takes other terminal to ground by my read of the diagram. If need be I can toss a test light on mine to verify. I know the relay is ground switched.

Tom S said:

No the hot will be hot all the time. The PCM takes other terminal to ground by my read of the diagram. If need be I can toss a test light on mine to verify. I know the relay is ground switched.

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Thank you for confirming that Tom I was pretty sure that was how it worked. It has been a while since I messed with a glow plug issue.

I looked some more and that circuit is key controlled then through fuse 22 underhood to power the glow, plugs, fuel heater, PCM, etc.

Tom S said:

I looked some more and that circuit is key controlled then through fuse 22 underhood to power the glow, plugs, fuel heater, PCM, etc.

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That follows what I've got on the hot side.

OK, so on the other side it's down to a chafed wire/backfed relay, or a bad PCM. After looking all this over I am leaning towards a chafed wire. Can anyone check their truck and tell me what the voltage difference between the two posts and from the switched lead to ground is? This would be when the truck is running and the glow plugs are off.

Someone correct me if I'm wrong, but since there is no current through the coil, the voltage on the two posts should be the same, right? The coil will just act like a piece of wire until the contact is closed, and then there will be a voltage drop across it.

My suggestion is to check out the wire that the pcm supplies the gound to. Disconnect it at the glowplug relay and also disconnect the 104 pin (pcm) at the firewall. Use the DVOM to see if that wire is shorted to ground.

Alright Tom, I got to looking at mine. My post is grounded all the time, even when the truck is off. It is not grounded when the PCM is disconnected, so the problem must be in the PCM.

Thanks!

I bet the PCM has a little relay in there that gets jacked up. I need to look the next time I have one opened up.