Help testing glow plug system
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Laughing Gas
Joined: Jan 2010
Posts: 854
Likes: 17
From: Florida
Help testing glow plug system
So I think I have a GPCM failure. I am getting p0678. I have read up on this, but have gotten myself pretty confused. From what I have read the odds of it being the #8 glow plug are small. Most likely is the GPCM, then the glow plug harness, then the glow plug (given this is the only code).
Yesterday I tried to do some testing and that was where the confusion started. Can anyone explain the algorithm or give me a link to the tech algorithm. I would like to start with the most likely and easiest to test first.
How do I test the GPCM?
I know the glow plug test - 0.5-2 ohms resistance. How can I safely remove the glow plug harness without the special ford tool? I probably need the wheel and wheel well off for #8, right?
I have contemplated just changing out the GPCM as a "test" method. If that does not work, then new glow plug. If that does not work, time for the pros.
Any help is appreciated!!
Yesterday I tried to do some testing and that was where the confusion started. Can anyone explain the algorithm or give me a link to the tech algorithm. I would like to start with the most likely and easiest to test first.
How do I test the GPCM?
I know the glow plug test - 0.5-2 ohms resistance. How can I safely remove the glow plug harness without the special ford tool? I probably need the wheel and wheel well off for #8, right?
I have contemplated just changing out the GPCM as a "test" method. If that does not work, then new glow plug. If that does not work, time for the pros.
Any help is appreciated!!
Fleet Owner
Joined: Nov 2003
Posts: 27,872
Likes: 3,592
Check your resistance at the red wire on the black GPCM connector. That is the circuit for the #8 glow plug. Each plug should read 0.1 to 2 ohms resistance. If that is ok, then you need a module. If you have a friend with a 6.0 then you can swap the module with theirs and see if the code goes away, otherwise:
GPCM is $114 from Tousley: YC3Z-12B533-AA (It is common for a bad GPCM to throw a single #8 GP code)
To remove the black GPCM connector, you will need to unplug the ICP sensor. The plug has a release on the side you will need to depress so you can pull it.
Drivers side glow plugs should be accessible from the top - by just pulling the factory air box and the degas bottle for the back ones.
I have read that the tools below work as well as the factory harness removal tool:
1/2in crows foot tool
Sparkplug boot remover tool (SPB1)
GPCM is $114 from Tousley: YC3Z-12B533-AA (It is common for a bad GPCM to throw a single #8 GP code)
To remove the black GPCM connector, you will need to unplug the ICP sensor. The plug has a release on the side you will need to depress so you can pull it.
Drivers side glow plugs should be accessible from the top - by just pulling the factory air box and the degas bottle for the back ones.
I have read that the tools below work as well as the factory harness removal tool:
1/2in crows foot tool
Sparkplug boot remover tool (SPB1)
Thread Starter
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Laughing Gas
Joined: Jan 2010
Posts: 854
Likes: 17
From: Florida
Mark - thanks!!!!
Congratulations by the way.
Ok, here comes a stupid question - I pulled the black connector on the GPCM and put a probe in the #2 hole which corresponds to the #8 glowplug circuit - Where does the other probe go? I put it on the battery negative and got 120 ohms. So is that a bad glow plug or did I just screw up the test?
I'm thinking the easiest thing is to stop thinking and replace the GPCM.
For the tool, did you mean a 1/2" crows foot? I got one of those
Congratulations by the way.
Ok, here comes a stupid question - I pulled the black connector on the GPCM and put a probe in the #2 hole which corresponds to the #8 glowplug circuit - Where does the other probe go? I put it on the battery negative and got 120 ohms. So is that a bad glow plug or did I just screw up the test?
I'm thinking the easiest thing is to stop thinking and replace the GPCM.
For the tool, did you mean a 1/2" crows foot? I got one of those
Post Fiend
Joined: Aug 2010
Posts: 13,710
Likes: 5
From: Manchester, CT
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Fleet Owner
Joined: Apr 2012
Posts: 27,307
Likes: 549
From: Spanaway
Miles,
You may want to read here. I posted a description of what each
wire on the GPCM goes to and the plug numbers. There is also
some testing info
https://www.ford-trucks.com/forums/1...ce-manual.html
Sean
You may want to read here. I posted a description of what each
wire on the GPCM goes to and the plug numbers. There is also
some testing info
https://www.ford-trucks.com/forums/1...ce-manual.html
Sean
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Thread Starter
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Laughing Gas
Joined: Jan 2010
Posts: 854
Likes: 17
From: Florida
In my case it was the #8 glow plug. I know everyone goes on and on about testing and I agree that is probably the proper way to do it - diagnose then fix. However, it will probably take you a lot less time to just replace the GPCM and #8 glow plug and call it a day. The testing is probably easy if someone shows you how to do it, but figuring it out and getting to these wires is a bit of a PITA.
I would try replacing the #8 glow plug first ($10). See if the code goes away. If it is still there, then do the GPCM - $110.
Just last weekend I got a P0674 - a #4 glow plug. Replaced it and all was good.
Be gentle and only pull straight up on the glow plug harness - use the wire loop method
I would try replacing the #8 glow plug first ($10). See if the code goes away. If it is still there, then do the GPCM - $110.
Just last weekend I got a P0674 - a #4 glow plug. Replaced it and all was good.
Be gentle and only pull straight up on the glow plug harness - use the wire loop method
Thread Starter
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Laughing Gas
Joined: Jan 2010
Posts: 854
Likes: 17
From: Florida
BTW I am not knocking testing - I just could not figure out how to do it easily. I spent more time on that then just replacing the parts. I was really not that concerned about replacing a GPCM that was 7 yrs old either. I am not a tech and I do a much better job when I don't try to pretend to be one (according to my initial "testing" the GPCM was broken and the #8 glow plug as fine).
Post Fiend
Joined: Aug 2010
Posts: 13,710
Likes: 5
From: Manchester, CT
Alright, I'll have to replace that glow plug. Hopefully I don't break the rail lol, it's fairly new. I have a fluke meter with an inductive ammeter for work so I might try testing it anyways
Thanks guys
Thanks guys
Thread Starter
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Laughing Gas
Joined: Jan 2010
Posts: 854
Likes: 17
From: Florida
Wrap a 16 gauge wire around the GP harness and twist the wire until it is tight around the harness, then pull straight up - it is held in with an o-ring and will require a fair amount of pull. The key is firm and only up - do not rock back and forth. A deep 10mm socket is a must - it helps to have a 1/4 in socket for this. My 3/8 socket barely fit.
GP goes in 14 ft/lbs. Don't over tighten.
GP goes in 14 ft/lbs. Don't over tighten.
Fleet Owner
Joined: Apr 2012
Posts: 27,307
Likes: 549
From: Spanaway
I know that you not knocking testing. But with that info I posted in that other thread and
a multimeter you can test a plug in less than 5 minutes and if you happen to have an
inductive amp meter also you can also check the amp draw in a few minutes more.
That being said all the testing in the world won't fix a bad plug,wire or GPCM.
If it has to be replaced then it has to. But if you not sure if it is the GPCM
or the plug then testing is needed to pinpoint it.
Sean
I am not the type that uses the shotgun method of repair that often
a multimeter you can test a plug in less than 5 minutes and if you happen to have an
inductive amp meter also you can also check the amp draw in a few minutes more.
That being said all the testing in the world won't fix a bad plug,wire or GPCM.
If it has to be replaced then it has to. But if you not sure if it is the GPCM
or the plug then testing is needed to pinpoint it.
Sean
I am not the type that uses the shotgun method of repair that often
Thread Starter
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Laughing Gas
Joined: Jan 2010
Posts: 854
Likes: 17
From: Florida
I can never seem to figure out the numbers on the plugs without confusing myself. If I saw a good video on it, I would be good to go, but when the computer is inside and the truck is outside and I'm running back and forth trying to figure out pin 1 is gp 5 and am I on the green or black connector - I go nuts!
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