Glow Plug Control Module
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Lead Driver
Joined: Jun 2004
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From: Amarillo, TEXAS!
Glow Plug Control Module
This one has me a little confused. Late 2004 F550 had codes for all 8 glow plugs. No start when cold. No codes for the module itself. I went through the pinpoint tests and all the continuity tests are good and no voltage where it shouldn't be. Replaced the module and the truck started repeatedly with no problem. Cleared codes and none returned. Ran the glow plug system self test, which completed successfully with no codes.
That was yesterday. This morning, it was a cold no start again, and codes for all 8 glow plugs were present but again no module codes. I'm at the point in the pinpoint tests that says "Replace PCM," but I wanted to see if anyone else has encountered this and how you resolved it.
That was yesterday. This morning, it was a cold no start again, and codes for all 8 glow plugs were present but again no module codes. I'm at the point in the pinpoint tests that says "Replace PCM," but I wanted to see if anyone else has encountered this and how you resolved it.
Hotshot
Joined: Apr 2010
Posts: 19,061
Likes: 238
From: Saratoga Springs,UT
Seems like electrical problems seem to show up when it gets cold
Whats you using for code reader
what happens if you move harness around it worked day before but the harness and connection were moved around some maybe temp is playing into this and getting open circuit in colder parts of day
Whats you using for code reader
what happens if you move harness around it worked day before but the harness and connection were moved around some maybe temp is playing into this and getting open circuit in colder parts of day
Last edited by BLADE35; Dec 4, 2025 at 04:33 AM.
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Lead Driver
Joined: Jun 2004
Posts: 6,343
Likes: 589
From: Amarillo, TEXAS!
I was using a SnapOn Modis Ultra, but I have my laptop with me today so I can use ForScan. I've wiggled and waggled, backpin probed and removed and reinstalled the module. I have a spare PCM Imma swap in today to see what happens.
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Lead Driver
Joined: Jun 2004
Posts: 6,343
Likes: 589
From: Amarillo, TEXAS!
I put my spare PCM in and same results. Retested all the circuits and they all seem to be good. The PCM is commanding glow plugs for 30000 ms. I can see serial data, but have no way of decoding it. I'm starting to lean toward a bad new module, even though it's Motorcraft.
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Lead Driver
Joined: Jun 2004
Posts: 6,343
Likes: 589
From: Amarillo, TEXAS!
It's from a local parts supplier. I know the parts they sell are genuine. At any rate, the new module is going to be installed on a different truck later. That should give me a definite answer.
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Hotshot
Joined: Apr 2010
Posts: 19,061
Likes: 238
From: Saratoga Springs,UT
A bad new GPCM sure would complicate things
When I look at wire diagram for glow plug I can’t tell or remember if there is a fusible link going to battery or not
I suspect there is a fusible link
have you done any testing with DVM on power supply
battery cables clean tight
Check fuse F-22
When I look at wire diagram for glow plug I can’t tell or remember if there is a fusible link going to battery or not
I suspect there is a fusible link
have you done any testing with DVM on power supply
battery cables clean tight
Check fuse F-22
Last edited by BLADE35; Dec 5, 2025 at 08:24 AM.
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Joined: Jun 2009
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From: Michigan
The glowplugs get their power from the positive battery post on the passenger side battery, it’s a stud with a 10mm nut. I would look at that and make sure the connection and cable are good.
I know if that cable isn’t connected it will set codes for all 8 glowplugs.
I know if that cable isn’t connected it will set codes for all 8 glowplugs.
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Lead Driver
Joined: Jun 2004
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From: Amarillo, TEXAS!
The glowplugs get their power from the positive battery post on the passenger side battery, it’s a stud with a 10mm nut. I would look at that and make sure the connection and cable are good.
I know if that cable isn’t connected it will set codes for all 8 glowplugs.
I know if that cable isn’t connected it will set codes for all 8 glowplugs.
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Lead Driver
Joined: Jun 2004
Posts: 6,343
Likes: 589
From: Amarillo, TEXAS!
Alright, this is resolved. The root of the problem was my multimeter was giving me bad readings on the glow plugs. I was getting readings that were in spec on 7 of the 8 glow plugs when in reality they were reading close to a dead short. When I started to question the readings, I borrowed a meter and checked them again. Replaced all 8 glow plugs (and my multimeter, lol) and everything is now working. This is officially the first time in my life I've seen codes for all 8 glow plugs at once and they were actually valid.
FYI: the glow plugs in the engine were junk Autolite, and they were all installed barely finger tight . . .
FYI: the glow plugs in the engine were junk Autolite, and they were all installed barely finger tight . . .
Hotshot
Joined: Apr 2010
Posts: 19,061
Likes: 238
From: Saratoga Springs,UT
Them multimeter are touchy
I fried 1or2 of them multimeters before
turns out they don’t like power feed to them on certain settings they were cheap unit higher end meter may come with protection
I fried 1or2 of them multimeters before
turns out they don’t like power feed to them on certain settings they were cheap unit higher end meter may come with protection
Last edited by BLADE35; Dec 8, 2025 at 07:51 PM.
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