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.

 

Forgot to mention, the EOT, ECT and baro readings are all normal. I can also see the data stream passing between the PCM and GPCM with my oscilloscope.

 

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

 

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.

 

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.

 

I would agree it's the GPCM, where did you get it from?

 

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.

 

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

 

Yes, there are fusible links. Unless it’s an actual Motorcraft GPCM, I wouldn’t say it’s 100% not at fault.

 

I have power to both plugs, so the links are fine. I may have stumbled across a resolution, but I'm not sure yet. I'll update when I know one way or the other.

 

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.

 

Yeah, I've already verified all of that.

 

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 . . .

 

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