Great issues

[psduser1]

I'm checking voltage across my gpr, and its not right.
Power in is 12.8 ish on the main lug-battery voltage.
Turn the key on, the wts light is lit, and voltage on the scanguage drops to 11.8 ish, for about 2 minutes, just as it should.
Trouble is, with the key on, at the relay, I have 12v on both small posts, and 0.00 on the terminal to the glow plugs.
I've tried with a two year old stancor, and a new parts store relay, so I don't think its a faulty relay.
I've checked the harness to the junction between the heads, and all the wiring looks good. A quick look at the rest of the wiring to the 42 pin connector looks good, as well, short of opening up the whole loom.
What am I missing?

 

[6.0 Tech]

I'm checking voltage across my gpr, and its not right.
Power in is 12.8 ish on the main lug-battery voltage.
Turn the key on, the wts light is lit, and voltage on the scanguage drops to 11.8 ish, for about 2 minutes, just as it should.
Trouble is, with the key on, at the relay, I have 12v on both small posts, and 0.00 on the terminal to the glow plugs.
I've tried with a two year old stancor, and a new parts store relay, so I don't think its a faulty relay.
I've checked the harness to the junction between the heads, and all the wiring looks good. A quick look at the rest of the wiring to the 42 pin connector looks good, as well, short of opening up the whole loom.
What am I missing?

12v on both control sides of the relay indicates you have a bad ground. Something is not telling the pcm to ground the relay, the ground circuit to the pcm is open, or the ground in the pcm has failed for it. But it’s typically not the pcm.


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[co04cobra]

It’s not my first go to either for the Pcm but I have replaced quite a few through the years that the pcm was the cause.

 

[psduser1]

So whats the easiest way to check?
I think I've got an 01 pcm from the same family on the shelf, can I just swap?

 

[6.0 Tech]

Should be able to


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[co04cobra]

Yes you can use the 01.

 

[psduser1]

Thanks fellas, I try it out tonight.

 

[psduser1]

I swapped pcms, no change.
Swapped back to the original pcm, everything is same same.
I pulled both small terminals at the gpr, and I have continuity between the wires.
I disconnected the 42 pin connector, and no more continuity.
I pulled the harness apart between the 42 pin connector and the firewall, and found a bare wire in the shielded harness, which I assume is a ground?
It has a large area of rot where the loom had rubbed on the steering shaft, however the wire was intact, and had continuity across the corroded section. Guess I'll replace that while I'm in there.
I assume those wires will need to be shielded when I get it back together, also.
I guess I need to pull all the oe grounds and clean them as well?
Any other suggestions?

 

[6.0 Tech]

I swapped pcms, no change.
Swapped back to the original pcm, everything is same same.
I pulled both small terminals at the gpr, and I have continuity between the wires.
I disconnected the 42 pin connector, and no more continuity.
I pulled the harness apart between the 42 pin connector and the firewall, and found a bare wire in the shielded harness, which I assume is a ground?
It has a large area of rot where the loom had rubbed on the steering shaft, however the wire was intact, and had continuity across the corroded section. Guess I'll replace that while I'm in there.
I assume those wires will need to be shielded when I get it back together, also.
I guess I need to pull all the oe grounds and clean them as well?
Any other suggestions?

Ok, so with both the control sides (small wires) disconnected off the glow plug relay they have continuity between them, without them touching the gpr? That typically means they’re shorted together. However, goofy **** happens in the pcm, so you should unplug the pcm when doing the continuity check. Also, ohm readings should always be taken on an unpowered circuit, otherwise it reads wrong. I would need to look at a wiring diagram to tell you which pins on the pcm are for the gpr.

However, even with the screwy ohm readings, it could still be something not telling the pcm to turn the relay on. I don’t remember off the top of my head what that is, but I would guess it’s either iat, eot, or ect, or a combination of the 3 after it does a check to make sure one isn’t really off. (I can’t for the life of me figure out what one word I’m looking for to take the place of the 5 words I had to use on the description).


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[psduser1]

Yes, I tested the wires thru the pcm, and disconnected from the gpr, and had continuity.
Then I disconnected the 42 pin connector, and no continuity.
I did take a look at a wiring diagram-on the phone-and it looks like there is a fusible link in the signal wire. I'm not sure exactly on its physical location, yet. The bare wire i mentioned in my last post had what appeared to be a oe style soldered connection, and I'm wondering if thats a fusible link. Of course I didn't think to test continuity thru that. Also, that looked to be actual bare wire, as in, no burnt insulation, just corrosion from being exposed to the elements. I'll look deeper at that tomorrow.

 

[psduser1]

As far as sensors for the pcm to look at, my iat has been mia for 10+ years, lol.
I think the only temp the pcm looks at to control gps on a 7.3 is the eot. That controls the duration of glowing operation, up to two minutes, afaik. According to the scanguage oil temps are within 5-10* of ambient, at cold start, so I'm kinda ruling that out, for now.

 

[6.0 Tech]

Yes, I tested the wires thru the pcm, and disconnected from the gpr, and had continuity.
Then I disconnected the 42 pin connector, and no continuity.
I did take a look at a wiring diagram-on the phone-and it looks like there is a fusible link in the signal wire. I'm not sure exactly on its physical location, yet. The bare wire i mentioned in my last post had what appeared to be a oe style soldered connection, and I'm wondering if thats a fusible link. Of course I didn't think to test continuity thru that. Also, that looked to be actual bare wire, as in, no burnt insulation, just corrosion from being exposed to the elements. I'll look deeper at that tomorrow.

Ok, thanks for the clarification on that. So, either the short together was due to the pcm being in the equation, or the short is from the 42 pin to the pcm. So, try unplugging the pcm and checking continuity between the wires again, see if they still have continuity. Also, unhook both grounds off the battery when checking, to rule the powered circuit out of the equation.

As far as fusible link, that is on the hot side, not the ground side, and if I remember right, the gpr is fed directly from the battery, like the glow plug side of it and the alternator. So, if the fusible link was blown, you would not have power to the control side of it, and possibly no power to the alternator or the load side of the gpr.


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[DEEZUZ]

Send your PCM to Johnathan and have him test it.

 

[DEEZUZ]

Also that bare wire, probably silver? Is a Noise reducer I think

 

[psduser1]

Send your PCM to Johnathan and have him test it.

At beans?

Also that bare wire, probably silver? Is a Noise reducer I think

I think that is correct, from my hazy memory. Still gonna change it, the corrosion is that bad.

Ok, thanks for the clarification on that. So, either the short together
assume your mean was due to the pcm being in the equation, or the short is from the 42 pin to the pcm. So, try unplugging the pcm and checking continuity between the wires again, see if they still have continuity. Also, unhook both grounds off the battery when checking, to rule the powered circuit out of the equation.

As far as fusible link, that is on the hot side, not the ground side, and if I remember right, the gpr is fed directly from the battery, like the glow plug side of it and the alternator. So, if the fusible link was blown, you would not have power to the control side of it, and possibly no power to the alternator or the load side of the gpr.


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My next step will be checking for continuity with the pcm disconnected, since there doesn't appear to be any obvious problems in the harness between the engine and the pcm.
Assuming the harness checks out, I'm kinda at a loss, since the second pcm was completely functional when I parted the truck. Its been clean and dry on the shelf since then.
The fusible link im looking at is on the signal wire to the pcm,not the main power link between the gpr and the battery. Its only a 20 ga link.

 

[psduser1]

So my brother and I spent some time on this yesterday. Ohmed out the wires, checked everything in the harness, etc, repaired a bit of damaged wire, and put everything back together.
We didn't really "fix" anything, lol, yet now the gpr is happy again. No idea why.
There wasn't any obvious problems that actually would cause the bad ground issue, so I don't have a clear answer for the original issue! So frustrating!
Take it all apart, put it all back together, now it works.:fustrate::shrug:
Hopefully 20 year old, worn out rig, doesn't have more phantom issues.:doh: