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2002 F350 7.3. 320k miles. Now that it’s getting cooler, truck doesn’t want to start. Found GPR not working. I was also checking out a EBP sensor code at the same time. It all seems to be ground circuit related. The EBP sensor ground is 20 ohms with KOEO and 2 ohms with key off. GPR will engage when key on terminal grounded. Ground at all 4 wires at PCM is 2 ohms with the PCM disconnected and 42 pin connector disconnected.
Is the GPR getting a signal to activate but not switching power to the glow plugs? Did you ohm the valve cover harness and see if the glow plugs are good and or harness is bad? Is the EBP tube clear?
I don’t have any knowledge about the ohm results so can’t help you there.
Just wanted to make sure you’re aware that the PCM activates the GPR on the ground circuit of the relay. Something I didn’t realize until the guys around here told me.
As you're bending over the grill, if I'm remembering things..
The 9 o'clock thick cable goes to the batt pos.
The 12 o'clock thin cable goes to the pcm to turn on the relay.
The 3 o'clock goes out to the glow plugs themselves
And I'm not sure I've actually metered out the 6 o'clock connector... It should be a positive.
When you switch the key on, the 12 o'clock pcm wire should go to ground and cause the gpr wire to go hot positive for a period. If the pcm wire does go ground, then look at the relay and the other 3 wires. If it doesn't, trace that wire.
The relays themselves do die occasionally and there are poor quality replacements out there.
I’d start with a new GPR and go from there. Cheap and simple to install.
Smart folks on FTE have described how the relays can carry low amps but still be bad on the plate inside and not carry large current loads (like glow plugs).
This was the best deal I found on a GPR. I'm still lost on the PCM not providing enough ground, how much is enough? Low amperage circuit and ground is ground, it's not a pulse width modulated circuit.
If you're in an area where snow is a thing, the Fisher snow plow motor relays work great and last a long time. Plus, I've never had a Fisher dealer tell me they didn't have one in stock.
Upon closer inspection, the GPR was cracked. Replaced the relay, and made no improvement. I did find the glow plug system test on FORScan, PASSED! Also has P0380 glow plug circuit. How can both be true?!?!
I would try crossing the large lugs with a screwdriver for about 20 seconds before a cold start and see if that helps. That should lead us in a direction.
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