totally stumped, GPR issue - Diesel Forum -
Power Strokes 1994-1997 General Technical discussion of topics related to vehicles powered by the Power Stroke engine in 1994 through 1997 models.

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post #1 of 1 (permalink) Old 08-09-2019, 03:52 PM Thread Starter
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totally stumped, GPR issue

I am struggling to make my GPR work. It works if I supply a manual ground to the solenoid. Just to be clear, when the ignition switch is on the solenoid has good 12v to one of the coil terminals the other coil terminal is connected to pin 101 of the pcm which is supposed to ground when the pcm decides to. The PCM does not supply the needed ground making the truck harder to start even in this hot weather we have here.. The pcm has good signal from the baro, about 4.6V, and EOT is reading correct starting off at ~70deg in morning. These circuits have been checked for open and short to B- and to other wires. All good . Also I misdiagnosed the issue thinking the pcm was at fault. I just installed a reman PCM from Chattanooga Diesel and same result.
The only other thing that I see on the wiring diagram is three wires from the switched power side of GPR connecting three pins on the pcm. The two glow plug supply branches that have the fused links each have a feedback line to the pcm through a small gage wire. The center switched power terminal also has a fused link there is a small gage wire connecting to the pcm. This may some how communicate to the PCM that the circuits to the glow plugs have power or maybe detect some resistance fault ? So to that end I just checked all the glow plugs for resistance and they were all in the 0.6-1.1 ohm range= good? Then I checked the wiring harness from all glow plug connectors to the switched power terminal of the GPR, all good. I was thinking that possibly the pcm must have good feedback from these three wires in order to supply the ground “signal” to the GPR. So, everything that I can imagine I have checked and either fixed or found to be good. The two wild cards are the PCM ugh, it’s brand new and the GRP it’s self. The GPR is new but it is not oem which should not cause any of this unless there is some internal logic circuitry that I am not award of (i’m really crazy now). So to be sure I just ordered a new motocraft F81Z 12B533.
Just a recap of things checked so far:
1. verified the EOT for proper range and signal to pcm
2.verified the Baro of output in hz and volts (~4.59v) and connection to pcm
3. ignition switch power to coil of GPR ,12v.
4. supplied manual ground to other side of coil, GPR worked
5, checked gp's, all 8 in 0.6-1.1 ohm range
6. checked glow plug engine harness, all good to switched power terminal of GPR.
7. checked the main signal wires for short to ground and to other wires, none found.
8. checked fuses and grounds
9. the GPR is new but is a cheap aftermarket, I really can't see how that could be an issue since it can work with a gound but I just ordered a motocraft oem just to be sure.
10. this all started after this van had set up for several years and the GPR that was on it had busted it's case. I had made a mistake in trying to start it on weak batteries which may have caused the GPR to overheat but it definitely fried the IDM and the starter failed as well.
11. I would say that I ordered the reman pcm due to a misdiagnosis, but since then, after the "new: pcm had no effect, I have ripped deeper into the engine wiring to the point that I absolutely have no idea except the new GPR which is on order, and dare I say, the pcm. When I ordered the reman PCM , there was that voice in my head saying "it's never the PCM". But two pcm's, sounds crazy but I can't help but wonder after two more days ripping open harnesses, checking and rechecking.
12. I have the factory wiring diagrams so it wasn't all guess work. Everything makes sense but the GPR besides getting the ultimate ground "signal" has three other connection to the pcm from the switched power side. No idea what they do except maybe verify the relay has connected the always hot to the switched side or maybe senses a faulty resistance in the glow plug circuit?

what to do next, no I don't want a dash switch, at least not yet anyway.

2003 (03.25MY) F-250 6.0, XLT long bed 2WD, auto. 147K, owned since new. work done; cooling system flush and fill with ELC-1, coolant filter, oem oil cooler, BP EGR cooler, turbo rebuild with powermax GT40 compressor, Dieselsite CACC boots (hot side), cat delete, blue spring kit, Edge monitor (no tuner), transmisson filter up grade to 08 style.

Last edited by hammer30; 08-09-2019 at 04:08 PM.
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