GPR kickingmy butt
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Mountain Pass
Joined: Dec 2004
Posts: 195
Likes: 0
GPR kickingmy butt
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 see how that could be an issue since it can work with a ground 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?
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 see how that could be an issue since it can work with a ground 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?
Tuned
Joined: Apr 2018
Posts: 329
Likes: 17
The three wires in the diagram may be from the shunt on the California truck. Check the diagram for mention of CA or 49 state.
Don't know where the computer grounds the GPR. Maybe the diagram shows? Maybe missing a ground. My guess would be a ground lug to the body over by the computer. A wire back to the battery would be better but not necessary.
You should be able to hook in a test light in that circuit. Might make testing a little easier.
Double check yourself on the EOT. Maybe to test you can unhook it? Check diagram.
Did you mention the wait to start light?
Don't know where the computer grounds the GPR. Maybe the diagram shows? Maybe missing a ground. My guess would be a ground lug to the body over by the computer. A wire back to the battery would be better but not necessary.
You should be able to hook in a test light in that circuit. Might make testing a little easier.
Double check yourself on the EOT. Maybe to test you can unhook it? Check diagram.
Did you mention the wait to start light?
Lead Driver
Joined: Sep 2002
Posts: 9,539
Likes: 2,030
From: North Texas
Ran a continuity test on that ground signal from GPR back to Pin 101 yet and are you 100% certain it is Pin 101? You sure that pin is fully seated in the EEC connector? I had a pin retract for my A/C circuit and it would constantly blow fuses...it is a 12V signal from A/C to PCM to increase idle. Pin beside it was a ground, so popped fuse every time. I would manually ground Pin 101 at EEC connector to verify it closes the GPR. Easy to do with key off. Just jump switched 12V signal on GPR to the +B and ground that Pin 101 on EEC connector side.
Does your EOT sensor output correctly? Have you checked from cold starts up to freeway cruise? I usually show about 100F upon start this time of year and 220s cruising, unloaded at 70mph around 2400rpm.
Return the Motorcraft unit, buy that big ******* GPR from Riffraff or Diesel Orings. Stancore?
This problem is easier than you think to me.
If you have issues starting without GPs in the summer, you are going to be in worse shape in the winter. Figure that out too, starter and/or bad battery cables. I have not run my GPs in months, GPR is disconnected for me this time of year.
Does your EOT sensor output correctly? Have you checked from cold starts up to freeway cruise? I usually show about 100F upon start this time of year and 220s cruising, unloaded at 70mph around 2400rpm.
Return the Motorcraft unit, buy that big ******* GPR from Riffraff or Diesel Orings. Stancore?
This problem is easier than you think to me.
If you have issues starting without GPs in the summer, you are going to be in worse shape in the winter. Figure that out too, starter and/or bad battery cables. I have not run my GPs in months, GPR is disconnected for me this time of year.
