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 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?

This thing has me stumped. ideas?

 

I have also gpr not functioning.
Torgue pro reads vehicle power running 7.2-8.2 volts and key on engine of 15.9v....???

I have a trouble code P0562,low voltage.Scanned with ForScan.
Tested with voltmeter, system voltage is within normal 12.5-14.8

Can you check your system voltage from obd vs voltmeter?

I can drive normally and it puls just like a freight train!

 

Knotreel...you putvmore emphasis into finding a cure than i did. I just ran a wire from the ground lug on gpr to a switch, then switch to ground. Manually operated relay. Just keep the orange wire unhooked

 

I don't remember using my forscan for system voltage. I know I had a meter on the battery as well as the GPR terminals. But that is something to check. Maybe I'm not getting 12+ to the pcm. when I get it put by together with new relay, I will look into that. Actually I've checked so many things I could not remember for sure. I see that I put this in the wrong forum. Should have been in the 94-98 year group. I lost this thread.

Tim,
I guess I should have done that at the get go, but it's too late now, 400+ for the pcm and 100 or so, or sensors and 50 for the relay. It's personal for me now. I want to sell it as it is no longer needed for vacation trips and people are funny enough about diesels and my experience has been when I start to explain something like why there is a button, they excuse themselves and are gone. If I were keeping it it would have been a button the dash.

 

An interesting link here:
https://www.ford-trucks.com/forums/8...-solenoid.html

I know you said the EOT reads fine. A suggestion in the link is to unplug the EOT, and see if the GPR fires up. Worth a try?

 

@knotreel I can move this thread over to the 94.5 - 97 7.3L sub-forum if you like, just let me know here or via PM.

 

Sous
I seem to be getting some ideas here, thanks but it is working ok as is.

Jeff
that's a good idea. it will be a few days until I get my new geunine FOMOCO GPR, so I will try it and report back. Thanks

 

My experience has been a pcm uses multiple small grounds [94 ranger] as opposed to 1 large 1. Check them all.

 

will do that too. I can't say i checked them all, looks like there are 5 grounds to the pcm.

 

Hard to start in hot weather means you have bigger issues than glow plugs relay.

 

OK, I checked all the grounds on the pcm connector ,(5) and all good. there is a B- buss pin that is open but it's main connection is to the DLC so I am ignoring that. I think that rules that out . As soon as I get it running I will unplug the EOT ans see what that does. I have been working on this for a couple of week in and off.

 

When I get it back together I will report this to you.

 

I had this problem, the solenoid appeared to be working when I probed it with a voltmeter. Turns out, the ground on the small terminal which is just the switch that actuates the GPR wasn't grounding with the wire that was attached to it. Ran a grond wire to the other relay next to it and fixed it. It works fine because the power side is actuated by the pcm, ground is ground.

 

On mine the power side is switched from the ignition switch ans is always energized while switch is on.. the ground is supplied by the pcm if the the pcm "thinks" it's needed. my pcm is not thinking it's ever needed. The wire ground on the small terminal is good to the pcm.

 

The PCM uses oil temp and intake air temp to determine if the glow plugs are needed and for how long. You've checked your oil temp sensor, how about both intake temps.

I say again that if your truck is hard to start at 70 degrees, you have other issues. Glow plugs should not be needed above 55 degrees.