There should be a fusible link and a wire from the left and right shunt posts and center relay post going to the PCM on pins 34, 8 and 9, respectively. The PCM uses those to monitor the glow plugs by watching current draw on each bank. If a bank draws less than 32 amps, or in excess of 8.5 amps less than the other bank, it should set a code and MIL, but only if the glow plugs were commanded on for a period of time greater than 35 seconds.

 

Your voltage drop is a reliable indicator that the glow plugs are not all working properly. 70 plus amps out of the dual batteries with a normal alternator will definitely cause more voltage drop when everything is cold. If you have dual alternators, different story--dual alternators would handle that load without much of a voltage drop.

 

Funny how old topics keep coming up.
Ten years ago now, my GPR failed on my 2000 miles from home.
Moved the wiring from the glow plug relay to the relay that powers the intake air heater. The intake heater had been removed before, anyway, so it was just sitting there doing nothing. It's been that way ever since. Later on, I disabled computer control of the glow plugs by disconnecting the little signal wire that comes in from the ECU and ran that wire to a momentary on switch that is mounted next to the steering wheel. Hold the button down for a 10 count and turn on the key. Wait another 5 seconds for fuel pressure to build and start it up.

 

I checked this today, and got a reading of 0.40v, with the key on. So according to your test criteria, my GPR is bad. The question is, at 0.40v, how bad does this indicate the GPR is? Just starting to go, change it when I get the chance, bad enough to affect other systems and/or change it immediately or risk getting stranded or doing other damage?

Also, can please explain why we should see those readings when good, and what is actually happening, when it fails, that would cause the "bad" readings?
I'm a little confused as to why I wouldn't want to see battery voltage on both big studs, when the glow plugs are energized. I thought the whole point of the relay was to "transfer" the voltage to the second glow plug power supply stud, when the relay is activated?

 

The reason you are not seeing any voltage between the two studs when the relay is energized is due to the fact there is no potential between them, it is like they are two different points o. The same wire referencing each other.if you see the full battery voltage it means that the voltage is not pushing through.

Imagine a pipe with a valve (just like a wire with voltage) if the valve is open then the pressure on both sides of the valve is the same. If you try to read the difference between them (like you are trying to do with your voltmeter) then your reading will be zero. If you close the valve and have 12lbs of pressure on the one side, and then again read the difference between them, then you will see 12lbs/volts.

Now if you went from one stud to ground you should see 12v and if the relay is closed you would see the same 12v to ground on the other stud, and would read 0 if the relay is open.

Hope this helps?

 

I did the led mod recently. We have not had much cold weather down here yet to time the GPR. But when I installed it the plugs stayed on for 42 seconds, on a cold engine, at 80* outside. I was surprised.


The mod is easy to do. Takes less than an hour. And cost less than 10 bucks.

 

i have seen my glow plugs stay powered for over 2 minutes when normal temps, and close to three minutes when temps are in the teens or lower.

 

My glow plugs are completely manual control now. Put it on a push button years ago. You only really need them to get the engine started.

 

Glad to see you posting Dan, it's been a while on this section. God bless all and Merry Christmas. Chet

 

What he said. The multimeter reads the difference in voltage between the leads. If you stick both leads to the positive battery terminal, it will read zero. We all know there is 12v there. Put one on the positive and one on the negative, and the meter will tell you what the difference between the two points are. So when when you check voltage along the path of electrical circuit and there is a voltage drop, there is some form of resistance between the test points.

I don't have the experience the rest of these guys do on these systems, but I wouldn't think that would be enough of a drop to affect the glow plugs. I could be wrong though. I'm surprised I haven't seen any one mention any of the other dozen possibilities that go along with a hard start. With 8 good glow plugs and a strong starter I don't recall having to cycle the ignition twice. The coldest I've seen in my truck are the teens. Now when I had an Autozone starter, which didn't turn as fast as the Motocraft, I had to cycle my ignition a couple times.

Also, did you ever confirm which GP's the shop installed? Around here Autolites are about $2 cheaper per plug. Autolites are also the plugs every one will tell you to steer clear of.

 

Dan... great to see you posting in here. Hope your Christmas was excellent and that you and your family are all doing well.

 

Finally got some weather cold enough (<30 F) to really need glow plugs and see how the truck behaves. Turned key on, waited about 45 seconds. Truck cranked 10-15 seconds, sputtered, died. Did that a couple times until it finally started, ran rough, then smoothed out.

Meter across glow plug relay showed battery voltage (about 12.5) with key off. With key on, voltage dropped to about 11.5. That's not good, right? Should be zero across those terminals with relay active?

Meter attached to hot side of glow plug relay and ground showed battery voltage. Attached to glow plug side and ground showed ~0, with key on or off.

ELM-327 plug and Forscan showed a glow plug cycle time of 85 seconds. With meter showing glow plugs getting no juice, watched Forscan, and heard the relay click when Forscan indicated PCM had turned it off.

So GPR is apparently "functioning" enough to make an audible click, but there's no juice flowing through it when it's "on". I replaced that relay myself this fall, and shop that replaced glow plugs last month said it was good. Maybe they're idiots, or maybe it really was good then. I guess it's time to order one of those Stancor relays, since this one is obviously worthless.

 

The relay is toast. You should read nearly zero volts across it when it's active. The Western plow relay is a good relay to use, and it's in the $20-$25 range.

 

Do you have a part#? I was thinking about ordering the Stancor from Diesel O-Rings, but if there is another good one I can get locally, that would be preferred.

 

Pretty sure it's 56131K-2 . . .