Glow plugs always getting power!?

[rammertide07]

I have both negative battery terminals disconnected and have already pulled the gpr. I can touch the ground lead from the volt-meter to the negative battery terminal and the positive lead wire to the wire that feeds the gp's (wires that get voltage when relay is engaged) and I get 13v. Doesn't that mean my gp's are getting voltage akl the time?

Batteries haven't failed in the 3yrs I've had them, but starting is a little slow. GPR is about 8 months old.

[cleatus12r]

No.

That means you're completing a circuit through the meter.

If your glow plugs were "on" all of the time, you wouldn't be able to let your truck sit more than 10 minutes at a time without draining the batteries (not to mention it would stink like hell after a while).

[rammertide07]

Thanks for the reply Cody...both of them.

What circuit am I completing? Shouldn't I be getting 0v since I had the gpr out? Again, I'm probing the wires that go to the glow plugs...not the wire from the battery.

I put the GPR back on and wired it back up...pcm ground is still unhooked. I hooked up one of the negative battery terminals and tested for voltage again at the "glow plig feed" wired. This time I got 0v. Doesn't make sense because I was getting 13v just minutes before at the "glow plug feed" wire.

[cleatus12r]

Yes, it does make sense.

With the batteries' grounds disconnected and you using the DVOM to engine ground (glow plugs) and the battery ground, you are completing the ground circuit to the truck. By reinstalling the negative ground cable, you are no longer completing a circuit to ground through the glow plugs. The technical term for what you have experienced is called "voltage drop".


I'll draw a diagram and you tell me whether or not I'm correct in how you have the DVOM hooked up.

[cleatus12r]

Here's the diagram that I came up with (15 minute drawing in MS Paint). The blue lines are the DVOM leads and they are labeled next to the picture of the DVOM. Also notice the broken battery cables above the negative terminals.

Is this correct?

If this is how you had your DVOM hooked up and how the truck wiring was configured, then what you see on the volt meter is exactly what should be showing on the volt meter.

[rammertide07]

Yep, thats it. But where is the + voltage coming from the glow plug side of the open circuit. Also, when I had the GPR out I was getting continuity between the "glow plug feed" wire and the positive battery terminal.

[rammertide07]

Cody, here is a thread I did on TDS..made a post each time I found a new piece of info. Takes too long to type all of that out on my phone so figured I'd share the link. Please note that the initial test of the relay showed it was putting out .7v before I ever did anything with the wiring. Then after digging deeper it showed I was getting 12.5-13v.

GPR continuity question - Diesel Forum - TheDieselStop.com (http://www.thedieselstop.com/forums/f24/gpr-continuity-question-300436/)

[cleatus12r]

But where is the + voltage coming from the glow plug side of the open circuit.

Between every circuit in the truck that is on the power distribution "tree".

You're looking way too far into this.....
It's easy.

[cleatus12r]

Please note that the initial test of the relay showed it was putting out .7v before I ever did anything with the wiring.

So that was between the ground terminal of the battery and the post of the GPR that has the yellow and brown wires?

Normal. You're measuring voltage drop across a resistance (glow plugs).

[rammertide07]

So that was between the ground terminal of the battery and the post of the GPR that has the yellow and brown wires?

Normal. You're measuring voltage drop across a resistance (glow plugs).

After I hooked everything back up, I measure the relay output again. The volt meter was reading 0v on the second go-around....first time it was measuring .7v.

I appreciate all your help and time. I know some mechanically inclined people that can give me some hands on explanation. Just talked to an ASE mechanic, now he's confused:doh:

The way I'm looking at it, the glow plug side of the "tree" is a dead branch...not connected to any type of power source, since the negative cables are not connected. The "glow plug feed" side of the circuit is not suppose to have continuity with the + side of the batteries until the GPR is engaged. Thats the way I look at it...

I'm going to do some research on automotive 12v systems and relays.

[cleatus12r]

Because you don't have the negative battery hooked up, some of the circuits/accessories that are on the constant (non-keyed) power side of the truck WILL conduct B+ voltage all the way through.

Since the volt meter draws VERY LITTLE current, you WILL see battery voltage regardless while doing a VOLTAGE DROP TEST (which is what you're doing; even if not on purpose) because the positive voltage doesn't just say, "Gee, I guess I'd be trespassing on Negative's property if I actually went through this circuit since he's not here.....".

Seriously, if your ASE mechanic buddies don't know anything about voltage drop tests (or theory), they better stick to tire rotations.


Edit:

I'm not trying to come across as a jerk, but why in the world are you chasing something that is not a problem?

[rammertide07]

Because you don't have the negative battery hooked up, some of the circuits/accessories that are on the constant (non-keyed) power side of the truck WILL conduct B+ voltage all the way through.

Since the volt meter draws VERY LITTLE current, you WILL see battery voltage regardless while doing a VOLTAGE DROP TEST (which is what you're doing; even if not on purpose) because the positive voltage doesn't just say, "Gee, I guess I'd be trespassing on Negative's property if I actually went through this circuit since he's not here.....".

Seriously, if your ASE mechanic buddies don't know anything about voltage drop tests (or theory), they better stick to tire rotations.


Edit:

I'm not trying to come across as a jerk, but why in the world are you chasing something that is not a problem?I understand you are not being a jerk. In my head it is a problem just because I don't understand. I am understanding a little bit more now...just trying to wrap my head around it. I really wish I had someone in person that could explain it to me, hands on. Hands on experience always beats text book experience. I've accpeted that everything is as it should be. It just bugs me to not know why...

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