2003 7.3 PowerStroke Glow Plug Relay Issue - Ford Powerstroke Diesel Forum
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Old 11-03-2013, 06:26 AM
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2003 7.3 PowerStroke Glow Plug Relay Issue

OK...so I tested the GPR and found that it was indeed bad. Purchased a new one for $56.00 and installed it. The only difference was the mounting bracket was 90 degrees out from the original. So I completed the install and checked the two large posts, once with key off and once with key on, all is good....well not so much. Came back out to the truck a few hours later to take off and the batteries were dead and the main power lead to the GPR was HOT. Apparently the GPR was pulling power the entire time I was on the house? Disconnected the two wires that go to the GP harness then started the truck....started fine, but now the truck WILL NOT shut off....hmmmmm. Disconnected one of the smaller wires to get the truck to shut down...what the heck is going on.
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Old 11-03-2013, 07:42 AM
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First, everything on the relay will read Hot except the large terminal going to the plugs themselves. IF the relay was actually activated, and powering the GPs, your batteries would be completely Dead in just minutes because they're are drawing 100-150Amps

Relays are usually activated by supplying Ground to the coil. In our case that's done by the PCM but, you'll see the 12v on Both of the coil terminals (sm ones) once You supply ground through a meter or test light.

I have No Idea why it wouldn't Shut Off though!
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Old 11-03-2013, 08:21 AM
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OK, now I am really confused...I now have taken the old one and bench tested it...it works properly on the bench, but when I tested it while it was installed in the truck it did not work property. Key on the rear (to GPs) did not read any voltage. Now it would seem that I have fried the new one I installed yesterday. I assume that there is something simple that I am missing here just not sure what it is yet.
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Old 11-03-2013, 09:24 AM
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You can't really "Fry" anything, don't worry

Maybe I can increase your understanding of how it all works though...

First, you need to know that the WTS light on the dash is not an indication of the GP system. It's OFF sometimes when they're ON (the GPs) and ON sometimes when their OFF. So don't think if the light in on you should be seeing power to the plugs.

Next, The PCM decides when to activate the GPs mostly by engine temp (Oil Temp for the most part) and decides how long to leave them on by monitoring what happens there. It turns them ON by supplying a Ground Path to the, already powered, coil.

This means, you have 12v supplied to one of the sm wires and the other waiting to see ground from the PCM. BUT, because the coil is just wire connecting the two inside the relay, if you test for power it will be on Both terminals. You need to disconnect them to find which one is the Supply. And, really, it doesn't matter which wire goes where, it's just a coil of wire and as current flows through it, it creates a Magnetic Field that acts on an Arm to close the connection and send Big Power to the plugs (that's what Relays do best, use a Tiny amount of power to control a Large amount of power).

And even the Big terminals don't care which Big wire goes where. They're just like your light switch, they connect the two through contacts inside (that again, the coils magnetic field closes)

The other aspect of those contacts is that your old relay may look like it's functioning OK, when in fact it's internal contacts have gotten "burnt" to the point that, although it shows it works on-the-bench, brake-down under the 150A load of the plugs.

OR you may have other problems and didn't really need your new one. Those can be bad wiring, PCM, Temp sender (I think the Oil on an Automatic but Coolant on a Manual)

What you need to test for is, Does the PCM supply ground to the sm wire that isn't the supply wire (disconnect them from the relay to know which-is-which) when the engine's Cold?

AND Does the relay throw when YOU ground the sm terminal with the OTHER 12v (supply) side connected and the ground side (to the PCM) Not? The next test is to measure the Voltage on Both of the Large terminals (one side Supply from the Batteries, the other to the Glow Plugs) and compare that reading when the plugs are ON. If you are losing voltage through the contacts, they're bad inside (a little bits OK).

Last edited by NoRalPh; 11-03-2013 at 09:31 AM.
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Old 11-03-2013, 09:59 AM
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I think that I did fry the new GPR...when I went back in the house for a few hours and the GPs were still pulling eventually killing my batteries I think that the copper coil welded together inside the GPR. Bench testing this one we can't get it to work...it won't pull in. We just finished re installing the old one which I thought was originally bad and everything is working properly....could the connections have been so corroded that it was giving me false info, leading up to me replacing it. This whole thing is baffling to me.
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Old 11-03-2013, 10:19 AM
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You mean the contacts - right? I guess it could happen but I don't think it would keep it from starting (or was that from the dead batteries? - didn't understand...) Still Not sure what you could have hooked-up wrong though...
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Old 11-04-2013, 04:36 PM
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Just replaced my gpr. Mine gets replaced once a year. Starts to fail when the temperature drops here in Co. The $50 - $60 gprs made in china don't last long. Make sure your clean your battery terminals even if they look good.


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Old 11-04-2013, 05:34 PM
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I just did the Stancor mod and I'm happy with it so far. Time will tell. After pulling my old relay out I ran a quick bench test, and it sure sounded like it was working, but, the contacts were too burnt up to make contact anymore. Clickity-clickity, no worky-worky.
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Old 11-04-2013, 06:01 PM
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What relay did you replace? Are you sure you replaced the GPR and not the AIH relay?
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