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Old 07-27-2011, 04:09 PM
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Glow Plug Relay Problem

Hello!
I think I have a problem with my glow plug relay.
Hard starts in colder weather.

So I searched the forums and found a thread on troubleshotig the relay.
It is a 2001 F350 with the 7.3 motor.

Ignition off:
One big terminal 12v all the time.
Other big terminal 0V.
Two small terminals .8V

Ignition on with Wait to sart light lit
One Big terminal still 12V
Other big terminal still 0V.
Two small terminals 12V.

AhA!
Bad glow plug relay!
Other big terminal should be 12V when ignition switched on according to the forum thread I found.
I pulled the relay and replaced with new one.
Absolutely no changes on the voltage readings...
Still 0V on big terminal with key to ignition.

What am I missing guys?
Is there a bigger problem?

Thanks in advance!

Last edited by vegasmike; 07-27-2011 at 06:18 PM.
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Old 07-27-2011, 04:14 PM
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Sounds like a grounding issue on the small terminal. Check the ground side terminal wiring for a probable open circuit.
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Old 07-27-2011, 07:08 PM
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You are right.
I am not getting the ground from the PCM when the key is turned.
One of the small terminals should get a ground to trigger the relay.

I looked at the wires coming off the small terminals and they both look fine and go into a harness thats wrapped up.

I guess that leaves the PCM?
If so...where the heck is that?
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Old 07-27-2011, 08:54 PM
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Disconnect the two small terminals and determine which one is the "hot" positive side when key is on. Reconnect that one and then run a new separate ground to the other side. The relay should now work. Problem could be an open circuit in the harness, and not the PCM.
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Old 07-27-2011, 09:28 PM
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Yes, i did that and the relay clicks very loud.

So that must mean somehow the ground from the pcm isnt getting to the small terminal.

I was planning on running a ground to the terminal on the pcm that outputs the ground.
If the relay clicks on then i know the wiring is good and the pcm is bad.

Only problem is that i have no idea where the pcm is.
I have a diagram showing which wire on the pcm outputs the ground...i just need to find it.

Also...does the pcm ever NOT send the ground? Like if your engine is hot?

Thanks!
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Old 07-28-2011, 06:39 AM
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Well, it's definitely not my relay.
Hopefully I save some people some timewth this post.

There are a few threads on the forum that say the GPR alway activates no matter what.

I can tell you for a fact that it only activates when the engine is cold. There must be a temp sensor somewhere.

Yesterday I was testing on a warm engine because I read that it always activates. And it would not activate. Therefore I thought I had a bad GPR.

On a whim I decided to try on a cold motor and presto! The PCM sends the ground out and the relay activates like it is supposed to.

My problem isn't over though...if the GPR is good, I now need to troubleshoot the glow plugs...

Thanks!
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