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Old 10-20-2021, 11:32 AM   #1
DeadheadNM
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Why might this relay be in use?

Iím slowly unwinding all the electrical changes made to my 99 Suburban that was formerly operated by a municipal fire department.

This relay appears spliced into an engine harness bundle at one end and the passenger side battery at the other end (red wire with an in-line fuse). The truck dies if this wire is removed at the battery. At the harness end it looks like a red harness wire was cut and spliced to the blue and yellow wires from the relay.

Not sure why this modification was done other than a possible anti-theft option. Any other thoughts? I donít have the schematic for this truck like I do my 72s and want the wiring back to original.
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Old 10-21-2021, 05:22 AM   #2
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Re: Why might this relay be in use?

I would look at the coil side of the relay and see where they go to determine what relay is for. One side should go to ground and the other to +12v. Either side could be switched depending on application. One side may go to 12v and other switched to ground or one side to ground and other switched to 12v.
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Old 10-21-2021, 11:20 AM   #3
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Re: Why might this relay be in use?

Where does the yellow wire go??
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Old 10-21-2021, 10:18 PM   #4
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Re: Why might this relay be in use?

Thanks guys. The yellow and blue wires out of the relay each join separated ends of a red wire at the harness. The green wire out the relay is a ground and the red out the relay is wired right to the pos battery terminal. I’m combing through some GM literature trying to nail down what the red harness wire supplies.
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Old 10-26-2021, 07:38 AM   #5
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Re: Why might this relay be in use?

Quote:
Originally Posted by DeadheadNM View Post
...The yellow and blue wires out of the relay each join separated ends of a red wire at the harness. ...
What does that red wire at the harness feed or do?
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Old 10-27-2021, 12:50 AM   #6
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Re: Why might this relay be in use?

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What does that red wire at the harness feed or do?
From the battery goes to #30 on the relay.
Yellow is the question after it turns to pink according to the picture
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Old 10-27-2021, 06:28 AM   #7
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Re: Why might this relay be in use?

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Originally Posted by kwmech View Post
From the battery goes to #30 on the relay.
Yellow is the question after it turns to pink according to the picture
Quote:
Originally Posted by DeadheadNM View Post
...The yellow and blue wires out of the relay each join separated ends of a red wire at the harness...
Not according to post #4 that I quoted above.

Last edited by The Rocknrod; 10-27-2021 at 06:38 AM.
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Old 10-27-2021, 09:36 AM   #8
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Re: Why might this relay be in use?

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Originally Posted by The Rocknrod View Post
What does that red wire at the harness feed or do?
Unsure - I need to track that down still and I imagine that's key to solving the puzzle. If I d/c the hot from the battery to the relay the engine dies. I'm clueless with diesel engines to be honest.
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Old 10-27-2021, 04:25 PM   #9
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Re: Why might this relay be in use?

It may be a glow plug relay. Possibly a fuel shutoff relay. Electric fan maybe. It is not factory installed.
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Old 10-28-2021, 09:16 AM   #10
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Re: Why might this relay be in use?

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Originally Posted by Wrenchbender Ret View Post
It may be a glow plug relay. Possibly a fuel shutoff relay. Electric fan maybe. It is not factory installed.
George
Thanks.

I'll d/c the added wire it when I get the chance and restore continuity to the OE harness wire and see what happens.
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Old 10-29-2021, 03:07 PM   #11
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Re: Why might this relay be in use?

If the engine dies when you d-c the wire then it most likely is for the fuel shut off.
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Old 10-29-2021, 09:21 PM   #12
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Re: Why might this relay be in use?

I'm looking at a picture where the yellow and blue wire both turn to pink at the overheated solder/shrink connector
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