Why might this relay be in use?
 

I’m 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 don’t have the schematic for this truck like I do my 72s and want the wiring back to original.

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.

Re: Why might this relay be in use?
 

Where does the yellow wire go??

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.

Re: Why might this relay be in use?
 

What does that red wire at the harness feed or do?

Re: Why might this relay be in use?
 

From the battery goes to #30 on the relay.
Yellow is the question after it turns to pink according to the picture

Re: Why might this relay be in use?
 

Not according to post #4 that I quoted above.

Re: Why might this relay be in use?
 

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.

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.
George

Re: Why might this relay be in use?
 

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.

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.
George

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