Small electrical fire following glow plug relay replacement - what are these burnt wires?

Found the wiring diagram, looks like it’s line 38, fuse link B and C

these feed the two sides of the glow plugs, could bad glow plugs be the cause??



Edit : I have both the ford dealership wiring diagrams for 1989 and most non ford shop manuals for this truck. If anyone that sees this ever needs a good pic of a wiring diagram, slide thru those PM's

Original plug relay back on, wires fixed how I had them before

no fire ..... starts easier.

I definitely have some bad glow plugs, could those cause a short?

planning on keeping it as it is while I build up a new harness.

https://www.ford-trucks.com/forums/1427991-perfect-replacement-glow-plug-harness-for-50-a.html

This post describes using the 6.0 harness, and very lightly modifying it to work with our trucks, I think this is the way for me.

ComatoseLlama said:

I definitely have some bad glow plugs, could those cause a short?

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I'm not going to say that this is impossible, but extremely unlikely. Glow plugs just burn out when they go bad. Power no longer goes through them to get to a ground. On the other hand, I guess anything's possible.

Thanks for all the help!

One last question before I really tear into it, I can't figure out where these two pop back out of the harness. They turn into the yellow wires that attach to the top of the glow plug relay, correct? NOT the wires that go next to the heads and splinter off into the glow plug wires.


If I am correct on this, I can take a thick cable from the always positive starter solenoid post (where the black/orange is located), put a 40AMP fuse inline, and then feed that to where the two yellow wires attach to the glow plug controller, and forget about the looming. Both are battery positive terminals. Does this check out?

CL

ComatoseLlama said:

Both are green wires

Here are photos of where it come out of and goes back into the loom

they have some shrink wrap in them for some reason
This pic makes me think it’s happened before and they shrink wrapped it

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chillman88 said:

I'm wondering if they're fuseable links. Possibly ones that have already been replaced once.

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Selahdoor said:

I'd agree about the fusible links. And being shrinkwrapped because it happened before.

Also, yes, it will still work if only one of the fusible links survives.

I would simply pull them out of the loom, all the way to both ends, and replace them.

Click to expand...

That's original, it's how the Factory installed the fusible links in the harness. Crimp connection that is also soldered and then a heavy piece of adhesive heatshrink.

On the 87-91 trucks both wires and links need to be in good servicable condition. Those two blk/org wires go over to and through the main engine connector harness next to each other on one end of it. Then they go to the feed side of the GP relay where they are recombined into a single lug. If one fails and the GP system is used the other one will fail in short order. The best course of action for these trucks is to connect a single piece of 4awg or larger wire to the bottom of those links and run that over to your relay. This is what Ford did on the '92 and up series and it eliminates the problematic engine harness connector which usually by this time is worn out and melted.

Here's a pic of a harness I have out of a '91:

Selahdoor said:

Well, that or just remove them on both ends, and add new wires in their place.

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Don't have to remove them, just clip them short where they enter the harness and leave them alone. Just do as above and run a single run of 4awg from the bottom of the links.

ComatoseLlama said:

One last question before I really tear into it, I can't figure out where these two pop back out of the harness. They turn into the yellow wires that attach to the top of the glow plug relay, correct? NOT the wires that go next to the heads and splinter off into the glow plug wires.

If I am correct on this, I can take a thick cable from the always positive starter solenoid post (where the black/orange is located), put a 40AMP fuse inline, and then feed that to where the two yellow wires attach to the glow plug controller, and forget about the looming. Both are battery positive terminals. Does this check out?

Click to expand...

They pop out at the main engine connector. Then I think they turn yellow on the engine side over to the controller. What you are proposing to do is exactly what Ford did on the later trucks. However I would recommend just using the 2 runs of 14awg fusible link wire into a 4awg run to the GP solenoid instead of finding a fuse and holder rated for this kind of current. Just do a good crimp and solder job and cover it with adhesive heatshrink just like the factory did.

And then throw away your other GP relay that created this mess.

Here's where they pass through the engine harness connector.


Here they are at the relay.


This is what Ford upgraded those two wires with. It's right about 60" total length and it's 16mmsq wire which is in between 4 and 6awg so 4 will work fine for this application.

ComatoseLlama said:

Thanks for all the help!

One last question before I really tear into it, I can't figure out where these two pop back out of the harness. They turn into the yellow wires that attach to the top of the glow plug relay, correct? NOT the wires that go next to the heads and splinter off into the glow plug wires.

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If I am correct on this, I can take a thick cable from the always positive starter solenoid post (where the black/orange is located), put a 40AMP fuse inline, and then feed that to where the two yellow wires attach to the glow plug controller, and forget about the looming. Both are battery positive terminals. Does this check out?

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CL

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Click to expand...

You are on top of it, but a 40 amp fuse is not going to hold. Use some fusible links like the other poster said or a very very large fuse. We had a discussion about this in another thread, and from the experience of another user a 150 amp fuse would not hold on this circuit. Somewhere between that and 200 amp probably would. Easiest would be to duplicate what the factory did and use those two fusible links.

I think the inrush current through the controller is over 400 amps (but only for a very short time) and drops to ~150 once the GP's start to heat. A fuse won't tolerate that which is why a fuse-able link is used.