The what's this (these) wire thread.

[INSIDIOUS '86]

Well the wiring under my dash and all around my truck is garbage and its a real pain to put something back together when you don't know what you may or may not be missing or aren't familiar with the specific model. So anyways my first question among many is

What the @$$@!? Is this bundle for? My truck is a 78 heavy half comes out near the brake booster and throttle cable, thanks.
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[Greenlee]

The plug on the firewall in the first picture looks like tail lights.
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[INSIDIOUS '86]

Hmm we'll it doesn't go to any part of the fuse box or whatnot. The plug on the firewall disconnects easy. Could this be for the tank selector?
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[texas gmc]

I can't remember, but you maybe right about it being for the tank selector.
My truck had duel tanks and I deleted one of the tanks and all the wiring.
I have the same bare plug as your picture.

[wraprail]

The 5 wire plug goes to the dual tank selector
The firewall plug is the other end of it and goes to the tank harness.
The 5th wire goes to ign.+

You should have a hole in the dash for the tank switch next to the ash tray.

And a second fuel tank..

[INSIDIOUS '86]

Everything under the dash is cut and spliced and moved and re soldered 100 times. Nothing's plugged in. It's so bad I'm almost considering getting a donor harness out of the scrap yard or just pulling an 80s wiring harness and updating under dash
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[wraprail]

Looks like the paint is nice.
Right there behind the dash anyways....love that brown metallic

[hatzie]

That's the dual tank selector in-cab sub-harness. Looks like you're missing the under cab half. If you still have the SPID label in the glovebox you should find RPO code NL2 listed.

Tan goes to the fuel gauge.
Blue and wire with a stripe are to the tank senders.
Green goes to the selector valve.
Pink goes to the fuse panel.

The 1978 GM factory wiring diagrams are in my signature.

[INSIDIOUS '86]

Thanks hatzie! Yah I'm going to send in for a spid at the heritage center but I know it's a heavy half at least and was a 454th400 truck original paint too
I have a build thread going on in the projects section. Now on to the next rat nest of wires...
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[INSIDIOUS '86]

Alright we'll I pulled the added wires on the truck have it back to what's left of the original wiring. My volt gage jerks very minute amounts at 8volts and when key is off it rests at 9:00. The seatbelt lamp stays on and the gas gauge is held at 3:00 and the blinker lights both stay lit when lights are on but I'm sure that's just the fact rear lamps aren't hooked up.

Have a couple more mystery wires that need homes but right now my biggest issue is fuel gauge and the wipers don't work

Anyone with ideas? I checked all fuzes with a meter on both sides. They all check out. I just need the wipers at least for Washington weather haha don't think the wipers get power to the intermittent wiper switch


Update: I added a ground strap to the wiper motor and the washer button when pushed makes the assembly click but not do anything. With the switch on the wiper motor gets hot
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[wraprail]

Would love to help but We don't use wipers down here. Got rid of that useless motor and wiring years ago. Truck is much lighter (and faster) now!
Sorry insidous.
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[motornut]

i'd go in, check the cluster plug order,is the oil sender a real thick wire(idiot is thicker then gauges)
you can remove the whole seat belt buzzer
gas at three ,sender wire off

[INSIDIOUS '86]

Originally Posted by motornut

i'd go in, check the cluster plug order,is the oil sender a real thick wire(idiot is thicker then gauges)
you can remove the whole seat belt buzzer
gas at three ,sender wire off

I got the volt gauge to work since I ha an odd feeling it was a bad connection the way it was acting. Found what two of the mystery wires were(blue ones) and foun them to be oil press and water temp. All three of those gauges work which is nice. Now just the fuel gauge/system and wipers and getting tail lights wired is left to do.
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[INSIDIOUS '86]

Alright I think I got it fixed now. The cluster harness had a pin that wasn't in the right spot. Coincident that the brake light was on? Nope. Pin 18 is for the brake warning lamp, pin 19 is for the fuel gauge. Pin 19 had some scratches in it when I looked and it looked like a pin had been there before. I just happened to move it over and the gauge went from 3:00 to e

So far wiring has been a real pain but I'm getting it fixed one wire at a time

Now for this wiper motor...
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