Scott's 1967 GMC 1500 pickup

[Scott Liggett]

I went through a lot of fuses last night trying to figure out what is causing fuses to blow. I was pretty methodical in an elimination process, disconnecting one thing at a time in order to figure out what is not causing the problem in the wiring. I started at tail lights and moved forward. I even pulled light bulbs one at a time. These 1157, dual filament light bulbs have been known to overheat and melt one filament into another causing a short. Since this problem happens when the parking lights and dash lights are turned on.

Here is the list so far: tail light bulbs, tail light harness, brake light harness, dome light harness, steering column harness, replaced headlight switch, front parking light harness and bulbs,

I got as far as pulling the gauge cluster last night, but it was still connected to the wire harness by one wire. I will have to cut it because I didn't use a quick disconnect to remove it.

[DanStokes]

Dang, Scott - this IS a puzzle. All I can offer is support and good wishes - you're doing everything right.

Dan

[Scott Liggett]

It is just a process of elimination, like ole Sherlock Holmes said. I am thinking that it is something to do with my wiring on the gauge cluster. I have not found anything wrong with the dash harness so far. There are some strange things in there. The lighter is an later add-on and has a light surrounding it. It was tapped into the light bulb to the heater controls, but only came on with the headlights, not the parking lots. I fixed that. Someone may have replaced the steering column harness. Every wire was cut and butt-connected back together near the column harness plug. I will redo that at a later date.

Here is a weird part that might lead to the cause. When the brake light fuse is blown, the dash light/tail light 3 amp fuse no longer gets power. If I turn on hazards, they work normally, with turn signal indicators working, but if I hit my brakes, the dash lights suddenly get power again and flash with the hazards. Somehow the brake lights and dash/tail lights wiring is getting a feedback, or connected somehow, but so far, I have not been able to find that.

I may head to the junkyard to look at another '67 truck's fuse box and wiring. There are four of them at the yard. My truck's wire harness is in such good shape, I don't see why I should replace it all. I may see if I can find a better taillight harness, as the previous owner did butcher it pretty good for the trailer hitch. I double checked my work on that when I fixed the mess, eliminating the trailer hitch harness for now since I am not currently towing anything.

[Scott Liggett]

The tail light harness is split up in three pieces: Under dash to firewall bulkhead, firewall under truck to tail lights under bed, under bed to lights.

I discovered it is the middle section that is causing the problem today. Now to find exactly where in that 10 ft long section and fix it.

I will be looking into getting replacement middle and back harnesses in the future.

[Scott Liggett]

Success!! At least so far. The problem was where I suspected. The previous owner had spliced into the middle section of the taillight harness near the front of the bed to power what I am guessing was camper shell lights. There were wires coming up through the bed when I bought the truck. I removed those wires, but the original wire harness had cuts in it that I missed causing arcing between the power wires for the taillights and brake lights. For now, I cut out about 2 inches of the wires, separated the four wires and spliced them back together individually.

I looked at getting a new tail light harness $90.00 and the middle harness $50 and decided not at this time. This weekend, I will see if any of the 4 '67 C10's and GMC's at the salvage yard have decent ones. I will probably be cutting up the tail light harness anyways because I want to rewire for trailer lights. The middle harness just runs from the firewall inside the frame rail back to near the rear bumper. As long as it isn't hard and cracked, a use one will work fine.

[mopar57]

Nice detective work...persistence pays off

[DanStokes]

YEA Scott!! Those things can drive one nuts.

Dan

[squirrel]

Also check the wiring at the rear of the truck, it's usually a kluge from folks wiring in trailer lights.

I was off by 8 feet...sorry....

Glad to hear you found it!

[silver_bullet]

Yeah...what Squirrel said... damned P.O.s...

[Scott Liggett]

I was off by 8 feet...sorry....

Glad to hear you found it!

You were close. It was a little farther forward, but still modified wiring previous owner. I forgot that I removed the wires that were attached there. It is partially my fault for not fixing correctly to begin with.

I will get pics later tonight.

[silver_bullet]

Can't blame yourself for issues caused by the PO... It's the lot we've chosen...

[TheSilverBuick]

Nice! Looking for shorts sucks.

I've put three turn signal fuses in the Skylark in the last two weeks. Since it's not taking out the brake light fuse, I'm assuming the issue is likely somewhere in the front turn signals, and I'm leaning towards the left one, as it seems I notice they are blinking when I'm going to turn left. Coincidentally the first time it popped I was towing a U-haul trailer, so thought it could be related to that, but it's happened twice since and I went over the visible rear wires (and again isn't effecting the brake light fuse). Every time I stick a new fuse in, I visibly watch the fuse when I cycle between the left and right blinkers a few times before hitting the road. So far no bites.

[milner351]

this reminds me that I was happy about putting all new wiring in the falcon, sometimes I'm afraid that tracking down mysterious problems takes longer than starting all new from scratch.
Nice work Scott - your patience exceeds mine!

[Scott Liggett]

A couple months back I installed a Fitech TBI style EFI on the truck. Had some issues with it out of the box. The Fuel Command Center fuel pump was DOA. They sent another one. The temp sensor included with it was reading 40 degrees lower than actual. I replaced that with one I had gotten for future EFI upgrades later. But, it had a major problem on anything above idle and part throttle. It would dump fuel in killing spark plugs and turning oil into gasoline. No amount of tuning on the handheld would solve the problem.

DieselGeek here helped me by tuning it and learning how their tuning software worked. He manually tuned the system and could email me the new calibration file. I could easily upload the new tune, but the self tuning software would take over and go back to what was happening before. The truck drove great until the Fitech tuning screwed it up. Even beat my coworker in his fancy new Dodge pickup on our new four lane road near work. Geek finally figured out how to completely disable the self tuning part of the EFI and now I can drive my truck. He says the software is functioning correctly, but it just can't keep it working correctly in self mode. I was about to pull the system off altogether unless he could solve the self tuning, or turn it off because it would eventually kill the engine due to fuel wash.

My plan was always to go to port EFI, but this opportunity came up to test a unit out, so I jumped on it. Unfortunately, it did not work correctly with my engine.

[DanStokes]

Glad you and DG Scott got it sorted. I hope FITech learns from your situation and fixes their software. You ARE a tenacious rascal!

I'm almost ready to try to fire up the Volvo but I have to wait for my fingers to heal up from the "dog incident" (could become a novel.....). Eager to hear it run!

Dan