Blowing fuses?

[JAKES 68GMC]

I'm having a problem with my rear taillight/parking/brakelight fuse blowing. When it blows, I still have my parking lights and blinkers, but not brake or taillights, or lights on the inside of the cab.

I was coming home from a friends house and went to turn my lights on. I had headlights, but no inside cablights or rear taillights. So I figured "Ahh, just an old fuse that blew, no big deal". So I changed the fuse the next day, and it worked fine. But the next day, I want to try my lights and the fuse blew again, immediatly. So I stuck another fuse in, same thing. So my dad and I looked some of the wiring over, and looked for bare wires, and we couldn't find anything.

So we took it to a garage and had someone smart work on it. He fixed it. He said "I just shook the hell outta the wiring in the cab, and put another fuse in, and its fine." So I took it, this was last Wednesday. I drove it a few times since with the lights on, and it was fine. But, I went to try my lights today, and the fuse blew immediaetly. I tried shaking the wiring harness, but that didn't help any.

Why does the fuse keep blowing on me, any ideas? Thanks for the help guys, this is driving me nuts.

[DavesRide]

If i might teach you a trick that old techs use to find a Short to ground. They can be tough to find especially when they only happen at certain times.
OK, here goes. To test the circut without wasting numerous fuses, plug in a headlamp bulb where fuse goes. Yes you will have to make an aparatus to adapt the headlamp and wires to where the fuse went. The load that the lamp has will keep the rest of the wires from "letting out their smoke" as we call it. If the headlamp is lighted, you have a short. When the bulb goes out, you have no short. Once Ifound this out, many shorts were found in short time. We call it the Ol' Ugly. Hook up the Ol" Ugly and start to move those wires around. When the light goes out. Look in the area of where you were when it did. Works every time if you have the patience. David

[cdowns]

the wiring harness runs down the drivers side frame rail and its quite common fo the isullation on the wires to breakdown and short out to the frame crawl under the truck and inspect it inch by inch to find the bare wires dont quit if you find one spot because lots of times theres alot more than one bare spot

[jay6891]

When mine did that it was the wire going to the interior light. Runs right past your foot on the left side under the rocker cover. (the thing that is supposed to hold your carpet/mat down). at the door opening. Unplug the wire going to that at the fuse box and see if that fixes it. I believe mine was orange in color.

[JAKES 68GMC]

I checked under the truck and the wires look fine. They're covered in so much old gear grease and crap, I don't think they could chafe anyway.

[kxnomore]

I'm watching this thread as my truck started doing the same thing a couple of weeks ago. As soon as I turn on the headlights the tail lamp fuse blows. With the headlights off it doesn't blow the fuse.

The stop lights and blinkers and headlights always work, too.

So far I have not been able to find any bad wires either, but I like the headlight idea. I'll have to try that tomorrow.

[DavesRide]

Unplug the ones that are swithed on with the Ol Ugly in line. Did it go off? The light of the Ol" Ugly I mean. If you pulled the headlamp switch on when the light (OLD UGLY) lite's up, inspect the switch also .

[JAKES 68GMC]

Dave, how do I plug a headlight lamp into the fuse box? I get what your saying and it make sense, I just don't see how you hook the bulb up.

[Longhorn Man]

first, I hope you didn't pay the 'smart' mechanic very much money... and I hope you know some better mechanics to go to.
The very first place to look when you are popping tail light fuses, is under the rear bumper. Look at where all the trailer harnesses have been spliced in over the years. look for where the harness is pinched under the frame maybe, look for green crap growing out of the stupid scotch locks that so many ppl think are such a good idea.
This is where you'll probably find your problem.
And so you know, if that fuse pops, you have no brake lights either.

[JAKES 68GMC]

One thing we did do, which I forgot to add in, was that we unplugged the wiring harness that goes to the taillights (there was a plug in the cab) and when we tired the lights again, the fuse still blew. That would mean that the problem lies in the cab somewhere? Thats kinda what we concluded.
I may check out the dome light wire today like Jay mentioned. seems as though that wire was kinda rotten IIRC but maybe not, it sounds like a good place to look though.

Thanks for the help guys.

[Longhorn Man]

ok, you are right, you can rule out the rear 1/2 of the truck. You DO still have the front markers on that circuit though.
Also, try unplugging the turn signal switch too... esp if the switch is sloppy.

[JAKES 68GMC]

I tried unplugging the dome light wire, the fuse still blew. What would the turn signal switch have anything to do with the taillights? The switch is a little sloppy.

I've got 20 new fuses to play with so I should be all set for awhile.

[JAKES 68GMC]

I just fixed it, I shook the wiring harness up around the light switch and the gauges and now it wont blow the fuse. So I guess I can shake the harness if the fuse blows, no biggie.

Thanks for the help guys.

[68 P.O.S.]

I wouldn't call that "fixing" it. I would continue to look into fixing it correctly...it would suck pretty bad if you got into an accident just because you thought what you did is "good enough". JMHO.

[bad6772]

my 67 did this earlier this year replace the headlight switch and it fixed it.....seen it happen to a few other trucks also..

[JAKES 68GMC]

Well the 1968 Chevy truck service manual says to shake the wiring harnesses when you have a fuse that keeps blowing immediatley.

I did the factory prefered method, what more can you ask for??? Maybe when I can find some time I'll attempt to fix it correctly.

[Andy4639]

Making a light bulb for fuse block.

Get a old bulb with wires coming off of it and put a aligator clip on both wires then hook it to the 2 ends of the fuse holder in the truck fuse block.
If you have head lights on and you have added other lights to the circuit it maybe overloading the switch. 30 year old wiring with a high load could have shorten the life of the switch.

[Boog]

I wouldn't let it go for long like that. It would be a real shame for the truck to burn down because of letting an intermittant short go on.

[Longhorn Man]

the brake lights go up through the turn signal switch, and the brake lights are on the same circuit as the tail lights which you were working on.

[JAKES 68GMC]

Well, maybe I'll look deeper into it sometime soon. But for now its working, and sorta safe , so we'll be headed to school tomorrow morning.

[kxnomore]

I had the same problem so I was watching this thread for tips. It took 3 or maybe even 4 hours to track down my bad short.

I actually found 4 spots where the rear harness was chafed bare and rubbing the frame:

The first was where the flat 4 wire harness went around the parking brake. Every time the brake was applied it moved the wires a bit.

The second was at a support by where the exhaust hanger was.
The third was where longhorn man called it- right where the green wire to the backup lights goes up through the rear of the bed. Both sides where actually chafed.

And after fixing all those the fuse still blew instantly.

But the fourth was the culprit. The friggin license plate light wire had a long black wire pigtailed to it that went around by the bumper bracket, and I had wrapped it around the bumper bracket when I put the bumper back on after paint, not knowing where it went. And that was of course the one causing all the problems.

But during all that I also found out that the backup lights run off that same circuit so maybe try disconnecting those by the neutral safety switch under the dash?

[Longhorn Man]

I never realized the back ups were on that circuit.... but then again, I've never had working ones.

[Yukon Jack]

Originally Posted by DavesRide

If i might teach you a trick that old techs use to find a Short to ground. They can be tough to find especially when they only happen at certain times.
OK, here goes. To test the circut without wasting numerous fuses, plug in a headlamp bulb where fuse goes. Yes you will have to make an aparatus to adapt the headlamp and wires to where the fuse went. The load that the lamp has will keep the rest of the wires from "letting out their smoke" as we call it. If the headlamp is lighted, you have a short. When the bulb goes out, you have no short. Once Ifound this out, many shorts were found in short time. We call it the Ol' Ugly. Hook up the Ol" Ugly and start to move those wires around. When the light goes out. Look in the area of where you were when it did. Works every time if you have the patience. David

I need to make an "Ol' Ugly" and find the culprit of my identical problem with my K5 - everything is fine until I turn on the headlight switch then I blow that same fuse - this keeps me from driving the K5 at night and with daylight savings coming I need to get this problem fixed.

Am I correct that after I have Ol' Ugly hooked up, I turn the headlight switch on and because of the short the headlight will light up - then when I start wiggling the wires and Ol' Ugly goes out, that area of the wire is where the short is?

Just wanted to make sure I should have the headlight switch on when i start wiggling the wires.

[DavesRide]

Yes, you want to recreate the condition that blew the fuse. if it is when the lights are on then turn them on. It will save you many fuses if you don't find it right away.
Before the guys in the shop found out this method to diagnose shorts, I watched a tech that had been chasing a short that was causing him grief put a shunt in instead of fuse. Waited for smoke and found the short. Not a good idea but he did find it. That was over ten years ago. I would much rather keep the wires from melting and destoying other items.
Try it out. For instance, if you have a cig lighter, connect the leads from headlamp to where fuse should be. Stick a screwdriver in lighter socket to create short to ground and watch the light lite up. Remove screwdriver and light will go out.
Then find your short with same method of hooking up the bulb with the circut LIVE. We did create some different variations to keep the bulb itself from melting plastic and carpet by containing it in a can with many ventialtion holes. David

[72BLUZ]

in my line of work we find bulbs in the tail lghts can cause this problem. and you may find a rubbed wire causing it as well. check behind the fuse block for loose connections and start there where the fuse keeps blowing and trace all the way to the lights. you will eventually find a short. these old fuse blocks on these trucks can be the reason for your problem. they also make breakers that push into the slots where the fuses go. i am going to change mine to these breakers. it could just be a dry cracked wire touching the frame some where!!