dash/tail/brake circuit short

[webfoot]

Here's some background info: pickup is a 71 k-20. It had an old junky stereo and aftermarket speaker in the center, neither of which ever worked. I thought it was the stereo that finally gave up the ghost, but when I got under the dash, I noticed that the speaker had fallen down and of course landed on the red wire for the cigar. (yes it looked sweet!)

So of course I pulled that junk out of there. But, I still have a problem.

Whenever I turn my headlights on, the dash, tail & brake lights go out and the fuse almost instantly burns. If I don't have the headlights on the dome light will even work and presumably the brake lights.

Does this sound like it could be a defective headlamp switch or do I need to dig for a big fat short somewhere? I tried removing the dash pad but had to post a new topic on how to do that.

[Longhorn Man]

if the fuse blows as soon as you turn on the parking lamps (the actual head lamps aren't on a fuse at all) then the problem will be in the parking lamp circuit.
Odds are, it isn't going to be the headlamp switch.
You'll need to check out the harness going to the tail lights (brown wire is the one you are specificly looking at) and then also the wires going up to the parking lights and markers.

[webfoot]

OK, so I think the best bet is just to replace the taillight harness. Is this available, and how hard would it be to do? I think not that hard but I've been wrong before.

The PO that made all the wiring "adjustments" in this should be hung!!

[72lb4x4]

First unplug the taillight harness at the firewall in the engine compartment and retest.

You should also probably try to find the problem rather than replacing parts randomly.

[dammitmitchell]

troubleshooting and fault isolation is what serperates a real mechanic from parts swappers..
the few minutes or hours finding the real problem could save you alot in the long run... never know.. that wire might be shorting out into something very important.. and maaaby that something important could set your truck on fire..

I would start by getting yer friendly wiring diagram..
and removing positive lead from the battery..
then check for a short to ground through all of your positive wires..
they have "inexpensive" multimeters at sears.. you dont have to go exxtravagant like some of us..
30 -50 bucks can get you what u need =-)
good luck

[dammitmitchell]

oh.. crap forgot..
if yoru problem is happening "after" you turn on the lights.. focus your search on the power leaving the switch
pull the switch harness off it and just start from there

[VetteVet]

Reading this thread I just couldn't resist commenting. This is one of the best threads on this forum that I have seen since I joined the boards. It starts with a very good description of the problem with inclusive information about the vehicle which gives the responders a lot better chance to diagnose the malfunction.
The information in the replies is exactly right and they cover it well. All I can add is that the common places to look are under the door sill and carpet in the cab and the harness in the rear along the frame.
Kudos to all you guys. Reps to all. LHM has too many LOL

[webfoot]

Well, I found the problem and it cost me $2 to fix.

I'm not sure exactly what led me to even look there, but when I removed the dome light lens (ok it was so brittle it went into a million pieces) I saw that the bulb looked like it had been very hot. It also looked very old. Removed dome light bulb and turned on marker lights, fuse does not blow. Replaced bulb, markers work.

It was out of pure frustration I entertained the thought of rewiring the thing, I'm glad I didn't!

Anybody recommend a good vendor that would have a new lens for me, and a new clutch and brake pedal cover? The bare metal gets kinda slick when its wet out!

[Longhorn Man]

for items like that, they all have them.... order from someone close to you with a good rep to keep your shipping costs down.
The only one I am real familiar with is classic heartbeat... and he is highly recomended

[webfoot]

How rare of a deal was that, on a scale of 1-10? I've never heard of it happening. I suppose a lot can happen on a truck pushing 40, that may have been the original bulb!

[Longhorn Man]

it happens. Nor real common, but like you said, it IS a 40 year old assembly.

[stealthy '69]

I'm new to this board and I've been having the same problem. I just bought my truck in december and have been working a little here and there to fix it. I was reading this and it hit me like a kick in the face. my dome light has been cut out and the bare wires i finally found last night, with power still flowing, shorting inside the cab. I think that's the first place I'll start looking. HOLY CRAP I LOVE THIS PLACE!!!!!

[stealthy '69]

Well, I pull the dome light wires and spent about a week tracing and testing others. And with no success, still blowing fuses. I did find that the previous owner cut out a section of wire to the right headlight and spliced in some 10 ga industrial wire. It looks almost the same as what we use at work when we wire in new equipment that require 480 volts. Would that be causing it to draw a higher amperage and then overload the fuse? I did get a new headlight harness that I plan on installing this weekend. If any one has any other advice please help because I'm lost, and I'd love to be able to drive this thing at night. And I feel like a deer in the headlights when I'm working on electrical troubleshooting.

[rick1956]

Ah, this takes me back to when I was younger and did a bit of "guerrilla troubleshooting" on a similar problem.

I had a 64 Comet Cyclone that had the same sort of problem. I'd replace the dome light fuse, and as soon as I'd open the door the fuse would blow.

My way (I was 17 then):
I took both door sill plates off (I had them off already actually)
I cut a piece of 1/4" bolt to fuse length and put it in the fuse block
I sat in the back seat and opened the driver's door (on full alert)

After only seconds, smoke started coming from below the driver's floor carpet. I hurredly closed the door and pulled the "fuse" back out. When I peeled the carpet back after taking the driver's seat out, I found a spot where two wires had crisscrossed and been hammered into each other by years of foot traffic. Problem found and no damage done!

Obviously, I wouldn't recommend this method, but hey, it works too!