Electrical problem...

[71Blaze]

Hi all,

My 71 Blazer has suddenly developed some electrical problems. First and foremost the tail and brake lights quit working the other day. I checked the fuse and it was blown. Naturally I figured all would be good if I put in a new fuse and the instant the ends of the fuse touched the clips, it popped! I tried another one and it did the same thing. With that in mind, I ordered and installed a new rear light harness (it needed it anyway). The problem is still there. Since all the other wiring has been replaced, I figured I should probably replace the flat 4 wire harness that runs from the firewall back to the rear light harness. I don't know if that will fix it, but it's worth a try.

The other problem (I think it's related) the parking lights don't work, front or back. The turn signals work all around though.

Any ideas? Thanks!

 

[BobK]

Have you visually checked the harness from the firewall back to the rear harness?
Most likely ione of the hot wires is ground out somewhere along the way.

Do you have trailer wireing?
I had a electrical gremlin like yours once.Everything checked out great wiring wise.Turned out to be one of the terminal screws in my trailer wiring socket was grounding out on the houseing,blowing the fuse.

 

[71Blaze]

There was a trailer harness hacked in to the old wiring but when I installed the new light harness I left the trailer harness off.

I have a bad cold at the moment, but when I feel better I plan to check the wires going to the back.

 

[ryoken]

check where the clips hold it to the frame... sounds like a dead short...

 

[Yukon Jack]

I have a similar issue with my K5 right now. If I turn on my parking lights I blow that fuse. If I disconnect the connector for the back lights at the firewall I don't blow the fuse so I know where to start.

Have you unhooked the connector at the rear most crossmember? If it still blows you know the problem is between there and the firewall connector.

 

[71Blaze]

I have a similar issue with my K5 right now. If I turn on my parking lights I blow that fuse. If I disconnect the connector for the back lights at the firewall I don't blow the fuse so I know where to start.

Have you unhooked the connector at the rear most crossmember? If it still blows you know the problem is between there and the firewall connector.

I just replaced the rear light harness, so I'm pretty certain that's not the issue. The original one was in pretty sad shape. I haven't disconnected the rear light harness from the firewall yet. I'll try that after my cold goes away.

 

[68MUDSTUD]

I have a similar issue with my K5 right now. If I turn on my parking lights I blow that fuse. If I disconnect the connector for the back lights at the firewall I don't blow the fuse so I know where to start.

Have you unhooked the connector at the rear most crossmember? If it still blows you know the problem is between there and the firewall connector.

I have a problem Exactly like that in my 71.. it does it either when you turn on the park lights, or use a turn signal!

 

[Greg72]

Maybe it's just the headlight switch itself? Does it blow the fuse when the switch is in the "OFF" position also, or just when you pull the lights on?

I solved a bunch of odd lighting behavior when I swapped out my old steering column. The directionals would only blink when the brakes were not applied, and I think the right directional turned on both sides (like the hazards look).

Electrical problems suck. I have decent schematics in my Misc Folder photo gallery if you don't already have those. Might help.

 

[71Blaze]

Mine blows the fuse with the lights off or on regardless of ignition key position. Everything was working fine for the year I've owned it until recently. My problems started with the underdash courtesy lights not working one day. I don't really need them so I didn't pay any mind to it. Then one day the heater blower motor stopped working. I don't think that's related though because I used some jumper wires directly from the battery to the blower motor and it still didn't work. Then most recently the tail, parking, and brake lights quit!

Before my cold started kicking my axx, I replaced the rear light harness. Hopefully this weekend I'll feel good enough to trouble shoot the harness that goes from the firewall back to the rearmost crossmember as suggested above.

 

[71Blaze]

Electrical problems suck. I have decent schematics in my Misc Folder photo gallery if you don't already have those. Might help.

Yeah electrical problems suck! The only schematics I have are the ones in the Haynes manual and they suck as bad as the problem! How do I get into your photo gallery? Anything has to be better than what Haynes provides.

 

[Greg72]

I've got the nice factory ones in my gallery.

Just click the blue "hyperlink" looking part of my .sig and navigate to the Misc Stuff folder.

 

[Blazin'4x4]

How about pullin the bulbs and cleaning the bulb retainer. Worked on an old hot rod with the same symptoms. Good luck

 

[71Blaze]

How about pullin the bulbs and cleaning the bulb retainer. Worked on an old hot rod with the same symptoms. Good luck

Good idea - always start with the simple things first. However, it's N/A to my truck, it has LED tail lights, new light buckets, and a new rear harness. I intend to thoroughly check the intermediate harness between the firewall and rear light harness this weekend. I'm almost positive my problem is there - somewhere.

 

[bigblock72]

Usually the harness gets cut/chafed/smashed around the transmission crossmember(s). Be sure to check real close there. Not uncommon for the wiring to fall out of the clips (if they are even there) and get burned on the exhaust. I'll bet the problem will be fairly obvious whatever it is.

 

[71Blaze]

Greg72- Thanks for the schematics! Do you have any higher res versions? The ones I downloaded are kind of small and blure when printed.

Progress update - I didn't fix anything, but everything is working as of this afternoon. The first thing I did was remove the harness that goes from the firewall to the rear of the truck. I tested it and found the brake/taillight terminal was grounded (or so it seemed). I then climbed under the dash and traced the four wires through the dash harness, through the headlight switch, and eventually I ended up at the firewall again. I got under the hood and disconnected the forward light harness and engine harness from the firewall. Using Greg72's schematics, I was able to determine which wire powered the brake/tailights. It too seemed to be grounded. Referring back to the schematics, the brown wire worked its way around the truck feeding various items such as front & rear park lights and all side marker lights. The brown wire also went to the voltage regulator. I converted my truck to an internally regulated alternator with a M&H wire harness that was modified (by M&H). So, after looking at the alt, I noticed that one of the wires going into the voltage regulator was brown. Hmm. I unplugged that wire and checked the harness again and the ground was gone! Natually my first thought was that I had a bad voltage regulator, but with it still disconnected, I installed a fuse and plugged the harnesses back into the firewall. As I snugged down the bolt I actually heard the fuse pop (I don't recall what rating the fuse was but I do know it wasn't a 20 amp).

At about that time, I was getting frustrated (and hot thanks to the great weather) so I put everything back together and pretty much gave up. After I was done, I decided to try my last fuse, which happened to be a 30 amp. I plugged it in, and surprise, surprise, it didn't pop! With the help of my wife, I checked all the lights and everything was once again working: tail lights, brake lights, front and rear parking lights, and all of the side marker lights. I know it's supposed to have a 20 amp fuse, but for now I intend to use the 30 amp. Maybe with the changes I've made, it needs the bigger fuse? I don't know.

Any thoughts?

 

[1972dozer]

i had a problem with my parking lidghts and like to have never found when and wat it was is the dash lights would not work at all and the parking lights wouldnt go off at all and you could control the parking lights brightness with the dimmer switch so check every wire on that thing and then put a dimmer switxch in it and it did the same thing but come to find out . aftrew giving up for a while . i had to take the fuse panel off and where all the wires are on the back of the fuse box one of them had a short . so if it does it again you might want to try checking behind that fuse box .

 

[71Blaze]

so if it does it again you might want to try checking behind that fuse box .

As I followed the wires through the dash harness I ended up at the fuse box. I removed it from the firewall and checked each wire on the back side to make sure they were all in good shape. They were, and that's what led me into the harnesses on the engine side of the firewall.

It's seems strange to me that a 30 amp fuse will hold up but a 20 won't. Since everything is working I hate to mess with it, but I might pick up some 25 amp fuses today to see if they hold up. I've always been told to use the lowest amp fuse that will get the job done.

 

[Greg72]

71Blaze,

If you click on the schematic photo in my gallery it will "grow" to a larger view which can then be saved in a higher resolution. The "preview" one you were probably looking at is decent, but it will be fuzzier if you try to print it as a full-page document.

30A sounds like an awful lot of fuse for a taillight circuit. I think you still have something else going on that needs to be resolved. What did you add to the circuit that could be drawing 10 additional amps? Offroad lights, a stereo amplifier...??? It's going to have to have been something significant to draw 10A.

 

[BobK]

The circuit's are rated at a certain amperage for a reason.

If your tail light circuit is sucking 30 amps it prob just a matter of time you'll have some type of meltdown,somewhere.

That "could" result in a fire.

As Greg suggested,"unload" that circuit if something extra has been added.

Check all the sockets for corrosion and well as check the grounding points for corrosion.This can increase the resistance in the circuit and end up drawing more amps.

As wire harnesses get old,the wire insulation can crack.This allows condensation into the wire,the wires then corrode and get brittle inside the harness...again leading to higher resistance and more amps being drawn.


I don't mean to scare you with the word "FIRE", but that's how guys end up toasting their trucks.

A new harness will solve your problems,won't waste your time,isn't that expensive, and something your truck most likely needs anyway.


Electrical gremlins can be a b#tch to resolve.


Now...if you like to solve this kind of stuff,you know...like "I WON'T LET THIS BEAT ME!" type of thing...get a multi-meter,a test light and a 6 pack of your favorite beverage and have-at-er!


Good luck.

 

[71Blaze]

BobK - You bring up all good points. However, I've already replaced almost every harness on my truck with M&H products. The only two harnesses that are original are the flat 4-wire going from the firewall to the rear light harness and the underdash harness, which by the way I found is in excellent condition thanks to yesterday's wire tracing adventure! Oh, and potential fire has crossed my mind!

Greg72 - I totally agree 30A is a lot for a tail light circuit. I haven't given up yet.

Electrically, this is what I've changed:

Dakota Digital gauge cluster
LED tail lights
LED underdash courtesy lights
HEI
Internally regulated alternator
Optima yellow top battery
Twin 16" electric fans (via relays)
New M&H harnesses - front light, engine, A/C, and rear light

Honestly, I feel I'm pretty good (and thorough) with auto electric. All grounds are clean and solid. All connections have been soldered and covered with heat shrink.

One more thing I did yesterday that I forgot to mention - when I had the engine harnesses disconected from the firewall, there was an overabundant amount of that black goey grease in the plug. I used a small flat screwdriver and removed the excess. I know the grease is there to protect from corrosion and that it's not conductive, but there was just way too much.

For shxts and giggles, I'm going to pull the 30A and try a 20A to see what happens. Who knows, maybe I inadvertantly fixed the problem by moving/unplugging/plugging the wiring?