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Electrical short thats driving me NUTS!

joez

joez

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Ok, ive been having a problem with blowing one fuse continuously. Now its gotten worse, and i cannot figure out what is causing it. Heres what its doing.

The T/L CTSY fuse blows as soon as i pull the headlight switch. This causes me to loose parking lights, license plate lights and dash lights. I still have brake lights when i hit the brakes, i still have headlights, and turn signals work. Ive ripped the dash appart, and can find no loose wires, or anything out of the ordinary. The wires leading back to the tailights all look good, i can find no damaged wires. Anyone have this happen to them? Im stumped. I thought maybee the switch was bad, but then wouldnt i loose headlights as well?
 
Have you checked all the ground connections to the bad circuit? Track down all the grounds for all the lights that are on that circuit, and make sure your getting a good clean connection.
 
I had it happen when I replaced my tailgate and pinched the wires with the mounting plates going in the tub.
Bill
 
I would be looking for continuity to ground on all wires that that fuse feeds. (the ones you lose when you turn the lights on)

IIRC the heeadlight switch has a built in circuit breaker, so it's almost impossible to lose lights unless you lose the fusible link to the fuse panel.

You might also de-pin the wire that feeds varying voltage (dimmer) to the fuse panel from the headlight switch, and re-connect the plug, try lights, and see what happens.
 
The first place that I would look is all the way at the back of the truck. Hack job trailer light wiring is often the culprit for that fuse blowing. /forums/images/graemlins/frown.gif
 
I agree. Try unplugging the tail light wiring from the harness, install a new fuse, and see if you get anywhere.
 
Try to remember what work you have done to the vehicle lately. Sometimes a wire can get pinched or just bared when you work close to it.
If this doesn't help you find the problem it is time to get the creeper out and follow the wires to the back of the vehicle. Anything out of line should be visible with minimal effort.
The next place to look is at the very back of the vehicle. There is some rough routing back here. If you still can't find the short try pulling the taillights apart to see if a wire has corroded or worn through.
A rusted lamp socket can cause a short also. Give them all a good shot of grease when you have them apart just to keep any further corrosion at bay. Don't forget tail lamps, parking lamps, marker lamps, any lamp.
Sometimes it can take years for wires to wear through.
OOOP's one more place to look is in the forward lamp harness. This harness is a prime location for corrosion or a pinched wire... maybe under the battery.
Chances are it is not a ground as the ground is after the load and if the ground wire is shorted or pinched this wouldn't make any difference. If the ground was cut then the fuse would not blow, the circuit wouldn't work either.
 
I'm not sure if my post was unclear or not, but if it was, and thats what you were talking about ground for, let me clarify.

Checking continuity to ground means making sure wires that aren't supposed to go to ground, aren't.

There is no reason to trace wires if you know what the wires do, and whether they should have continuity to ground or not. If one wire on the headlight connector (making up a scenario here) is the ground for the switch, and yet a second wire on that switch shows that there is a connection to ground (continuity) then that second wire is bare and touching ground somewhere.

Conversely, if the wire is broken, without apparent damage to the jacket, checking continuity from one end of the wire to the other proves whether it is broken or not. No reason to chase wires if the problem is constistent and 100% repeatable. Obviously blowing fuses won't be the result of a broekn wire inside its insulation, just showing how effective continuity tests are.

You can also make sure that there is not continuity between wires that shouldn't have it between them, say two wires that are supposed to feed different things. (backup lights vs. tail lights)

I do agree with the others, previous owner installed trailer wiring is probably the #1 cause of light issues.
 
Im now thinking it is the switch itself. I have unplugged the tail lights, disconnected front parking lights, disconected my visor, and also the t-case light and dash lights. Ive checked and re-done every ground i could find, i soldered on new ends on the two that had rust on them. I traced all wiring inside and out of the truck, cant find anywhere it is bare or could be grounding out.

The trailer lights were actually my first suspect, but the wiring job is actually pretty darn good. I was surprised to say the least. I ran out of daylight yesterday, so ill go back at it today and see what i can find.
 
Dont spend all the time in the back of the truck.
Also check the parking lights and any running lights in front of the truck.
 
No rust or corrosion on any of the wires. Only two grounds had some rust on them, and that was just surface. Tim has seen the truck, he knows how clean it is. Tailight sockets were my second suspect, but they are clean as a whistle. Ill check all of the forward connections again today, as i only briefly went over those yesterday before it got dark.
 
Disconnect the stereo. If the factory harness was used there is a wire for the dash lights on the dimmer. With lights off it acts as a ground through the bulbs. Turn lights on and it's dead short. The radio will already have ground through the antenna coax. There is no ground in that harness. Three postives, Batt, Ig and dash lights.
Your welcome /forums/images/graemlins/histerical.gif
Let me know if I'm right. /forums/images/graemlins/deal.gif
 
There is a ground at the radio, although your scenario sounds good considering that it will find a ground if one is not there and hooked up correctly.
 
I had the same problem on a friend of mines Dodge P/U. It turned out to be an old license plate light that was cut in order to make room for a trailer light plug. The wire looked fine from the outside, but was corroded and causing a short exactly as you desribed.
 
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