Taillights don't work (fuse blows)

[DaveBen]

I have a 2004 F-350 diesel truck that I bought new. I have added two items that are in the tail light circuit; clearance cab lights and a rear stop/tail lightbar. 5 or 6 months ago I noticed the tail lights were not working. I checked the fuse and it was blown. I changed out the fuse (15 amp) and turned on the lights. The fuse blew immediately. I removed the headliner from the cab and checked the wiring to the cab lights. It is perfect and not nicked or cut or chaffed. I used the factory harness when I put in the cab lights. I did the fuse thing again and it still blew. I remover the rear light bar and checked it again, and it still blows the fuse. I have crawled under the truck and looked for animal chewing marks, cuts and shorts; nothing. I am not a hack when it comes to wiring, I can build computers and radios, using the radio shack kits of the old days (wired and no circuit boards) I have wired several Jeeps in the past, so I know something about how vehicles are wired. Does anyone here have any ideas? Thanks.

Dave

 

[BIG JOE]

Dave.. disconnect the 7 pin trailer plug, and pull the tail light and front parking light bulbs. put the fuse in.. and see what happens ? Go from there ?

OR.... yer 10 year old light switch could be shot ??

Joe

 

[DaveBen]

Joe - I have pulled all of the lights and inspected them. No joy. I have pulled both 5 pin and the 7 pin trailer connectors and they are nice and clean. Tomorrow I was going to replace the headlight switch. It i s the only thing I have not looked at or messed with. I am betting the headlight switch is bad.

Dave

 

[DaveBen]

I got a new headlight switch and that is not the problem. Still blows the fuse when the headlights are turned on. I guess I will start to remove every light on that circuit. I will plug them in, one at a time, until I can find the culprit.

Dave

 

[JLDickmon]

Dave,I'd tell you to build a short finder out of a circuit breaker and a magnetic compass or inductive analog voltmeter and start running the wires.. how about didn't you tell me something about a pinched wire behind a fender due to collision damage a year ago or so?

 

[DaveBen]

I did not find anything pinched or chaffed behind the fender. I was very careful to make sure I got it all. I removed the battery and the battery box and inspected under these items. I traced the wires going to the finder (harnesses). I was proposing that a wire might have been pinched, but no wire was found. I have taken every light bulb out of the tail lights, running lights, clearance lights and marker lights and it still blows the fuse. I know there is a short to ground and I have to find it. Down to looking now that the plug-in things didn't work.

Dave

 

[trackspeeder]

Have you tried disconnecting the clearance lights?

Your wiring could be fine. A bad socket or bulb could pop the fuse.

 

[DaveBen]

I got to looking at it this morning and I found one clearance light that had a blown bulb. It blew and shorted out to ground, causing the fuse to blow. I changed the bulb and all is good right now. I have never seen a bulb blow to a short, most blow open. You are correct trackepeeder. Any way it is fixed now.

Dave