Running Lights Won't work

[Irishcream]

Recently I blew a fuse in spot no.4 in the fuse box in the cab of my 97 OBS F250. It killed all my exterior running lights and also all the instrument panel lights on the inside. The headlights, turn and brake lights all still worked but none of the running lights including the tail lights. I replaced the stock 10 amp fuse with a 20 amp and it lit up all the lighting but only lasted for 3 days. Last night when I started up the truck before going out I tapped the brake pedal and the fuse popped and all the lights went out. Where do I start to look to find this wiring issue?

 

[Bugman]

First off never replace a 10 amp fuse with a 20 amp unless you want a truck fire. Fuses are there to protect the wiring and if you have too many amps going through a too small wire you will create a fire hazard.

As for where to look for a problem check the rear end wiring to see if a previous owner has wired a trailer connector into the harness without using the factory harness. Other than that it is just a hunting expedition trying to find where the problem is.

 

[cfdeng7]

I would start at the easiest places to look. Follow the harnesses from each light as far as you can looking for chaffing, melted wires etc. Check all the wiring you can see under the truck. And I wouldn't keep putting bigger fuses in. Fuses are there to pop before the wiring does. You could make a much bigger problem by doing this.

 

[Irishcream]

is it possibly the Headlight switch? I picked up a auto reset breaker and through it in spot no.4. It lit all the lights but for only a couple seconds. It got pretty hot even after the lights were out but when I turned the headlight switch off it cooled down. I have a new switch sitting on my toolbox at work but of course, that's at work.

 

[SPEEDDEMON31176]

It could be the headlight switch, the turn signal switch assembly, a short in the tail light wiring, etc. I would start by checking the wiring at the rear of the truck first.

 

[Irishcream]

Well, I found what the problem was. Because I don't have any mud flaps for the rear tires it was shooting all the snow and dirt into the bumper where of course my license plate light which hasn't worked in over a year was sitting waiting to become a problem.
While under there I also took the time to replace the trailer light cord harness which goes between the two tail light plugs. one of the posts had coroded and broken off in the adapter. It's been slow around the shop lately so the boss let me get it all back to good working and "legal". Replaced a 3rd brake light bulb which had burnt out so hot it actually melted the glass into a droopy bulb. I changed the headlight switch first hoping it was the problem but once it turned out not to be the issue I figured I'd throw the old one in my tool box along with all the other spare parts I have accumulated for my truck. Thanks for all the help fellas.