Hey all,
I've run into a snag with my trailer circuits. Currently I only have right side trailer lights. When I tried tracking down the issue I figured out my under hood fuse was blown again. I replaced it, hit the left turn signal in the truck and it immediately blew. So there is a short somewhere between the junction box and the grey connector by the spare that the oem 7-way extension plugs into (currently not connected). There is also a 7-way in the bed that was spliced into the truck->trailer wires by the spare. They were done poorly but don't appear to be shorting anywhere (no exposed wires). Does anybody have some tips on where there could be a short under the truck?

Thanks!
Chris

 

Be specific about what truck you're working on and the specific fuse number.

 

The spliced wired don't appear to be shorting or were they tested for shorts? I'd cut the spliced wires off,(especially since you say they were done poorly) make sure the harness wires were still in good shape, then try your turn signals. If you want or need an in bed plug, get a complete in bed harness that plugs in, no splices.

 

 

 

Pro Tip #229 - When blowing fuses is a problem. Take an old car horn, stuff shop rags into said horn, install 6 ft leads onto horn (ground and power). On the blunt cut ends of leads install crimp-on spade connectors. Insert spade connectors into problematic fuse. Listen to horn as you eliminate, tighten, remove, shake, etc all offending suspects (wire bundles, devices, etc.) Current draw of horn in conjunction with audio will work to identify short circuit offender.

 

Sorry, I'm working on my 00 F250 CC. Fuse #1 from the manual, under the hood junction box.

I'm considering doing this. I was hoping to use the 7-way that's there and wire it into a t-connection at the point the bumper extension connects to the truck harness by the spare. The splice itself has no exposed wire that would be the cause of the short when nothing is hooked up to the 7-ways (bed or bumper).
The splice was done by striping the insulation of the oe harness wires and wrapping the stripped end of each wire from the 7-way around its pair from the truck harness. Then each splice was wrapped in copious amounts of electrical tape. Not how I would have done it but none of these connections are the source of the short.

Right, no trailer is hooked up and the OEM 7-way bumper extension is disconnected from the truck.

Trailer has been eliminated from the circuit and left turn signal still blows the #1 fuse.

As soon as I track down a horn I may give that a try.

 

Tracing this simple circuit should be fairly easy (knocking on wood since I have been proven wrong before). Attached is what I believe to be your diagram, there are three (3) connectors tween fuse #1 and the bumper. Using the horn method, and breaking each connector, you should be able to quickly isolate the troublemaker..

 

Don't forget to check the bulb sockets.

 

I would pull both turnsignal bulbs and see if the short goes away, if it does replace 1 at a time to find which 1 is causing the problem, I have more than once had a shorted bulb.

 

Gent's, with all due respect, the fuse that is being blown, Fuse #1, (& Fuse #3) are trailer light fuses. One of the cool things about SD's is that the trailer light circuit is separated from the truck's tail light circuit. Looking at the diagram it is readily seen that the truck turn signal bulbs are not energized nor involved in the circuit past Fuse #1. The trailer has been disconnected per comments by OP. It is always a good idea to check everything and leave no stone unturned, but in this particular case the trailer bulb's are not a factor (at this time).

 

You are absolutely right and I was about to say the same thing. (BTW, the trailer light functions perfectly when connected to a different vehicle.)
The issue is definitely in the trucks trailer circuit wiring between the OEM trailer plug extension connector (by the spare under the bed) and the fuse box. I'm going to have to do as someone else suggested and check each connection from the box to the tailgate and the wires in between. Without the horn test, I figure I can test for circuit grounding after disconnecting each section of harness moving forward from the back to isolate where the short lives and then open up the loom and inspect the wiring in the harness section to find the short. There may be something I'm overlooking in the spliced wires so I'm going to start by eliminating that first and check for grounding.

 

Regarding that OEM trailer plug extension. Had a trailer wiring issue the other day myself, opened up the plastic covering on the trailer plug extension and found some apparently factory "butt splices" of very dubious quality. My wiring issue, which was an intermittent open involving the apparently factory butt splice on trailer ground wire.
No telling where/what you will find regarding the root cause of your turn signal wire shorting issue, but after seeing those butt splices on my trailer plug extension, nothing would surprise me.