Trailer Lights - Running lights blowing truck fuse only when connected via hitch

Trying to figure out where to start looking.

A couple weeks ago my 7 way connector on the trailer broke (old plastic just cracked) and all the wires pulled out. I was able to get a replacement and connected everything up and it looked to be working correctly - but I was just checking everything with the truck not fully connected - only the plug in - hitch not sitting on ball.

Everything checked out - brakes, blinkers, running lights - with not sitting on ball.

Before leaving and fully hooked up - brakes and blinkers worked no issue, and it was daylights so didn’t check running lights.

On the way home, once the sun went down, we realized the running lights where not working. We checked the connector (since we had just replaced it) and everything looked good. Used a multimeter to check and everything seemed good.

Checked the truck and found a blown fuse for the running lights. Replaced fuse. Running lights working again with trailer connected, but not sitting on hitch. Once lowered down to hitch, fuse blows again. I did notice we had an error message on brake controller when only connected via wiring but message went away when connected fully through hitch.

Where should I start looking? I assume it is a grounding issue, but all the wiring is ran through the trailer frame so I am not sure where to look there. Anyone seen this and know where they found the issue? Thanks for the help!

The fact that it’s only happening when the trailer is hitched makes me think you had more than just your connector crack. Look at the 7 pin wiring all the way to where it splits and heads off into a million directions in the trailer, I bet something in there is worn out. For sure look at the parts of the wire that get hit by the sun, unscrew any holders that guide the wire, etc.

You may need to splice from there and replace the entire 7 pin wire from the plug back.


If you are blowing the fuse in the truck, it is likely the trailer that is the problem. I have had this issue as well. Anytime an individual running light is not working correctly it can blow a fuse in the truck. They key to success with my trailer is making sure every single running light is functioning correctly. This meant replacing almost all the running lights with new ones. You can also replace the wiring harness, but that is much more difficult.

Climb under the trailer and check all the wires by hand. You are looking for damaged wires. The wire running under the floorboards on my trailer had actually gotten caught in the metal support beams that hold the floor. It was pinched so tightly I cut out that entire wire and ran a new one. This time with tip ties holding it safely away from any entanglement.

Running lights are a real pain to maintain. I certainly hope I never have to replace the wiring harness itself, as most of the wiring is hidden in the frame and not easily accessible.

If you decide to replace all the lights, you may want LEDs. I’ve heard they are less likely to cause issues, but I didn’t try them myself. I thought I was just replacing a couple running lights. Didn’t realize that eventually all the running lights would need replacing. I have 3 original lights left on the trailer and noticed today that one is cracked and needs replacing.


As said already, a trailer running light, or more likely a wiring splice or connection in the trailer tongue region dating to manipulating the wires to replace the trailer’s 7pin connector. If you or the trailer manufacturer used Scotch wiring connectors my bet is on one of those.

This is a Scotch wiring connector.

These are butt connectors, which are an order of magnitude better. The best butt connector types are watertight, with heat shrink tubing covering the otherwise open ends. They are no more expensive than Scotch connectors, but using Scotch connectors saves assembly time for the manufacturer. That is why so many horse and utility trailer makers use them.
butt crimp

I found my issue, so reporting back incase anyone else down the road is searching.

Like I explained my connector broke & the wires reconnected following diagrams & wire colors from the internet.

Turns out, whomever wired my trailer originally, swapped the running light hot & ground wire color it seems from standard.

I swapped the 2 wires and my issue is solved. Much simpler than having a wire worn out. I had already gone through all the visible wires and checked them and replaced some connections with butt connectors, but the issue was still there (could see voltage drop from trailer to truck hitch when checking with multimeter). Since the fuse would blow, the hot would drop out and then everything on my trailer (besides running lights) would work as expected using the ground through the hitch.