Trailer Lights Blowing Fuse When Running Lights Are Activated

Question:

2009 enclosed covenant and 2001 GMC sierra 1500. All other functions work except the running lights. Brakes and blinkers all good! We have a wishbone 5-wire setup installed using a 4-wire 4 pin flat and a round adapter on the trailer. It was originally a 5-wire adapter but it was modified for use with 4-wire? When the brown/green is activated it immediately blows the fuse in the truck. When the brown/yellow is activated it does nothing at all. When you twist the two browns and connect to the truck side brown it blows. The ground has been redone for security and still blowing fuses. Both brown wires have been manually and visually inspected. No bends, tears, or metal-on-metal from what we can see on the outside. The fuse even blew with the new red bulbs unplugged? The running lights worked fine last week. This week it seemed like the connection was wonky because the lights would go on and off when the plug was jiggled. A few of the running lights were out so all were replaced with LED. Replaced 4 pin wiring harness due to exposed wires. Replaced round trailer adapter just in case. Replaced socket on truck due to visible corrosion. After all this the lights still don't work, the fuse blows every time, and we are out of ideas!

asked by: Christina

Expert Reply:

This sounds like you have a short circuit somewhere in your wiring. The first thing you'll want to do is, disconnect the trailer from the tow vehicle.

Next, switch out the blown fuse with a new fuse and use a circuit tester like # PTW2992 to test the vehicle side connector while you test the running lights. If your circuit tester shows power and the fuse does not blow, then the issue is with your trailer. If it does blow a fuse, your issue is with the vehicle. If this is the case, you'll want to trace down the wire that is not functioning and look for a problem.

Now, if the vehicle side checks out, you'll want to inspect the adapter. If your adapter does not match up with your vehicle side connector you'll need to re-pin one or the other so they match up. If everything matches and there are no damaged wires plug in your adapter and test the pins on the back side of it. If there is no signal from your running lights then you have a problem with your adapter. If there is a signal then you'll need to check your trailer's wiring and make sure that it is pinned correctly.

If everything is as it should be then your problem is with the trailer. You may have too many lights on your circuit that will cause an overdraw; in turn causing a fuse to blow.

expert reply by: Bryce D