Trailer Tail Lights Not Working and Causing Other Light Issues on Truck When Connected

Question:

On vehicle I have a 4 way pin which I’ve checked with a test light to make sure left and right turn signal pins have power. On trailer I have a 4 way pin with 5 wires which makes it a wishbone harness I believe. Here’s the problem : when I first plugged in the trailer lights and turned on the right blinker all lights blinked including marker lights. When turning on the headlights the right blinker completely stopped on vehicle and trailer lights stopped too. I’ve disconnected all marker lights and looked for wires touching and no touching wires. I’ve checked trailer ground and it’s good. I disconnected the right trailer light and both wires. I reconnected the right turn signal wire to light and it works fine and the brake light works too with headlights on or off. But when I connect the taillight wire to the right light it will stop the right blinker on trailer, if the headlights are on the right blinker will stop on truck too. If I leave off the right brown tail light wire everything works on trailer and vehicle. Except the right taillight of course. Any suggestions

asked by: Tim D

Expert Reply:

You have done a really nice job of narrowing down the possible source of the trouble you're having with the trailer lights and the lights on your truck when connected. There certainly seems to be something going on with that brown tail light circuit wire running down the right side of the trailer.

Since you tested the 4-Way on the truck you know the truck's wiring is good. What I recommend doing, and this may be a bit of a pain depending on how the wire is ran, is to try and trace that brown wire on the trailer starting at the 4-Way all the way back to the tail light assemblies. Based on the fact that the issue is also causing problems with the lights on your truck, there is likely a short somewhere on the wire. Look for any pinched, exposed, or otherwise damaged portion of wire that needs to be repaired/replaced. I think there's probably an exposed portion of wire that's contacting bare metal on the frame so as soon as its connected and powered it causes a short.

If you still aren't able to find anything or resolve the issue please let me know and we can continue to troubleshoot.

expert reply by: Chris R