Trailer lights brain-teaser

[ferd]

I'm stumped! Please help

When running lights are on, left turn or brakes blacken the left lamp. With no running lights all lights work.

-voltage from truck coupler tests ok

-I replaced all left side wiring and the left tail light

Any ideas?

[Macgyver55]

Its definitly a ground problem. Make sure the trailer frame is grounded and that both lights are grounded to the frame.

[Giant_Jackpot]

I’d bet Mac is right. Remember you cannot rely on the trailer hitch/ball connection for the ground. Make sure the truck wiring harness is grounded.

[Jeremy airjer W]

I'll third the bad ground. The white wire is the ground wire for the trailer. Backprobing with a voltmeter with the red lead connected to power you should find 12 volts (or battery voltage) everywhere you touch the ground. I would start with the truck side of the plug in first. If you find 12 volts its o.k., if not you know its the trucks problem and not the trailer.If the truck checks out o.k then plug in the trailer connecter and check for 12 volts on the whiter wire on the trailer side. As soon as you loose the 12 volts you found your culprit

[FM_Mike]

My brothers trailer had the same problem. We found that the light causing the problem had a rusted socket were the ground wire connected. The seal on the light was bad and allowed water in and eventualy rusted the connection. Put a new light on and that solved the issue. Might be worth a look.