Trailer Lights Stop Working when Headlights on Tow Vehicle are Turned on

Updated 08/10/2017 | Published 08/09/2017

Question:

my lights on trailer will not come on on one side when my headlights are on but will work when headlights are off

asked by: Karen

Expert Reply:

There are a few possibilities as to why this is happening on your trailer. You either have a bad ground, a short somewhere, or your trailer has too many running lights and is overloading the wiring harness.

You will want to start out by testing the connectors on both your tow vehicle and your trailer. By testing them you can determine which end of your towing setup the problem is on. You can start out by making sure both connectors are clean and free of debris.

To test the connector on your tow vehicle you will want to use a Circuit Tester # PTW2993 and test each pin for a signal while a helper sits inside the cab and runs through the functions. To test the wiring on your trailer you will need to use an alternate 12V power source, like the battery from your tow vehicle, and go through each function here as well.

Whichever side the problem is on you will want to check all of the grounds (all of the lights on your trailer should have a ground) and make sure that they are secure and connected to a clean, bare metal surface. If they all look good, you will then want to check out the wiring itself. Look for any places where there is corrosion, damage, bent, or exposed wires and replace them.

If all of your wiring looks good and you don't have very many running lights, then you will need to take your setup to a shop and have them look at it since they can physically inspect it.

expert reply by: Jon G