Troubleshooting Trailer Lights Not Working

Question:

Hello. We recently replaced all our 1979 exterior lights for LED lights. Brake and turn lights work. Running lights work BUT turn and break don’t work when running lights are on. We checked the ground and replaced connectors to make sure there was a good, clean connection. Any other suggestions as to what we could check? The brake lights are LED bulbs. Maybe that’s the issue? Thanks in advance!

asked by: Anthony

Expert Reply:

You made a good first step by checking your grounds, and although it never hurts to double-check, I can give you some more troubleshooting steps to take which should prove helpful.

Start by taking a look at your owner's manual to locate any trailer fuses or relays and ensure they are all there and in working order. If they are, in order to determine the specific problem you will need to purchase a circuit tester if you do not have one - the very handy Cordless Circuit Tester # PTW2979 would be my strong recommendation due to how easy it is to use - and test the connector on your vehicle after disconnecting it from your trailer. Check the wiring going into your harness from your tail lights to see if those wires are carrying the correct signals, if not then the issue is with the vehicle wiring.

If the signals going to your harness are okay, you'll need to test the signals coming out of the harness, and if they are correct then your issue is likely going to be with the trailer side wiring. You'll want to go over your wiring to ensure there are none which are crimped, broken or otherwise damaged as that can cause interference with the signals.

I have added links to a pair of help articles on trailer wiring, including a troubleshooting guide which I highly recommend taking a look at.

expert reply by: Kyle S