Troubleshooting Trailer Taillights Where Only One Side of Trailer Lights Work

Updated 06/25/2019 | Published 06/24/2019

Products Featured in This Question

Optronics Trailer Tail Light - Stop, Turn, Tail - Submersible - Incandescent - Round - Red Lens

(2695)

$6.74

Hook Probe Circuit Tester

(72)

$13.44

Question:

We had the trailer lights working for one day break lights and turn signals on both side no running lights though now only one side works I got power in the tail light assembly on both sides one side there is power but no light

asked by: Harold

Expert Reply:

First thing you should do at this point is test the 4-way of the wiring harness with a circuit tester part # PTW2993 for voltage on all of the right pins when the taillight signals are being applied. Just like in the video I attached.

If the voltage checks out there but the trailer lights still don't function right then the problem is on the trailer wiring. That would mean that problem is either due to a ground issue or a short issue.

Make sure that the trailer side connector are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces. The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

If the problem is on the vehicle side then I would first recommend you double check how the harness is installed on your vehicle, make sure all of the connections are good and tight. You should check all towing related fuses and relays as well.

expert reply by: Jameson C