Troubleshooting Trailer TailLight That Is On At All Times But Turns Off When Brake Pedal is Pressed

Question:

I have a Carson utility trailer. When I hook the trailer up to the tow vehicle, the left tail light comes on and stays on even with everything shut off at the vehicle. The right tail light stays off. When we hit the brakes, the left tail light goes off, and the right tail light lights up like its supposed to. The hitch and lights are a new install on the vehicle. I originally thought that was the problem, but the shop that did the work has double checked their work and says all is fine. The problem seems to be with the trailer which is basically new but it has sat for the last 1.5 years. Blinders and tail lights work fine otherwise.

asked by: Tim

Expert Reply:

I feel like there has to be an issue with the taillight converter that was installed on your 2018 Ram Promaster City. A trailer wiring issue might be reasonable with the light staying on all the time but as the light goes out when you push on the brake pedal and the other light comes on. To me that indicates an issue with the converter. The shop may have correctly installed all the parts but if a part was defective from the manufacturer then that would explain what is going on.

I would use a Test Light # PTW2992 to test the connection on the vehicle. Below are some pictures of a 4-way flat connector and a 7-way connector to know what terminals control what on your trailer. Use the test light to confirm that the lights activate only when they are supposed to.

I have linked some of our trailer wiring articles below that may help you troubleshoot the trailer wiring if everything test good on the vehicle.

expert reply by: Bud M