Trailer Lights do not Work Properly When Truck is On

Question:

My trailer running lights, signals, brakes and four way flashers work fine on the left. On the right, however, the lights will work intermittently with the engine off and the headlights in the auto position. It doesnt work with the headlight or in the on position. Strangely, even if they are working in the auto position, as soon as I start the car, it goes out. So, with ignition on, car not running, left light and front marker lights work fine. Right tail light works intermittently with headlights set to auto, but not at all when st to on. Even if it is lit, with the ignition set to accessory, when I start the vehicle, the light stops functioning. Thanks in advance for your assistance.

asked by: Juan G

Helpful Expert Reply:

The light problems you are having sounds like the elusive ground problem that haunts many trailer owners and shops. Ground problems typically appear when more power is applied to the taillight circuits and can cause erratic lighting or no lighting at all.

The first step in troubleshooting is to check the trailer side and vehicle side connectors for any corrosion and repair or replace as necessary. Next, test the vehicle side trailer connector using a circuit tester like part # PTW2993. This will tell us if the vehicle side trailer connector is working properly. Test each of the pins for proper function. If the trailer side connector is not working properly, then we know the issue is on the vehicle side. If this is the case, I recommend checking the ground and other connections if the trailer wiring is not factory. If you have factory trailer wiring, then you can still inspect the wiring, but you may want to have a dealer repair any issues. I would test the connector with the truck on and running.

If the vehicle side connector is working properly, then the issue is likely on the trailer. Starting at the trailer connector, check all of the ground connections on your trailer as you work your way to the taillights at the back of your truck. Check the main trailer ground and the grounds for each of the trailer lights. The lights and trailer ground should be grounded to a clean and bare surface on the trailer frame. I also recommend inspecting the wiring. Make sure there are no areas that are pinched, exposed or damaged. If you find damaged wiring, repair or replace as necessary.

If your trailer grounds through the hitch ball, clean any rust or dirt off of the hitch ball and off of the coupler of the trailer so it is making good contact.

If you are unable to solve the problem or find a solution, let me know what you come up with after following these steps and we can take further troubleshooting steps. I have attached a trailer wiring help article for you to check out that should be helpful when troubleshooting your trailer wiring.

expert reply by: John H