Troubleshooting Trailer Lights That Work with One Car But Not the Other

Question:

Hi there. Awesome webpage here. If I hook up my trailer to the Fairmont all the lights work fine. If I hook it up to the 2011 Ford mondeo wagon the trailer brake lights don’t work, the other trailer lights are fine, the indicators flash fast and one of the mondeo brake lights doesn’t work. With the trailer not connected the mondeo lights all work fine. Any suggestions?

asked by: Kelly S

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