Troubleshooting Trailer Lights that Running Lights Only Function

Question:

I have a 2003 Ford Ranger and I am having a problem getting the trailer lights to function correctly. The truck has an existing 4 flat connector on it. The trailer was recently rewired and new LCD lights installed. When the truck lights are turned on the trailer lights come on but the brake lights and turn signals do not function. When the truck lights are turned off the trailer lights do nothing. Can you help me ?

asked by: Mike D

Expert Reply:

It sounds like your trailer wiring has only the running lights circuit active and the rest of the circuits are not turning on.

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.

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. What you described can sometimes be due to the converter box shorting out.

expert reply by: Jameson C