Troubleshooting Trailer and Truck Lighting Problems When Trailer 7-Way is Connected

Question:

We bought a used RV. We have a 2012 Dodge Ram 4500. Both vehicles are wired with a 7 way flat system. When we plug the trailer in, the running lights on the sides of the trailer come on, but the back two lights are not working. Also when plugged in, the top brake lights on the truck stay on bright. The turn signals on the truck work but not on the trailer. What could be causing this? Truck Harness? Or trailer wires?

asked by: Janet N

Expert Reply:

Thank you for providing a thorough description of the truck and trailer lighting issues you're having. The various problems you describe COULD be related to the same issue, corrosion in the trailer connectors, but there may also be multiple things you'll need to attend to. The best way to troubleshoot your concerns is to unplug the trailer from the truck's 7-way and then to test each one individually. Since your RV is used, it's wiring is my prime suspect.

Step one for troubleshooting is to check the ground wire connection at the truck's wiring harness and at the trailer's plug. Make sure both of these connections are secure and made to a clean bare metal surface. If that does not correct the problem then you will troubleshoot further.

Start with the trailer plug. Check for and correct any obvious corrosion on the plug contacts. Please refer to the linked photo that shows you a 7-way RV-style trailer plug and what function each pin/circuit is typically wired to do. You can use jumper cables and connect either to your vehicle battery or a spare 12V battery if you have one. Attach wire leads to the other end of the jumper cables (or connect your testing wires directly to the battery). Attach the battery ground wire to the pin at the 5-o-clock position on the trailer plug (see photo). Then one by one connect the wire from the battery hot terminal to each of the other pins and see what function engages on each pin. If the trailer lights, trailer brakes, etc all react correctly to this 12V input power then you know the issue is upstream of the trailer. If some of the trailer's lights do not work, you'll need to trace the wiring for each of those circuits looking for cracks in the insulation or worn spots where friction on the trailer frame may have worn off the insulation, leading to a short. If the wiring looks good, then check that the non-functioning lights have a secure ground connection to the trailer frame.

On the truck side you will need a circuit tester like # PTW2993 to test the various circuits on the truck's 7-way socket. The linked article includes a photo of the 7-way socket with indications for the common wiring scheme, which correspond to those on the photo of the trailer plug. Apply the grounded circuit tester to each pin, one at a time, while a helper activates the functions on the truck. Please see the linked video for a demonstration. If you do not detect the correct signals on the truck's 7-way pins, then you will need to again inspect the wiring to the 7-way looking for corrosion within the connectors or damage to the insulation of the wires leading forward on the vehicle.

It can also happen that a used trailer may have been re-wired by its previous owner in a non-standard configuration; this would easily explain why some functions do not work. Again, the linked article shows how 7-way connectors are normally wired.

expert reply by: Adam R