Troubleshooting Running Lights Not Working on New 5th Wheel Trailer With Headlights On

Question:

I have a 1 year old truck and a 2 year old fifth wheel. When towing, everything operates correctly until the headlights, or the running lamps are turned on then the service trailer brake light comes on, disabling the trailer brake, and the running lamps on the fifth wheel dont work. The turn, and brake lights operate as normal. I am suspecting it is something in the plug wire on the trailer side. I tested for continuity in the plug to the junction box located near the hitch and the running lamps have continuity, plus there is continuity between the running lamps at the plug, the ground, and the trailer brake wires. Is this normal? If so, what else can be troubleshot? Thank you.

asked by: Andrew

Expert Reply:

When the headlights being switched on cause other lighting functions to quit working we have found this to either be the result of a bad ground or too many lights on the trailer. Since it sounds like you have had a normal towing experience until recently it is probably a bad ground.

Start off by verifying that both your tow vehicle and 5th wheel trailer connectors are clean and free of any corrosion or debris. Once those are clean we can move on to finding the issue with the ground but I would also verify that the running light circuit wire in your junction box isn't damaged.

What you want to do is check all of the ground connections on your trailer. Start off with the main ground and then move on to all of your running light grounds since that is the circuit you are having problems with. Make sure that they are all secure and connected to a clean, bare-metal surface as even some paint can cause issues.

I have attached a helpful article on wire troubleshooting but if you are still having issues let me know and I can help you troubleshoot further.

expert reply by: Jon G