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Troubleshooting Steps When Trailer Lights Stop Working After Turning on Headlights

Question:

I bought and installed wiring 118649 on my 2017 Sprinter 2500. Everything works great until I turn on the van's headlights. Then my trailer wiring quits working. What's going on?

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Expert Reply:

We can absolutely help you with this issue. First, check the ground wiring on both the trailer side and the vehicle side. Typically when the lights work on the trailer initially but then malfunction once you turn the vehicle lights on it means that there is some sort of issue with the ground on the trailer wiring. Check to make sure that where the ground wire is connected to the trailer is clean and free from paint, debris/corrosion and fully connected; you'll also want to make sure no other wiring is connected to the same spot as the ground wire.

If that doesn't correct your issue, the next step is to inspect the wiring on the trailer to make sure that it is not frayed or exposed and touching any other metal parts of the trailer. Trace back through the trailer wires on each individual circuit. If wires are exposed and touching either another wire or the trailer frame it can cause more power to be drawn than required and an overload to occur.

If completing these steps with the trailer wires do not fix the issue, follow the same steps with the vehicle wiring working away from the wiring harness on the vehicle towards the battery. Make sure that none of the vehicle wires connected are damaged or exposed and touching any other exposed wire or metal, such as the tow hitch or internal metal framing. I have also attached a link to the troubleshooting instructions for you to refer to when going through this process on your vehicle/trailer.

If these recommendations do not fix the issue, please let me know and I will continue to walk you through some other troubleshooting steps that can help find the problem.

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