Troubleshooting Vehicle Lighting That Fails When Trailer is Connected

Question:

trailer un hooked to car lights work,trailer hooked to car lights do not work

asked by: Roy

Expert Reply:

Since your car lights work when the trailer is not connected, but not when the trailer is connected, then that additional power draw of the trailer is causing the vehicle lights to not function correctly. It sounds like you may have a bad ground on your trailer or on your vehicle. I recommend starting your trouble shooting by using a circuit tester like # PTW2993 to check your trailer connections when the trailer is not connected function by function. Check the ground on your vehicle side trailer wiring to make sure you have a clean bare metal to metal connection. If everything checks out there, then you've isolated the issue to your trailer.

To test your trailer lights, you can simply use jump wires and a battery. Start with your ground wire connection to make sure it is bare metal to metal and free of corrosion or debris. Then connect the negative of the battery to that ground and use the positive terminal jump or circuit tester to test each trailer wiring function. I've attached an article to assist.

Finally, clean your connections on each side and use some dielectric grease # 11755 to keep moisture and debris out of the connection.

If this doesn't solve your connection issue. Let me know the year/make/model of vehicle and we can continue to troubleshoot.

expert reply by: Jason S