Troubleshooting Utility Trailer Tail Lights Only Working On The Running Lights Circuit  

Question:

Expert Reply:

I would first start with doing a bench test on the lights to determine if there is something wrong with the lights themselves before diving into the wiring on the trailer and vehicle. This is easily performed by using a 12 volt battery and connecting the negative terminal whether it is a separate wire or built into the mounting hardware connection. You can then individually touch each red and black wire separately to the positive terminal to ensure that both lights are working. It is always good to verify that the lights are wired as you were told with black for the running lights and red for the turn/brake light. As that would explain why you cannot see the faint flash of the lights when they are wired backwards.

With good lights the next step will be to check the 4-way connection off your tow vehicle. A Test Light # PTW2993 will allow you to verify that the 4-way lights correctly. The male end closest to the female port will power the running/marker lights. The next male lug is for the left signal/brake lights and finally the end male lug is for the right signal/brake lights.

With a proper functioning 4-way and verified working tail lights you can turn your attention to the wiring. You can use the same Test Light # PTW2993 to verify that the wires are getting power on the trailer side of the connector and follow the wiring all the way to the tail light connectors to ensure that part of the harness is not damaged. If you find a damaged wire you will need to repair or replace the wiring.

The final test will be for the ground circuits. I find the best way to verify that you have a good ground is using a jumper wire to run from the trailer tail light to vehicle to eliminate the chance of there being bad connection. If your jumper wire allows the tail lights to work then there is a weak connection in the ground circuits on your trailer. You will need to check and clean the connections to determine if corrosion may be at fault. By using the same jumper wire you can test the ground circuit at different points of your trailer to locate where the connection is bad.

I have linked one of our self help articles below as well to help you with all your testing.

expert reply by: Bud M

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