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Troubleshooting Trailer Wiring That Does Not Allow Running Lights to Work

Question:

Hi just put brand new lights tail can get blinkers to work brake lights only have right tail light left hand blinker won’t work with tail lights on

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Expert Reply:

Most often when one trailer lighting function interferes with another, such as when your trailer brake and turn lights work fine UNTIL you turn on the running lights, the problem is caused by a weak ground. A loose or corroded ground connection will often allow one function to work but may not be able to pass enough current for multiple functions to operate at the same time.

You can check this easily by finding the main ground wires for both your vehicle's wiring harness and your trailer's wiring harness. Their ground wires are usually white and will connect to the frames with a ring terminal.

It can often happen that a bit of corrosion/rust can form within that connection point or that dirt or other foreign materials can get in and block the ground connection. The fix for this is easy. Unscrew the ring terminal, sand off the grounding point to expose clean bare metal, apply dielectric grease like # 11755 and re-attach the ground wire ring terminal. The grease will block water and dirt and help prevent future corrosion-related problems.

If you have a circuit tester like # 40376 you can test the vehicle by itself. Disconnect the trailer and apply the tester to the vehicle wiring harness. The linked video show you how to use the tester. If you get the correct signals on the vehicle wiring them move on to the trailer. Use the battery in the vehicle or a spare that is well-charged to apply power to the trailer plug, one circuit at a time. If the lights work with this direct application of battery power then this tends to tell you the issue may be on the vehicle side.

You can refer to the linked articles for additional troubleshooting help.

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