Troubleshooting Trailer Brake Lights That Flash When Turn Signals are Activated

Question:

When either signal light’ is being used and you use the brakes the brake lights start alternate flashing

asked by: Jimmy

Expert Reply:

The most common cause for trailer lights to act up like yours is a weak ground connection on either the tow vehicle wiring harness or on the trailer's main ground that connects to the frame. A loose ground wire or one that is connected to a spot that is rusty or greasy can cause all sorts of weird behavior from the trailer lights.

Find the vehicle and trailer main ground wires where they attach to the two frames and sand off and clean those points to expose clean bare metal. Once you re-work the ground connection you can protect them with dielectric grease like # 11755.

The problem could also be caused by corrosion inside the vehicle's socket or inside the trailer's plug. Check the socket on the vehicle and the plug on the trailer for any signs of damaged or corroded connectors and replace as needed.

If you have a tester like # PTW2993 you can check the vehicle connector for proper signals while a helper activates them. You can refer to the linked article to see which pin to test for each function.

expert reply by: Adam R