Troubleshooting Trailer 7-Way Wiring With Constant Power on Lighting Circuit

Updated 06/29/2017 | Published 06/28/2017

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Hook Probe Circuit Tester

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7-Pole and 4-Pole Trailer Connector Sockets w/ Mounting Bracket and Wiring - Vehicle End

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Question:

For some reasons there is constant power going to the right turn pin #6. I causes my trailer light on the right side to stay on all the time. Even when i use the right signal light in my vehicle, the trailer light stays on. It even stays on when my vehicle is off. Any suggestions on what the problem is?

asked by: Denis

Expert Reply:

Often when a trailer harness lighting circuit stays on continuously, even when the vehicle lights are okay and function correctly, the issue is corrosion inside the vehicle socket or the trailer plug that allows the 12V auxiliary power signal to bleed over to that other light circuit.

On a 7-way wiring setup the 12V power circuit is carried on the pin at the 1-o-clock position as you view the socket on the vehicle. Please refer to the linked photo. Just next to this pin, at the 3-o-clock position, is the right side stop/turn circuit; on the other side is the tail light circuit.

If corrosion has built up inside the socket or inside the trailer plug these circuits can become bridged together and cause the issue you're having.

You can carefully inspect both the vehicle socket and the trailer plug for any signs of even slight greenish or whitish discoloration. Note that can occur INSIDE the connectors too, not just on their visible contacts. There are various spray-on electrical contact cleaner products, such as Caig De-Oxit, that you might try before replacing the connectors.

For a replacement 7-way trailer plug you could use # H20046 and for a replacement 7-way vehicle socket an item like # HM40975-11998.

expert reply by: Adam R