7-Way Trailer Connector Sparked When It Touched Bare Bolt Of Trailer

Updated 03/20/2017 | Published 03/18/2017

Question:

After I unplugged my 5th wheel 7 pin wire harness from my tow vehicle I wrapped the cord around the king pin bracket of the 5th wheel then put the plug into the open back of the king pin bracket to protect it from weather and keep it off the ground. When I did this one of the plug contacts touched a bare bolt and arced and sparked! much to my surprise. Should there be power output on the trailer side ? I thought the trailer was powered by the tow vehicle? The 5th wheel does have its own battery which was on at the time this happened meaning I had not yet used the battery disconnect switch, but I dont think it is supposed to put power out to the plug?

asked by: Mike

Expert Reply:

The 12V power connection on the plug does go directly to your trailer battery positive terminal. So it completed the circuit when it touched the grounded bolt on your trailer. This is not uncommon, so I suggest you just be a little more careful when putting the plug away. Your trailer wiring is working as it should. The trailer wiring is run from your vehicle, but that trailer battery will still give you that charge when the circuit is completed.

expert reply by: Jason S