What is Purpose of Fifth Circuit Of Trailer Wiring that Runs to Reverse Light Circuit

Updated 03/21/2018 | Published 03/19/2018

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Question:

I will tow a trailer with surge disc brakes tie down brand. It has a fifth wire that I believe has to splice to the back up light positive wire. On 2015 Highlander the back up lights are on the tailgate. Where is the best place to splice in to this wire? The fifth wire has to do with releasing a valve or something while backing with the trailer. What would happen if I didnt connect this wire? Would I have brakes going forward?

asked by: Jon E

Expert Reply:

The circuit for the trailer is what allows your trailer to be backed up without the brakes locking up. When that circuit is active it prevents the surge brake actuator from being able to build pressure and activate the brakes. You want to have it connected to your reverse light circuit on your truck.

On the tailgate/rear door you'll need to look for a rubber grommet with that has wires in it. In that wiring bundle will be a circuit that carries the reverse light circuit. Tap into it with a quick splice like the part # SWC501746-1 and you'll be good. To test the wires use a tester like part # PTW2993. You might note the wire color there and then trace the wiring on your vehicle body to a more accessible area.

expert reply by: Jameson C