What Does The Fifth Wire Do On A Boat Trailer 5-Way Wire Harness Connector

Updated 12/08/2023 | Published 12/07/2023

Products Featured in This Question

Curt Trailer Connector Adapter - 4-Way Flat to 5-Way Flat - Vehicle End

(103)

$14.11

Question:

Hi, First of all, this is such a great site and your experts and have been so helpful, definitely appreciate it! We just bought a new used Lund Pro V bass with a Shorelander trailer. It has a 5-way plug and our vehicle has a 4-way harness. For now we connect them and all the lights work. For the surge brakes we use a manual lockout key when backing up, which works fine. My question relates to the blue wire coming from the trailer into the 5-way plug. What does it do? I have read it is for reverse lights, however I have also read it is for an electrical lockout of the surge brakes when in reverse. Could you clarify this? I recently purchased a 4-way to 5-way convertor and it looks like there is some splicing work to be done for the blue wire. I am not sure I want to do this given all the lights are working and we are using a manual lockout key for the surge brakes. What are your thoughts? Thank you,

asked by: Wayne

Expert Reply:

The fifth wire on your trailer's wire harness is for your reverse light signal, like you thought. It can get wired to both the reverse lights and an electrical lock out on your surge brake coupler. This makes it so that you don't have to get out of the tow vehicle and walk back to the trailer to manually lock out the brakes so that you can back the trailer into the water. It's really just for convenience so you don't have to use it if you don't want to.

If you do want this function and your tow vehicle only has a 4-way connector then you'll just need to install the Curt Trailer Connector Adapter # C57187. The 4-pole connector on this adapter will plug into the connector on your vehicle and the extra blue wire will get spliced into the reverse light circuit on your car. I've attached a short video below where Dave goes through the installation process on an Acura MDX that will give you a good idea if this is something you could tackle yourself.

I've got a ton of wiring experience and I like convenience so I personally would install it if I was in your shoes, but if you're happy with how your setup is then you don't need to change it.

expert reply by: Kevin C