How to Determine Functions of Vehicle and Trailer Wiring

Updated 04/03/2017 | Published 04/01/2017

Question:

Boat trailer has white/brown/yellow/green/blue. factory pigtail has white/blue/red w blackstripe/grey/ and black . I guess white and blue are matches. How to connect other three???

asked by: Colin K

Expert Reply:

Wire color alone is not a reliable way to make trailer wiring connections so I recommend you test the vehicle and trailer to confirm the function on each wire. Often some wire colors are common to a specific function, such as a blue wire carrying a brake controller's output signal for trailer brakes, but since this is not always the case it is best to test. On a boat trailer a blue wire could be for an electric brake circuit or it could be for a reverse lockout signal to disengage the surge brake coupler so you can back up. A white wire typically is ground.

The trailer can be tested simply by connecting a 12V battery to each circuit one at a time to see what lights up. Ground the battery to the trailer frame and then apply +12V to each wire on the trailer to determine its function. On the vehicle side use a circuit tester like # PTW2993 on each wire while a helper activates the various lighting functions on the vehicle.

I linked an article on trailer wiring that will help you.

expert reply by: Adam R