Which Wires Go Where When Converting Boat Trailer 7 Wires To 4 Way Flat?

Updated 03/10/2026 | Published 03/07/2026

Question:

I have seven wires coming out of my boat trailer going into four wires on a flat prong. I’ve connected three whites to one white one yellow to one yellow one green to one green and one brown to one brown lights come on, but I have no turn signals and no brake lights. What am I doing wrong

asked by: David T

7 wires to 4 wires on boat trailer
(click to enlarge)

Expert Reply:

Hey David, I see what you're saying.

Based on your wire count (three white, two brown, one yellow, one green), that’s typically a standard 4-way setup. Normally white is ground, brown is running lights, yellow is left turn/brake, and green is right turn/brake. It would be unusual for white wires to be brake signals, but wire colors aren’t always perfect - so tracing them or using a circuit tester is the sure way to confirm.

Since your running lights work but you have no brake or turn signals, the most common cause on boat trailers is a weak or corroded ground. Brake and turn circuits draw more current and will fail first if the ground isn’t solid. I’d double-check that all whites are securely tied together and grounded to clean, bare metal on the frame.

It’s also worth testing the vehicle’s 4-flat to make sure the yellow and green pins are actually getting power. Let me know what you find and we’ll narrow it down.

expert reply by: Andrew N