Ways To Convert A 5 Way Boat Trailer Plug To 7 Way  

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The blue wire on a 5 way plug can be used for reverse lockout function (on surge brakes), or can be used for electric brake signal depending on how your specific trailer is wired. Most 7 pole to 5 pole adapters are wired for the reverse lockout function for surge brakes, not for electric brake signal. So if your trailer is setup with surge brakes (which yours probably is, being a boat trailer) you could go with the 7 Pole to 5 Pole Flat Adapter # A75TB. You would not need a brake controller for this setup.

If you have electric brakes, I recommend going with the 7-Way Round RV Style Trailer Connector # A7WCB.
The adapter will open up and let you wire your 5 way to this adapter, making it a 7 way.
The best way to wire this is by first cutting off the 5 way adapter and exposing/stripping the 5 wires.
You will then use a secondary battery and hook up the connector's white ground wire to the negative terminal on the battery. Test each wire individually by connecting them one at a time to the positive terminal on the battery and determine which function each wire correlates to. Once you have that figured out, you will connect the wires to the appropriate terminal on the 7 way adapter; the wiring diagram that I attached will help you out with determining the function of each terminal/wire on the 7 way.

In regards to the Curt Echo # C51180, this will in fact work with electric brakes or electric over hydraulic brakes as long as you have a functioning vehicle-end 7 way with power and a working trailer-end 7 way. This does not work with surge brakes, as you wouldn't need a brake controller for that setup.

I have attached demonstration videos along with a link to help out with trailer wiring and testing.

expert reply by: Andrew N

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