Originally Posted by -twitch-
I am using to tow a car trailer. I am running the 12V wire because that's what the 7 blade plug said it needed. I have no brake away. It is for the lights and electronic brakes on the trailer.
Let's back up to the begining. I think you're confusing some of the terms being used. If all you want to do is power the electric brakes, tail lights, and brake lights on your trailer you have already identified all the wires you need.
The wires for the brake lights and tail lights are already in the socket at the rear of the motorhome. You found the wire in the harness labeled "electronic brakes". This is the "power wire" the instructions are referring to for the 7 pin socket. This wire needs to be extended and terminated on terminal #2 in the trailer hitch socket.
For simplicity sake I would remove the original socket at the rear of the coach and replace it with your new 7 pin socket. You're already going to have to take the original socket apart to terminate the brake power wire so it would eliminate several additional potential problem connections between the original socket and the adaptor by just replacing it.
The blue power wire will only be hot at the socket when the trailer brake controller is installed and you step on the brake pedal in the motorhome when the motorhome is decelerating.
The brake controller needs several conditions to pass current to the brakes through the blue wire. First it uses a built in motion detector (deceleratometer) to know when wnd how fast the motorhome is slowing down. It also needs a signal from the brake light switch in the motorhome to know you've actually applied the brakes.
You also have to set the dial (wheel) on the controller for the maximum voltage you want to go to the trailer brakes. For example if you set the maximum voltage to 8 volts thet's the maximum that will be sent under panic stop conditions. If you stop under normal conditions the controller will determine the rate of deceleration an apply brake voltage accordingly.
The plug on the trailer is in all likelyhood wired like the standard schemaitc Scarab0088 posted for the 7 pin trailer end. You need to open the original socket on the motorhome and remove the wires 1 at a time. They should match the schematic Scarab 0088 posted for the 6 pin car end. As you remove them label them to avoid confusion when moving them to the new 7 pin socket. Then terminate each of the wires in the new 7 pin socket per the 7 way car end schematic.
Disregard my previous post about the "power wire". I thought you were also looking for a wire to power internal lights or charge a battery in your trailer. This wire is present in the harness but has nothing to do with the brakes or external lights.
As I said earlier you have already iidentified all the wires you need for brakes and external lighting on the trailer. Don't be discouraged because you can't find power at the blue brake wire when the motorhome is setting in the drive. Once it's wired it'll work just fine when the motorhome is decelerating and you're stepping on the brake.
I would initially set the maximum voltage on the controller at 6 volts or less for testing purposes. When I pull an empty trailer weighing 2100 lbs behind my Ford truck that's enough voltage to feel the trailer stopping without skidding the tires.