Is there a Brake Controller Wiring Harness for a 2008 Mercedes GL550


How do I order the proper wiring harness for the trailer brake controller so that I can just plug it into my car. I have a factory installed brake controller port under the dash. The controller Im going to order from you is the Curt Triflex Trailer brake controller. Part # C51149 and C51515. My vehicle is a 2008 Mercedes Benz GL550


Expert Reply:

There isn't an aftermarket brake controller wiring harness that will fit your 2008 Mercedes GL550 so you have a couple of options. First you can see if there is an adapter from a Mercedes dealer with one end that plugs into the port under the dash and the other end with just wires. Then you could wire the controller to that adapter harness.

The other option is to determine the functions of the pins in that port using a circuit tester such as # 40376 or a meter if you have one. One pin will have power only when the brake pedal is applied and one will have constant power.

The ground pin and brake output pin are a little more difficult to determine. If you can run 12 volt power backwards (from the pin in the 5 o'clock position on a 7-Way trailer connector) then one of the pins will show power when it didn't before. That would be the brake output pin. The remaining pin would be the ground.

Brake controller wire functions are:

White = ground
Black is 12 volt power in to controller
Red is input from the brake switch (power only when brake pedal is pressed)
Blue is output to the trailer brakes

expert reply by:
Michael H

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