I'm installing a break controller & 7-pin receptacle for towing.

The wiring diagram I am following (below) shows the 7-pin and brake controller wired directly to the battery. I want to know if I can wire to a switched source (only when the car is on) and if that's doable, then what is the best switched source?

wiring diagram

  • 1
    I'm pretty sure the reason this is on a non-switched source is for safety purposes, so the trailer brakes will work even if the vehicle is off. Jun 2, 2015 at 21:30

1 Answer 1


It can be but you will need a relay with the capacity to carry at least 40 amps (based on your diagram, your brakes may require more).

I would suggest a relay similar to what is pictured below. I wouldn't go with anything smaller than a 50 amp continuous duty.

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Continuous Duty Solenoid 80AMP 12V

You could also use a relay like the one below, but I haven't had good luck with this style when used for high amperage.

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12 Volt DC 50 Amp Continuous duty

I would connect the contact circuit directly to the fuse that's connected directly to the battery, with the other end connected to the brake controler. The control circuit can be connected to almost any key on power source, as it only draws a small amount of current

  • That makes sense. My reasoning for wanting a switched source was for when I put the brake controller away, I plan on stuffing the wiring harness in the dash. I didn't want the hot wire to accidentally ground itself on something.
    – Jason
    Jun 2, 2015 at 20:18
  • @Jason it would be cheaper and easier just to pull the fuse at the battery, but if the plug in is the right style there is little danger of it coming in contact with anything. Ultimately if it did it would just blow the fuse anyway. Jun 2, 2015 at 20:42

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