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How to Find COLD Side of Brake Light Switch for Hardwiring Brake Controller

Question:

How do i get a hot wire off of brake light switch

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Expert Reply:

When you are hardwiring a trailer brake controller like the Prodigy P2 # 90885 the red wire connection that provides the triggering signal to the controller should be made to the wire on the COLD SIDE of the brake light switch circuit. This is the wire that carries 12V power ONLY when you press the brake pedal and at no other time. Please refer to the linked photo and article for more details.

If you were to connect the Prodigy to the HOT side wire then the controller would constantly generate brake output to the trailer and you would not get too far.

Applying a circuit tester like # PTW2992 is the best method for finding the correct wire at the brake light switch.

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Dan S.

2016 Thor ACE motorhome built on F53 chassis.Every one Ive talked to and everything Ive read online insists that I have a plug and play connector in the dashboard. However, after removing the dash and searching through it, theres nothing even resembling one. There are several unconnected wires, but they have labels for PTO and blower motors. Not sure what you mean by Brake Light Switch, do you mean the switch thats activated by the pedal? Those are the wires Im referring to. There are 4: one is constant 12v, one is constant ground, the other two are the ones mentioned above. Where else on the vehicle could I access the cold side?

Etrailer Expert
Reply from Chris R.

Thanks for the info! It's really odd that it doesn't have a factory port - take a look at the linked answer below that actually shows a picture of its typical location so you can check one last time. If you still aren't able to find it, your best option is to use the Stop Light Switch Kit # RM-751000 , which basically lets you wire in your own brake light switch to use for the controller.

Dan S.

Tekonsha says not to connect to a wire that goes to ground when the brake pedal is not depressed. I have 2 wires in my brake position switch that get 12v when brake pedal is pushed, but they both go to ground when it is released.Any thoughts?

Etrailer Expert
Reply from Chris R.

You might be able to access the cold side of the brake light switch somewhere else on the vehicle (you're right not to use either of the two wires you found). If you can tell me what model vehicle you have I'd be happy to provide a more specific answer. Another option is to add a Brake Light Switch - which again are vehicle specific.

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