What Does It Mean to Splice Into the Cold Side of the Brake Pedal  

Question:

Expert Reply:

In regards to installing a brake controller such as the Tekonsha Prodigy P3 Trailer Brake Controller part # 90195, there is a red wire labeled as the stop light switch circuit. This circuit is used to let the brake controller know when the brake pedal in the vehicle is depressed that way it can then send power to the brake output circuit so the trailer's brakes can activate. This red stop light switch circuit will need to be spliced into the cold side of the brake pedal on the vehicle. The cold side of the brake pedal simply refers to the wire that only has power while the pedal is depressed. If your vehicle does not have a plug-in port under the dash for a brake controller then you will need to hardwire the circuits into the vehicle. Basically, for the stop light switch circuit you will want to find the plug that goes into the back of the brake pedal assembly and put a circuit tester like part # PTW2992 on each wire. The wire that only illuminates the test light with the pedal depressed is the cold side of the stop light switch circuit and the one you will need to attach the red wire on the brake controller to.

Please note that splicing into the cold side of the stop light switch circuit works on about 75% of vehicle's however, there are some where this signal cannot be obtained here due to a lack of voltage etc. so please confirm the year, make, and model of your vehicle that way I can give you a heads up of what is the proper procedure for your vehicle.

expert reply by: Conner L

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Todd B.

7/23/2025

I have a 19911 Ford F150 and am hard wiring a brake cintroller in. Is it safe to wire to the cold side of brake light switch. I have read somewhere that it could interfere with antilock brakes on this vehicle.

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Robin H.

7/23/2025

@ToddB it is safe but if you're worried about it you can install the # RM89ZR and connect to that instead. It will act as the stop light switch for the brake controller.

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