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Hello!!!

I am trying to install a brake controller (Tekonsha Prodigy P3) to me 2016 X1 xDrive, US model in order to tow a trailer with electric brakes. Brake Controller instructions said to connect its red wire to the cold wire of the brake sensor switch. However, the break switch is electromagnetic and all I see are two wires – a ground (brown) and a hot (blue) with 8.9V. Pressing the brake produces a slight variation in voltage on the hot (from 8.9V goes down to 8.8V), ground shows no change. {Note: I’ve installed same controller on a Honda Odyssey which have a mechanical switch and the cold switch wire shows 0V (brake not pressed) and 12.6V (brake pedal pressed)}
Does any of you have any suggestion where I can get the input from for the brake controller? I was thinking that the switch must have some other wires (besides the two I mentioned that powers the electromagnet of the switch) that will send the break signal to the car’s modules, but I do not see any. I am sure somehow transmits, but I do not see how.

Can any of you suggest a solution? I was thinking to run a wire from the stop lights from the back of the car to the brake controller input, but I have a feeling it is not best thing to do.

Please help me solve this challenge!
Thank you,
-Alin
 

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This doesn't help you much as it doesn't go far from your last idea.. but on my other BMW's a lot of the voltages were 1) not full 12 V 2) oscillating power? This sounds like the exact same behavior as you're seeing. Most likely you will see something similar even with the back tail light.

The way to solve this is to use that brake switch and get an automotive relay from 12V line from your battery. The 8.9V will be enough to trip the relay and you'll get the full 12V from your battery. Of course you should get an inline fuse setup and running the wire. I seem to recall when you're installing trailer hitch connections you need to run a power wire from the battery anyways?
 
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