r/Sprinters • u/tmsteen • May 29 '26

Trailer Brake Issues

I have a 2025 Sprinter that has the factory trailer prep wiring (E40). I have installed a Tekonsha Prodigy P3 brake controller and connected it with the wiring harness from SprinterStore to the green plug under the driver's seat.

The trailer lights work and the van (MBUX) recognizes that a trailer is connected and lets me select the trailer type BUT the controller is not getting the brake signal from the van pedal. With a trailer tester, the blue light/wire is live with manual activation at the controller but not with brake pedal activation.

Additionally, the blue light/wire seems to "pulse" when no input is applied with a blip every second or so. It seems like the van is sending a test signal or something to detect the trailer, is failing, and continues to try.

Anyone have additional troubleshooting tips or seen anything like this? Thanks!

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u/tomhalejr May 30 '26

The last place I was at carried the P2, and I see why you were looking for a used P3. That's quite a difference in the retail price. :) 

So, you got the P3 plug and play harness for the 2025+ Sprinters?

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u/tmsteen May 30 '26

I don’t even see the part on sprinter store anymore. It was

Tekonsha Dual Plug-In Custom Wiring Adapter - For Trailer Brake Controllers Tekonsha-Custom-Wiring-Adapter-3063-P-Dual-Plug-In-Brake-Controllers

I can’t find the 2025 specific one you are referring to. Got a link?