Brake Controller Blows Fuse When Plugged Into 2012 Mercedes-Benz ML350

Published 06/18/2021

Question:

I just bought this brake controller along with an adapter to connect to my 2012 Mercedes ML350 which was pre-wired for a brake controller. As soon as I connect the brake controller to the pre-wired harness through the adapter, the 30a fuse blows. I discovered this after the brake controller would not turn on when I first plugged it in, then found the blown fuse. Replaced the fuse but it blew again immediately. Note that all of this is happening without ever connecting to a trailer. Sounds like a short somewhere, but I dont know how to find out if its a problem with the controller, adapter, harness, vehicle wiring, etc. How should I troubleshoot this? I am a beginner at towing and not very knowledgeable about auto electrical systems, so hopefully this question makes sense. My family wants to take our new travel trailer out this weekend, so Im in a pickle.

asked by: Mike

Expert Reply:

It's difficult to say, having not seen what you have going on in person but if I had to hazard a guess, I'd say that you're using the wrong brake controller wiring harness. To install the # TK90160 Tekonsha Primus in your 2012 Mercedes ML350, you'll want to use the # 3066-P wiring harness. This would explain the fuse popping as soon as the controller is plugged in.

expert reply by: Mike L