Where to Access Brake Signal for Curt Spectrum Brake Controller in 2010 Mercedes-Benz ML350 BlueTec

Updated 05/06/2019 | Published 05/02/2019

Question:

Curt Specrum - 2010 ML350 Bluetech I am having an issue installing this brake controller, Currently the Mercedes send a monitoring voltage of 3.6v, on the brake circuit wire, the issue I am having is that the controller thinks the brakes are pressed and outputs a signal to the trailer, always showing the trailer brakes activated. Can I add a relay / resistor to negate this signal? the voltage increases to 12v when the brakes are pressed. On another note, in my testing, if I connect a test light to the brake circuit wire and ground, the controller works perfectly....activating the brakes when I press the pedal, or manually with the knob......Remove the test light and I am back to square 1, power output to the trailer brakes...... Im guessing maybe the Volkswagon / porsch have a similar issue as I did see one review for installing on a vw tourig where they installed a relay per w TSB, but have not been able to find this TSB

asked by: Mark C

Expert Reply:

To correct the unwanted braking action from your Curt Spectrum # C51170 caused by the 3.6V monitoring signal generated by your 2010 Mercedes-Benz ML350 BlueTec you'll need to access the brake light signal at a location other than at the brake light switch. You can tap into the circuit at the CHMSL (center high-mount stop light) or at one of the two main brake lights and run that wire back to the controller's RED input trigger wire.

My contact at Curt confirmed this and that you do not want to insert a resistor or relay into the brake circuit.

expert reply by: Adam R