Hi all, I’ve got a problem with my 2015 (64 plate) bmw 125i M. Whenever I have the windscreen wipers on the car will randomly brake harshly. It drops by 10-30 mph each time. I’ve been able to replicate this issue in dry and wet conditions but all the mechanics I’ve been to including BMW cannot find the issue. I was wondering if anyone’s ever come across any similar issues or has any suggestions what might be causing this.

Morning Alex

Welcome to the Forum

If your car has Radar controlled Cruise Control that could apply the brakes if there is a Short Circuit somewhere while the wipers are operating.

If you don't have radar cruise control then the Brakes are a separate mechanical system that would not be affected by a short circuit. That leaves a sudden cut in power so back to a short circuit that affects the ignition/fuel injection looms

Just my initial thoughts

Dave

Could a failing abs circuit or ers stabiliser cause this? If it thinks the wheels are sliding or yaw out of kilt, then it would apply brakes etc?  

Hi Stu

There has to be pressure in the braking system (foot on the brake) before the ABS function can work releasing and reapplying pressure 

The vehicle needs to be fitted with Active Cruise Control with the Stop/Go function to allow it to brake the car (Stop/Go has an electric servo on the brakes) if Alex's 125i has that then it could be a short affecting it

You would expect BMW or a competent Independent  to be able to work it out though

Dave

Good thinking greydog. 
I have had an issue with abs where the speed sensor is faulty and the brake switch on the pedal shorts, car thinks it is braking…. Was on a French thing though, so it may have just surrendered. …. Can you tell which brakes are being applied? Do you have an automatic handbrake?