I had my ABS light which would go on sometimes. If the light goes on, then drive to your nearest MB shop and ask them to connect their computer to your car so they can see which sensor is bad.
They didn't told me that the first time I left my car to them, they "just" changed the batteri because they thought it was a low voltage supply problem... of course not :-(
If they had told me that in the first time I wouldn't had to leave my car to them again. This time they located the problem to the left wire harness going to the sencor=which was replace and they also greased the sencor.
Good Luck / Patrik from Sweden