Service centre brake problems

[ren]

I own a 2005 golf and had the front discs machined and new front pads put on at a vw service centre, upon recieving the car a grinding/scraping noise was coming from the front when applying brakes, i returned it to vw and was told it had a stone in the pad and was charged to have it removed, upon recieving the car for the second time the noise was even louder, i returned it to vw and i am now being told its a faulty abs sensor when i had no warning lights in the car, the sound only came after the service centre machined the disk and replaced the pads can anyone help or offer advice with this situation?

[gavs]

Possibly a fault in the machining, they might not have removed an even layer of material. First thing would be to inquire (polietly at first) with the service department manager as to what was exactly done because the noise was not there before the repair job and if it was a stone in the brake pad (highly unlikely, it would have either ground a mark into the disc or been disintegrated after a decent pedal application) then why is the problem not only still there, but is now worse than before.

I would doubt that a faulty abs sensor would cause a grinding noise, especially since it is not in contact with anything that is moving on the brakes, here's a photo of the abs setup on a front wheel:

http://www.fallastarmedia.com/vortex.../large/017.jpg

That disc with all the little cut outs in it is the abs sensor wheel. The little brown knob thing that has the wire on the left of the image runnig into the back of it is the ABS sensor. Essentially, when the disc is spinning around normally, the little sensor behind it records the timing of how often the cut outs flash by it at any given speed. When you brake hard enough to lock a wheel, the sensor notices that the cutouts aren't flashing past, meaning you have locked a wheel. When the sensor detects this, it backs off the pressure being applied to that wheel, making it turn again, hence, anti-lock brakes.

A faulty sensor would be detected most likely through a VAG computer because generally it would cause a fault which the car's on board computer would store. I'd say they have no idea what is wrong, or they do and are saying it's the abs sensor so you drop off your car with them and they have the time to be able to properly fix the issue, and then charge you an exhorbetent amount for it!

[ren]

Thanks for all the detailed advice gavs, the vw service centre changed their minds again and said it was the wheel bearings which i know is not true, they 'replaced the bearings' free of charge due to inconvenience even though i noticed a different brake caliper assembly on. Will never go to that vw service centre again. Its a shame when you cant trust a genuine VW service centre to carry out repairs.