[Brakes] Brakes feel rough until ABS light comes on

[flanagaj]

So my 2008 B9 has developed a weird sensation with the brakes when you first start and drive.  When you are driving and press the brake pedals, you get a rough sensation through the brake pedal and it feels like the brakes are completely knackered.   After driving for a few minutes, the ABS light pings on, and the brakes feel completely normal again.   The vehicle passed the MOT recently, so the brakes cannot be dangerous.  I need to get it fixed though as the wife refuses to get in it now.

Any ideas what is potentially happening here?

[shtu]

First off, assume nothing with an MOT and have a look for yourself for any excess wear or corrosion.

Itf that comes up OK, I'd suggest a senssor or tone ring problem. The rough feeling being the ABS triggering.

Check the rings are where they should be and intact, clean, not excessively rusty.

Remove each sensor and clean the mounting surface. A hammer and chisel might be needed to break out the corrosion.

[flanagaj]

I did have a stuck piston on one side and I replaced it, along with the calliper carriers. When you say sensor, I assume you mean ABS sensor, but can you post a link to 'tone ring' as I have no idea what that is.

Thanks

[geoff]

The ring is a “ toothed ring “ mounted onto the wheel hub that the sensor uses to detect motion by counting the “ teeth “. No picture sorry

That said, some cars have the ring built into the wheel bearing and cannot be seen.

Brakes may pass the MOT but can still cause issues. My old Berlingo had a max pulsing pedal and wheel lock ups when slowing on a bend - all down to worn pads discs coupled with a slightly sticky calliper.

Have fun and work through one bit at a time and then test and repeat as needed.

[geoff]

^^^ Mad not max !! Poxy auto text !

[Multispacer]

So my 2008 B9 has developed a weird sensation with the brakes when you first start and drive.  When you are driving and press the brake pedals, you get a rough sensation through the brake pedal and it feels like the brakes are completely knackered.   After driving for a few minutes, the ABS light pings on, and the brakes feel completely normal again.   The vehicle passed the MOT recently, so the brakes cannot be dangerous.  I need to get it fixed though as the wife refuses to get in it now.

Any ideas what is potentially happening here?

The brake pedal 'judder' is standard operation to inform you of the ABS not operating correctly. The ABS light only illuminates after you reach a certain speed [I think over approx 30mph from memory] As @shtu has stated the most likely problem is either a faulty wheel sensor, or even more likely the sensor not close enough to the sensor ring on the wheel. This is caused as there is a bit of a design fault on these vehicles as the plastic bodied sensor sits in a recess in the hub held in by one torx screw. What happens is that rust can build up in the recess underneath the sensor and flex the plastic body of the sensor just far enough away from the sensor ring, so that the ABS system cannot sense the wheel rotation and brings up a fault light and the pedal judder. What you need to try first is to remove each sensor in turn and chisel/scrape out as much of the rust build up under the sensor as possible and then refit the sensor. From my own experience it is normally a front sensor, especially the near-side that seems to suffer from the worst rust build up. 

Hopefully that will cure your problem, if not you need a diagnostic check carried out, to show which wheel is throwing up the ABS fault. Sometimes the sensors do actually fail and some owners have also reported broken sensor wires.

[shtu]

I've jogged my own memory - on the front, they tone ring is integral to the wheel bearing, making the rust-in-sensor-housing more likley.

Rears, dunno, not taken them off yet.

[flanagaj]

Thanks all.