The ASB wheel bearing has a magnetic encoder fitted within the seal which in conjunction with the ABS sensor measures the 
speed of each wheel independently.
This information is used by the ECU for the vehicle’s ABS (Anti Lock Braking) System.
The first vehicle to see this technology was the Renault Laguna in 1997 and today the majority of new vehicles now fit ASB wheel bearings as standard.
BEST PRACTICE STEPS
When you fit an ASB wheel bearing, in addition to following the vehicle manufacturers’ assembly instuctions, it is important to consider the following care points:
1. Ensure the knuckle, hub, the press etc. are clean and free from swarf and other metallic particles.
2. Remove the plastic cover just prior to installation.
3. Keep ASB wheel bearings away from other magnetic sources (a magnetic screwdriver for example) as it will erase the magnetic poles within the encoder.
4. Always install the wheel bearing with the magnetic encoder inboard side – you can check this with a metal paper clip.
5. Always follow the vehicle manufacturer’s instructions.
