The above wheel bearing has an integrated ABS-sensor with small magnets built into it. The card in the above picture has an inset window, if this window is moved over the bearing, then the magnets can be seen and their functional ability ascertained, almost as if under a magnifying glass. Should for any reason, one of them not be effective, it is noticed immediately. In most cases, the removal of the wheel bearing is not even necessary.
An ABS-system should actually, not pose many problems in the workshop, after all, it has only the wheel sensors and all the actuators compactly built into one device. Indeed, the former is more often a point for discussion, in fact more often than the normal user would like. Once again, if the ABS-warning light goes on, this does not influence the braking function. If this is the case, one must simply take care that the wheels do not lock up or keep a suitable distance from the vehicles in front of you.
You probably already know, that the warning light mostly goes on in cars which are somewhat older, and as a rule the error can be traced to one or more sensors. Dirt and an incorrect gap are sometimes responsible. Actually, the damage can be easily repaired, sometimes even without having to install new parts. Be thankful that your control device lit up to show that something was wrong.
Indeed, now we come to those damages, which are not indicated. Nowadays, the rings, whose gaps are scanned, are made of a rust-free material or they may even be built directly in to the wheel bearing, where they are completely protected. Should this not be the case, it may be that some gaps are no longer 'visible' to the sensor. That's when it will count wrongly and this may cause it to vary from the neighbouring cylinder on the same axle.
Why then, is the driver not informed through the error storage? Because the earlier (not quite as clever) control devices considered the difference to be plausible. It is unpleasant when an ABS-case is initiated, although there was no real reason to do so. Cleverer control devices may warn that a tyre is losing air. That should put you on the right track.
Indeed, perhaps not, because the blocking of gaps through rust or the distortion of such rings takes a long time and the inequality has already been compensated for long ago. This is where an error, which is difficult to localise, occurs. This is why easily accessible sensor-rings should, in all cases also be replaced, e.g., when the brake-discs or -drums are replaced. 09/12