[Hanuman]

The ABS is remarkably tolerant.

In the case of my ‘85 WR, the toothed ring on one drive shaft was so corroded, you’d have been hard pressed to see any teeth for about 1/8th of the ring, yet the ABS would still work normally most of the time.
One sensor has actually been damaged by being inserted without the ablative cap, and that still detects and I get normal operation.

If the system is not detecting any permanent error once moving, then I’d suggest that one or two sensors need adjustment, or the rings need cleaning or replacing. I’ve cleaned the rings before by pulling the sensor out, and gently feeling the ring teeth with a small terminal screwdriver. The teeth get clogged with general grime, and as they corrode, the gaps fill with corrode material deteriorating the signal.

The rings are about £10 from eBay. I managed to do one in situ on the car without removing the entire shaft.

The system has a ‘combination relay’, which is the item that ultimately results in the ABS light being turned off.

It will only turn the ABS light off if a number of conditions are met. My fading memory suggests seven required conditions to allow the light to go out at start-up, including-

System voltage normal (a dying battery and failed charging system can drop to 10v before this gives an error)
ABS on/off switch not pressed
System ‘healthy’ signal from the controller

The signal from the sensors isn’t checked until the car is moving. I think this is just a check that some signal is measured at each sensor, and the system simply compares the 4 signals as being reasonable. The bit about being very tolerant of poor condition rings!
There’s a distance over which this occurs. I think it equates to 60meters (x number of rotations).
If the light comes on, it can be reset by turning the ABS off then on, and it will again compare signals.

Once you get to the occasional solid pedal, sound of a rapid buzzing from the other side of the bulkhead without any heavy braking to induce lock-up, it’s likely that the system is now getting one bad signal and is ‘just’ enough to keep functioning.

In my case, once the rings were cleaned (not diligently, either) it was several years before it finally failed.

Damp/wet conditions will exacerbate a grubby ring (ooh-er). I found the ‘flutter’ and eventually ABS dropping out happened mostly on wet days, resulting in the ABS light coming on, until the dodgy ring was sufficiently eroded as to be useless.