[urq] Urq ABS

[Phil Payne]

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You need to fix that.

> Then when on my way home, I applied a medium braking effort and the car braked wildly to the
right, then suddenly to the left. Built up speed again and braked sharply to try and feel what
was happening.
It 'feels' like each caliper is braking at different times. Perhaps what would happen if
braking on ice so the abs would apply bracking to only the wheel that isn't skidding. Only
this was a dry sunny day.

Weird one.  I suspect that one of the front wheel sensors - probably the right - has a rotated
outer casing.

The very end of the sensor is chisel-shaped.  It must be aligned along the axis of the hub to
match the teeth on it.  Usually this is an inevitable consequence of fitting it - but it's
possible for the outer casing to get twisted round, especially during a difficult removal.
Instead of neatly facing one complete tooth or gap at a time, the sensor then 'bridges' two or
more teeth.

The result is odd - at speed, the sensor signal is good enough for the ABS controller to
accept it.  Moving more slowly, the amplitude of the sensor's output falls off much more
quickly and causes either the ABS controller to reject the signal or a premature indication
that the wheel has stopped.  This latter causes the ABS to open that caliper.

You can have this problem on the back without it having any effect on steering - the back end
is single-channel.

--
  Phil Payne
  http://www.isham-research.com/quattro
  +44 7785 302 803
  +49 173 6242039