Audi 90 quattro 10V - ABS light stays on

I am in the UK and own an Audi 90 quattro 10V (1988) that has developed a problem with the ABS braking system. I have made some observations and have
some questions listed below. Apologies for the length of this posting but this has been going on for a while now.... Problem - ABS lamp illuminates after driving a short distance. The ABS start-up procedure seems to be working normally, ie
ABS light switches on at initial ignition on
ABS light off after engine started but.
After a short drive (approx 20 - 30 mph) the light comes back on. Prior to the light switching on, the brake pedal has initially (slightly) excess travel and the ABS pump can be heard operating. The pedal then has reduced travel as the pump operates. Once the light comes on the brakes feel 'normal'.
A major side affect during the majority of the tests is that the O/S/F brake calliper has been (partially) activated resulting in the wheel/ brakes becoming hot.
The problem initially seemed to appear after some work on the front suspension but this may or may not be directly linked to the problem.
I have tried a few experiments and made the following observations
Test Situation Result
1 The problem seemed to start when the car required new roller top mountings for the front McPherson struts, this required the suspension to be dismantled ABS lamp illuminated after a short distance. Not aware if brake calliper was falsely activating at this stage or not.
2 Reset the O/S/F wheel speed sensor ABS lamp no longer illuminating but brake calliper (falsely) activating
3 Replaced brake calliper (suspected sticking calliper) ABS lamp not illuminating but brake calliper still (falsely) activating
4 De-activate ABS switch on dash before driving away. Brake calliper no longer falsely activating
5 Various positions of ABS sensor on O/S/F sensor tried ABS lamp illuminated after a short distance and brake calliper still (falsely) activating
6 Checked sensors for resistance All within tolerance stated in manual
7 Replacement ABS speed sensor fitted to O/S/F (as this calliper was still being falsely activated) ABS lamp illuminated after a short distance and brake calliper still (falsely) activating
8 Various positions of ABS sensor on N/S/F sensor tried ABS lamp illuminated after a short distance and brake calliper still (falsely) activating
9 Replacement ABS speed sensor (originally fitted to O/S/F) fitted to N/S/F ABS lamp illuminated after a short distance and brake calliper still (falsely) activating
10 Remove O/S/F and N/S/F driveshafts and inspect ABS sensor ring on CV joint for damage. Clean up ABS sensor ring and replace driveshafts No obvious damage. ABS lamp illuminated after a short distance but brake calliper no longer falsely activating
The current status is as per the last entry in the table, ie light is still illuminating and the calliper is working normally.
The next plan is to remove the O/S/R and N/S/R driveshafts and inspect the ABS sensor ring for damage (similar to test 10 but on the rear wheels). I would also like to arrange to observe the output of the ABS sensors on an oscilloscope if possible.
I have some specific questions -
General
a.. When the ABS lamp illuminates after a short drive, does this mean that the ABS is completely de-activated ? If so, why would Test 4 affect the brake calliper operation ? b.. Test 10 has resulted in the brake calliper no longer falsely activating, what could affect this (electronics and/or hydraulics/mechanical ?) c.. Can you recommend any publications or websites that explain how Audi ABS systems of this design work ? Electronics
a.. Why did de-activating the ABS with the dash switch (Test 4) result in the brake calliper operating normally ? - does this suggest an 'electronics' fault rather than a 'hydraulic' fault ? b.. Which pins do I connect the oscilloscope leads to ? c.. When the O/S/F calliper is falsely activated, which wheel (sensor) is most likely to be causing this to occur ? Any suggestions as to the likely cause(s) or clues as to what I should check next would be greatly appreciated.
Regards,
Bob
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