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