Finding bad ABS sensor

[Bernard Littau]

Eyvind writes:
> I'm having a problem with one wheel speed sensor on my '90 200TQ.
[snip]
> I've borrowed an old multimeter, but the lowest DC voltage setting I
> can measure is 10 volts, with the sensor output being 0.5 volts, I
> guess this won't work. I don't easily have access to an oscilloscope..
>

You don't mention if you tried positioning the sensors with the little
plastic jig thingies you can get from Audi -- you could just be having a
sensor alignment problem.

Audi sells and uses a plastic cap that covers the sensor.  The sensor and
cap get pushed firmly against the ABS timing wheel, with the plastic cap
touching the timing wheel, and bolted into place.  When the wheel turns, it
rubs away some of the plastic cap, leaving the sensor positioned correctly
and with a clear air gap to the timing wheel.

These little caps are inexpensive.  What boggles my mind is the local Audi
dealers always seem to have only one or three of these caps in stock at any
one time.  Sounds like you need just two, for your two front wheels.

If you have already done the alignment via the plastic caps, then the
oscilloscope is the tool of choice for further debugging.

You might try using the borrowed multimeter on AC, and spinning the wheel,
rapidly, by hand, and note the hopefully now steady (but low) voltage
reading.  Your could perhaps try to maximize the AC voltage reading on the
multimeter, and hope that is the correct alignment.  This may be futile with
a cheap meter and the low voltages.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq