<E34> Erratic Speedometer: Vehicle Speed Sensor???

["Theodore W. Matthias" <twm@xxxxxxx>]

Recently, I have noticed erratic speedometer behavior in my 89 525ia.
When the car crosses large expansion joints, the needle suddenly, albeit
momentarily, drops its reading though the vehicle speed and RPMs remain
the same. This is now beginning to happen without the aid of expansion
joints.

This becomes a real problem when I use the cruise control.  Evidently, the
drop in indicated speed is sensed by the cruise control controller (!) and
the unit accellerates then decellerates to compensate.  The net effect is
that I'm beginning to feel like a whiplash test dummy.

Since the tach remains unaffected when the speedometer exhibits this
problem,
I don't think it is the engine's RPM sensor.  From reading the electrical
diagrams, I am aware of a "vehicle speed sensor" located on the
differential.

Could this be the problem?  Does anyone have experience with one of these
do-hickeys?  How do I test it?  How do I find it?

Thanks in advance...

Ted Matthias

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