wacky speedometer

["Rodney Moore" <m6bigdog@xxxxxxxx>]

Paul,
>>>>>>>>>>>>>>>>>>>
Date:	Sat, 06 Mar 1999 14:10:27 -0800
From:	Paul <pault77@domain.elided>
Subject:	wacky speedometer
- -------snip-------stopped at the light, foot on the brake, i noticed that
the speedometer was was going out of control, it was going up and down, it
was acting like the rpm meter as if you rev'd you engine. it would go up as
high as 80mph and go back down to zero and would go up to about 80 again. i
didn't feel anything movement on the car---snip------
and as for my mom's car she tells me she noticed the same.
my questions are: is it something to worrie about?? is it a simple fix?? any
long term damages for leaving it alone?? perhaps a recall?? and anyone else
had the problem??
thanks for the help
paul
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

I would replace the magnetic reed switch pickup in the differential.  When
the switch contacts start to fail they will fail to make good contact and
this will cause the switch interface circuit to thing the axle is rotating.
The reason the speedometer jumps so high is failing contacts will arc at a
high frequency. A quick check is to let the car roll forward just around a
foot when this is occurring and this will cause the switch to open and stop
the intermittent contact.  I would assume the speedometer works normally in
all other respects.
My 1990 535 did that with only 8k on the odometer.  The dealer wanted to
change out the instrument cluster.  That would have been like heart surgery
for a stubbed toe.  I changed the differential switch and it never acted up
after that.
Regards,
Rod...
- ---
Rodney Moore
m6bigdog@domain.elided

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