Intermittant Speedo

Subject: Intermittant Speedo
From: Ian Milne at Upside Engineering <milnei@domain.elided>
Date: Mon, 23 Sep 1996 16:22:04 -0600

Help!  The VDO speedometer in my '84 318 (150Mm) has begun to fail
intermittantly.  When cold (below 32 DegF) the unit works properly. As the
interior of the car warms, the speedo and fuel economy functions quit and
the needle dives to zero.  If I am on the road for an extended period, and
the car interior gets toasty, the speedo will sometimes start to operate
again; often jumping between zero and the correct speed a few times.

I have only been able to make a number of measurements with the car
stationary. When stopped the speed sender unit appears as a dead short on my
Ohm-meter (Hall Effect device?).  Running it produces a 10 Volt square wave
whose Off duration remains constant at about 2.5 msec while the On duration
varies with speed.  The speedo worked consistantly the whole time I was
making the measurements so I was unable to determine the cause.

Has anyone else experienced and corrected this problem?  If so, what was
done  short of replacing the speedo?


Thanks for the info.
Ian Milne