Flakey 200TQ speedometer revisited

[jp@barclay.net (JP Timmerman)]

The electronic/analog speedo in my '89 200TQ is misbehaving. I combed
through the archives and found lots of knowledge on similar problems.

About two years ago the speedo became intermittent. Sometimes smacking the
dash would help. After about a week or two it died completely. This summer
I tracked the problem to a dead sender and replaced it. It worked perfectly
until two  weeks ago.

When I go to work in the morning, with the car cold (outside temperature
-5C downto -20C overnight), the speedo works. After a few minutes (3 to 10)
the speedo dies. When I come home at night, the same thing happens. How it
dies is the interesting/complicating part. At first I thought it just
stopped, but there is a speed threshold below which the thing still works.
The threshold drops from highway speed to zero in about a minute. Drive at
100 kmh till it stops. Slow down a little, it starts working. Speed up, it
stops. Slow down further it starts. When it works it doesn't bounce and
seems to be accurate. Once it has died completely it won't work till the
car is cold again. It would be easy to miss the decreasing speed cutoff
part of the evidence, so other people might not have reported it. Next day,
same thing. I couldn't decide if it is affected by time, temperature, or
vibration. All of the other unrelated instruments work fine.

I checked and cleaned the speedo sender wiring harness. I checked, cleaned,
and tightened the little pin connectors between the speedometer head and
the PC board in the instrument cluster. I'm confident that these areas are
not the problem. I have seen many flakey and temperature sensitive
electronic problems caused by cracked solder joints and loose connections.
A quick look on the instrument cluster PC board revealed nothing, but I
could have easily missed something. The symptoms all point to such a
failure. Smacking the dash doesn't seem to help anymore.

>From the files (Tue Jan  3 14:33:43 1995):

>There's an Audi service bulletin which describes an intermittently open
>wire on one of the circuit boards that make up your instrument panel.
>It has pretty good directions for removing the boards, re-soldering the
>bad connection, and reassembling everything.  One way to check is to
>give the top of the dashboard a good whack next time the problem occurs.
>If it goes away, it's probably the problem I've described.  I have the
>manuals on microfiche; let me know if you'd like me to look up the
>number of the service bulletin for this problem.
>
>Good luck,
>Mike Rottenborn

Does anyone have this info?

TIA

JP Timmerman
'89 200TQ
'86 5000CDTQ Avant
'85 5000S