Help with failing speedometer diagnosis

[MuffinMan]

Back in September, all was good in the world. I had a fully functioning 911 complete with a very healthy engine, a relatively fresh transmission rebuild, a new suspension, and of course bright orange paint. What more could I want? At that point, I was content.

So I entered the Northern New Jersey PCA region's Porscherama event...the event of events. It included an autocross (held on a small road course), open track time, a time trial, a rally, and a concours event. It was grand until my transmission self-destructed, my starter failed, I locked my keys in the car, and my speedometer flaked out. The latter being the topic of this thread.

Nevertheless, I was on the track when I noticed odd speedometer behavior. When I was on the throttle, the speedometer functioned perfectly, but as soon as I lifted, the needle would drop to zero. While it was amusing watching the needle shoot between 0 and 120 at a remarkable rate, I knew I would be in for a repair.

Later that day, my transmission failed via the carrier bearing. I didn't know if this had anything to do with my speedometer failure or not, but the speedometer was the last thing on my mind at that point. After transporting the car back home (which is a story in itself ), we rebuilt the transmission and got the car running again. The good news is the speedometer now functions whether I'm on or off the throttle. The bad news is it never displays the correct speed.

While before, it was either at "0" or the actual speed, depending on whether I was on the throttle or not, now it displays a speed that is much lower than actual. The needle is very jumpy/jiggly, and reads very low. If I'm doing 70mph, the needle will be at roughly 30mph, behaving in a very jittery fashion.

What does this sound like? A faulty speedometer? A faulty sender? Perhaps the magnets on the carrier plate were effected by the transmission failure (although, they were cleaned very well before transmission re-assembly). Your thoughts are appreciated.

[ianc]

I believe there is a test procedure in the Bentley manual for the sender, isn't there?

ianc