Odo works, speedo doesn't
 

On my second day of Porsche ownership, in my ignorance, I reset the Odometer. I believe I was parked at the time, but can't be sure. The car is an 84 Row Carerra.
So off I go and not far into the drive I notice that the speedo was not working at all, but the Odo was.
Reading on here, everything I see mentions the plastic Odometer gear going bad, but nothing that I can find talks about the reverse.

Any thoughts on what would be the cause of this? What drives the speedo? Is there a generator the drives the electric part behind the gauge?

Maybe these threads help:

http://forums.pelicanparts.com/porsche-911-technical-forum/736651-1980sc-speedometer-problem.html

http://forums.pelicanparts.com/porsche-911-technical-forum/682223-bench-test-electronic-speedometer.html

http://forums.pelicanparts.com/porsche-911-technical-forum/725419-help-odometer-repair.html

http://forums.pelicanparts.com/porsche-911-technical-forum/279093-86-speedometer-problem.html

http://forums.pelicanparts.com/porsche-911-technical-forum/766176-speedometer-clock-repair.html

http://forums.pelicanparts.com/porsche-911-technical-forum/684428-speedometer-question.html

http://forums.pelicanparts.com/porsche-911-technical-forum/632126-video-how-test-speedometer.html

But the fallback solution is always to send the speedometer to one of the meter repair shops. They can fix it.

I just did an article on gauges for Vintage Motorsport magazine. The folks at Seattle Speedometer say they see more damage from owners trying to repair their own gauges than from normal wear. Interesting.

They want the customers to call them before they do anything with their gauges.

Richard Newton
Car Tech Stuff

Thanks Porwolf for doing all that work, I had read some of those, but not all.

I guess the first test will be the finger flick, just to see if it's sticky. But I don't think so. Funny how I did the one thing thats supposed to break the Odo gear but it didn't.

update
 

Just to finish off this thread. The Odo continues to work fine. And for no known reason the speedo started working again as well.
I had done some replacing of headlight dimmer switch work, but that was after the speedo self repair.

It remains a mystery

Maybe the needle movement needed a little knock. Or it's summer time and the temperatures are better.