Another 3.2 tach pegged and running backwards
 

Pretty much what the title says. When you turn the key to on, the tach needle abruptly swings all the way to max. Then when running, it tends to work in reverse, but with at 2000 RPM lag.

I've read all the threads on this here I can find, and ther are several. None have indicated a true cause or fix. This issue seems to be somewhat intermittent, I had it for a week or so last summer, then it just as abruptly stared working perfectly again until three days ago.

As for some of the common thoughts, I've checked the grounds, and the system is not overcharging. The CASIS light has been disconnected for over 4 years, but I tried with it reconnected just to be sure, no change.

This would seem to be a failure internal to the tach itself, not the DME.

Has anyone actual found and fixed this? Or do I just need to wait for it to go away again by itself?

Bump so more people might see this. I figure some one has found a true fix.

Maybe I need to call NHS. But that will lead to removal of the upshift light, addition of a volt meter, etc.

No one?

Time to bite the bullet. You have all the classic symptoms of a failing board in your tach.
Disabling the upshift light is like a no-cost operation, as they will be inside there anyway.
Just got mine (85 Carrera w 156,000) back three weeks ago.
Bill K