After not being driven in over one month, I took my 82 380SL for some
exercise. When I started the car, I heard a strange knocking sound
that seemed to be from under the hood. I switched it off and checked
around, but could not see anything unusual, so I tried starting again.
When I got in the car, I noticed that the tach was on 1800 RPM. It
started and ran fine, with no unusual noises or behavior. I did see
the tach move above 1800 briefly, but after an upshift it just fell
back to 1800 and stayed there.

I also have had a jumping fuel gauge for a long time, so I'm willing
to spend some effort to remove the cluster if it is the gauge itself.
Before I do, can someone tell me if the tach is driven off of the
flywheel in this model, or is there some kind of amplifier under the
hood that I need to check out, or perhaps there is a fuse?

Thanks for any advice.

David