82 380SL tach stuck on 1800 RPM

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
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David wrote:

I am far from an expert, but I have had my '81 380 SL instrument panel off. It is super easy to remove- pull steering wheel (super easy w/ single center bolt) and pull the panel gently toward you. Enough leeway on the cables to lay it down on the steering column. Maybe 10 min job the 1st time, easy 5 min after that. Very logical (German design, nicht vahr?) and everything is self-evident. All?? connections are electrical except oil pressure. All seemed very familiar from my early days as self taught cheap fixer of old cars to keep them running.
Chip
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Thanks for the information, Chip. A long time ago, I rebuilt a flathead V8 in a 1951 Mercury, but I had a manual and a teacher to keep me from screwing up. I found some other posts about tachs on diesels from about the same era, so I think I have enough information to start tinkering on the next nice day.
Thanks again,
David
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