Thanks to all who have provided info regarding repair of tachs and clocks.
I followed the procedures in the threads and was successful in repairing the tach sender and clock in my 123 300D. I would offer my opinion on the use of a plastic peanut or filter from a cigarette under the cap of the sender. This is NOT a permanent fix. I found a number of poor or broken solder connections that can only be permanently repaired by resoldering.
The most time consuming part in the tach repair was removing the silicon on the bottom of the circuit board. I did not replace the silicon after resoldering. I am not sure why the manufacture sealed the unit in silicon other than to make repair more difficult. The clock repair was a piece of cake.
Thanks again to all who provided info. I do much more reading, rather than posting. Don't feel that I have much to offer considering the expertise of those who regularly respond to questions.
1982 300D Turbo 198k
1987 420SEL Sold
1993 400SEL 123k
MBCA Desert Stars