The tach amplifier is the likely culprit. The pins get loose solder
joints and create an open or intermitent connection. It is a black
cylindrical canister about 1 1/2" in dia and about 3" tall located on
the left front fender well. The simplest fix is to unscrew the
amplifier and place a small wad of papertowel in the center between the
pins and screw it back down. This creates a forced contact with the
solder joints and the tach should work again. I've had to do this 5 or
so times in the last 8 years with my 300SD. Let us know if that works.
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