My 300SD was behaving similarly, and I just got the Tach Amp from Bob, per the above link:OM617A Tach Amp. Has worked perfectly, and it takes all of about 30 seconds to replace it. (OK, 60 seconds if you clean off the plastic cover....)

FYI, the tach amp on the fender does not apply to '85 W123's or any W124's. It doesn't apply to most if not all W201's. Not sure about when the cutoff point is on the W126.
Pay attention to what car posters are referencing.......

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John