RPM gauge and manual trans

But they do work , but maybe not as it should . You say you changed the box to manual ...What about the flywheel and CPS ? ...same thing on my '87 190d--and I went thru all the same items, it turns out to be the "klima" relay(behind the battery). I replaced it with a used one and tach started working --for a while, (like 2 weeks) then it only "sometimes works", so I swapped the orig. klima back, and it works again, now it's been ok for 5 weeks-I suspect the contact (s) in the recp./plug.(poor contact)