Why would this happen?
 

Hey.
Why would the tach stopp working on a '87 300 TD?
The needle is sitting on the bottom peg.
Doesn't move.
Thanks in advance.

I think most of these Tachs are mechanically driven... find a book which shows how to take the cable out of one end or the other, whichever is proper... and see if it has broken...

It is electric and common cause seems to be that the tach looses it's grounding.
Solved by rerouting a ground wire on the back of the instrument cluster.

Does your AC work ok? If it does the sending unit on the engine is ok. If it also has failed the sender or it's cabling might be faulty.