Helmer,
for the most part they are sealed units,if there is a calibration port
on the back you may be able to use a precision lubricator on the
movement but not likely...
You can try moving the tach to a warm dry location to dry out any
condensation or residual moisture, but you will probably be best served
by sending it to a Speedo/instrument repair facility,and it may be
cheaper to replace it before all is said and done
Regards,
markvictor

http://groups.google.com/group/badboatbiz