There should be a gray signal lead on the back of the tach going to the coil. Don't remember for sure which side but you can verify continuity and connection.
There is also a switch on the back of the tach to set it for 4, 6 or 8 cylinders. That switch can get sticky or dirty over time. Cycle the switch several times to try and clean the contacts inside. I've seen that fix them albeit for a short term.
These tachs don't last forever and they do go bad. I've had to replace mine and I know of numerous others who have had to replace them.
