My tach intermittently works: thinking it’s a bad connection ? thoughts?

 

The Tach signal is from - terminal of ignition Coil, or the sources of ignition power to the + terminal of the ignition coil on the 6 cylinder XKEs. Not sure which way yours is wired, and I am assuming you XK 6 cylinder engine: Cleaning the contacts to all connectors on both the + and the - terminals of the ignition coil with contact cleaner and light squeeze with pliers on the female connectors would help to isolate your problem.
If the problem comes back, then checking all the way to the ignition switch to the tach connectors themselves will be next step. If it is internal to the Tach, then first guess would be the capacitor that is easily unsoldered and a new capacitor soldered in place.

See link: https://www.jaguarforums.com/forum/e-type-xk-e-59/xke-series-2-1969-tachometer-issue-234173/



XKE with Tach connected to negative terminal of Ignition coil.



Capacitor circled in yellow is common problem
Rgds
David

 

Does his 74 v12 have the same setup as the 6 cylinder?

 

95 is Tach, 164 Ballast Resister, 183 is Ignition Amplifier, and 19 Two Way Fuse
Again I do not know for sure this is the actual circuit for your car. If the car is running fine, then the connectors between 164-95-19 should be checked; otherwise it is within the Tach it self.
Rgds
David