[Ozarkguy]

Not sure on your setup. However I had a tach generator that drove my SW tach that was driven off the accessory drive. It was mounted in the engine compartment. I found it was froze up. Took it apart , cleaned and lubed the shaft n gears. It's about the size of your fist with cable shaft in and wires out to the tach. Also check the wire as they get old n break

Edit. Appears yours different.

There is a lot of info on the WOGroup about this issue