There are mechanical tachs that use a cable driven gear just like a speedometer, but they are few and far between and it would be obvious if you had one of these because of the large cable housing that comes off the distributor shaft that runs in to the back of the tach. However, the large majority of tachs that are sold and installed, including the factory style units, are electrical in construction.
If you've recently made changes to the wiring around the coil, you could have potentially disconnected your tach.
