It's more than likely in need of being rebuilt as the originals didn't have a long life.
It gets it's signal from a gray wire on the "-" side of the ignition coil. Is that wire connected at both ends? It's connected to a tab on back of the tach. There is a blue wire that provides the power. Is that connected? The boot slips over the longest bolt on the back of the tach.
If you have the tach out of the dash, you can conduct your own experiment under the engine hood. You can run a wire from the "+" of your battery to provide power. You can run another wire from the "-" side of the coil to the tach. Then lastly, it has to be grounded. In the dash, it's grounded to the dash frame, but here you need to run a wire from one of the nuts on the back of the tach to either the "-" side of the battery, or a valve cover bolt, or anyplace you can get a ground.
