Hey 1022fan (Ruger??),
According to my manual, AKfish is right on. The tach and the hour meter are fed by the same circuit. In fact, they share the common 10 amp key switch fuse. So if your tach is working, then you probably have a fault somewhere in the cluster itself.
Key Switch --[red]--> Fuse Block (key switch fuse) --[red]--> 10 Wire Instrument Cluster Connector --[black]--> Ground
The tach and hour meter get fed through red wire in the 10 wire connector on the back of the cluster. They both get grounded by the black wire in the same connector.
Sounds like if your tach is working, you're going to have to crack the cluster open.