Are you sure it's hooked up right? I know that sounds stupid but tachs are pretty basic in what they do...i've never heard of one going bad. If all is right i'd at least try to get it replaced especially if it's not even a year old.
im pretty positive its hooked up right. white wire goes to the positive on the coil. i have the ground and light source so it should be working right. i must have gotten a bad one from the factory. idk