Me thinks the tach be bad. There is a small hair spring that keps the tach at zero, its possible that it broke. you would have to take the cluster out and apart to see. They are fairly simple to take apart and easy to remove. i dont remember if its the same as the eairly models, but if it is...take the covers off by the trip meter reset and the clock set knob. remove the screws, and unhook all the wires in back. most of them will only go in one place, make notes if you need or if you think you may forget location.