My tach on my 83 245GL did the same thing. It worked intermittently and was somewhat annoying. When the needle didn't move, I would whack the dash and the needle would jump right up--sometimes. Quite often it would take a couple of whacks. The needle would bounce really high and then settle at the appropriate range. Another trick I did was press this button near the tach. For some reason it worked just as well as whacking the dash. I doubt that button was for making the tach work, but for some reason putting pressure on it got the tack to move :o)