Actually it is a 1981 300SD; was mistake. Now that I am OK with pulling out the instrument console I am wondering about the tach. While it does not work 95% of the time, the other times it goes crazy - back and forth like a whirligig.

So I am thinking maybe the problem is the electrical connection behind the instrument console. There is a large circular plug with about a dozen pins in it that is unplugged to remove the instrument console. I am wondering if one of those pins runs the tach.

Anyone know?