I have a problem with the tach in my '82 300D. It fluctuates between 0 (most of the time) to 5000 rpm even when at idle. I also notice that when I turn the key to "on" before I crank the engine, it fluctuates as well. I tried the "cigarette butt" trick on the sender and even replaced it with another one but it is still erratic. It sounds like some sort of electrical problem. I don't know how the tach circuitry is wired. Any suggestions?