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?

Dear mb300d,If you've already replaced the sending unit,the only other thing left is the wiring. I would check the power to the gauge itself, by pulling out the cluster and using either a volt meter, or a test light to ensure you have juice. After that, maybe disconnect the wire that runs to the sending unit, and temporarilly run a jumper wire, outside and seperate from the car's wiring harness. If the tach works now, you'll know that the factory wire is no good.

Three Parts To The Circuit...
 

There's the TDC sensor in the motor, the TDC sender on the fender well, and the tach itself. Along with the wiring to connect it together, the problem could be anywhere.

If you've replaced the sender with a new one, I would try giving it some time to adjust. Sometimes these things seem to fix themselves...

My next suggestion would be the tach itself, and then the sender in the motor. Good luck, and let us know what works!