I've seen some auctions on ebay about a spaztic tachometer, but this one really has me wondering. First, the tach will NOT work up until you get about up to 3000rpm(it just jumps right to it), an when it gets to 3000, it quivers up and down a bit. heres the catch! It works absolutely FINE when the A/C is on?!?!?!?! Someone at least provide an explanation of this!
BTW this is a 1982 300D Turbo Diesel with 195k on it

At least in the case of my 1981 300TD, wierd stuff happening in the instrument cluster was due to cold (i.e. bad) solder joints on the circuit board at the back of the cluster. I removed the cluster and re-heated the main ground connection - you can easily spot it as it is one of the larger areas of solder and is where there is a connection to the metal of the cluster which is ground. While I was at it, I touched up all the rest of the solder points on the circuit board and all worked fine thereafter.
John