I am re-posting just in case someone may have an answer:

I have installed a tach from an '82 Mercedes 300d Turbo in my '77 Mercedes 280E and have wired it exactly as per a 300d turbo Mercedes wiring diagram. I have used my existing TDC sensor, connected to an additional diagnostic plug containing a new tach amp. The result is just as if the tach amp were bad - a wandering & sometimes dead tach. I bought the new VDO tach amp on e-bay and it looks new. ?

Additionally, one of the terminals on the 300d tach amp diagnostic plug had an extra wire, not shown on the wiring diagram, so I discarded it. Did I screw up? So I am wondering if I need a new TDC sensor, or to reconnect the mystery wire.