(JimmyL, thanks for asking about the "trimming plug"- that was going to be my next question. And thanks to DeliveryValve and Jeremy5848 for answering and guiding us all to it and the control unit.)

I was able to clean out both ends of the rpm sensor (a grimy, awkward job at the south end of it). This didn't change the multimeter readings I am getting from it.

Yes, my a/c works, as does the cruise control and everything else- just not the tach.

This morning I cleaned the contacts in the control unit (part # 002 545 6032)- they had some green oxidation. I also pulled out the instrument cluster and cleaned the tach contacts there, too- but still no working tach.

I posted earlier that as far as I can tell the turbo kicks in, though perhaps the tranny is simply dropping down a gear (this being the only 300TD I've ever driven, it's hard for me to know how the turbo should feel).

Since this tach worked briefly a few months ago (as described earlier in this thread), I'm now at a loss of what to do... as far as I can tell, all of the component pieces are working or are able to work, just not collectively.

I haven't found any cut or frayed wires, though I can't trace them entirely since they are wrapped as a bundle in plastic sleeves or pass behind the center pods in the dash.

__________________
Charlie
Federal 1985 Mercedes 300TD, 220k
Blue exterior
Palomino interior