Manich, thanks for your input. I was about to buy a tachometer (if I could find one used). I tried a brand new OVP, and it did nothing for the tach. There could some inline fuse somewhere that blew or burnt a ground or worse yet blew a circuit in the computer. I see that you have a 300E, I'm not aware of the differences in electronics between the diesels and gassers so your solution may vary but it seems unlikely because this a fairly unique problem.

I'm ordering a manual that includes w124 so maybe it will give some wiring diagrams or troubleshooting and i will break out the multimeter and see if I'm getting grounds. My glow plug light does not work and that seems to be grounded through the metal tach assembly itself (all other instrument lights have separate ground wires) so I 'm leaning towards a bad ground. I will definitely look into the tach pickup. I'll let you know if I find anything.