First a tip of the hat to everyone here. I've only recently acquired an '87 300E w/155k and the info here has enabled me to fix the windows, power steering pump and few other odds and ends.
The latest problem to tackle is summarized above: no tach, no a/c and the CC jumps from the set speed up and back again repeatedly. I'm not sure if these all point to the OVP or if they are independent of each other.
I've also disconnected the coolant level sensor to turn the light off on the dash if that is of any consequence.
I searched the posts here and it seems as though it may be the OVP but I'm not sure.

Re-attach the coolant level sensor, if there is enough coolant in there and the light stays on you might need to replace the thermostat.

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"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL

Ok...but is that related? And why wouldn't that indicated a bad sensor?

Try giving fuse #7 a spin...

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A Dalton

Cleaned of fuse #7. No go. Still interested in the thermostat theory.