Hello,
My 85 300D has an interesting new problem. The tachometer and the AC stopped working at the same time. I never knew they were related! Some symptoms:

At idle (relatively smooth): tach reads zero, AC clutch does not engage
Step on accelerator, revs up to about 1500 rpm: AC clutch kits in, tach reading goes up to about 1000 .
Hold engine about 1500rpm for a few seconds: tach drops back to zero (not gradually) and AC clutch kicks out. It's like a relay somewhere gives up.
Repeat with same results.

I've visually inspected the KILMA relay. Looks fine
I've resoldered the OVP relay.
I resoldered the CCU a couple years ago and it has worked fine ever since.

Other info:
AC works great when the clutch will stay engaged.
Tach was always fine until a month ago. Then for a few hundred miles it just read low. Then it started reading VERY low for a bit longer. Now it does as I've described above.
AC compressor clutch started being very erratic about the same time tach started acting up. I've witnessed it clicking on/off very rapidly. It was making a noised that reminded me of a loose spark plug wire.

Any help will be greatly appreciated.

Brian