I've just reheated all the solder joints on the OVP. Tach, A/C still didn't work. Pulled the KLIMA and upon opening got a whif of burnt electronics.
Question is, can the KLIMA keep the tach & A/C from operating?
Should just replace them both?

The klima relay can keep the A/C compressor from comming on, not the tach.

WHAT year & model car?

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It's an '87 300E. The OVP was checked by a tech about a month ago for an intermittent no-start problem which turned out to be the ignition coil.
So I'm starting to think the OVP is OK. Can I troubleshoot and repair the tach regardless of the OVP's state? I've seen some your previous posts, MBDoc, and I know I need to pull the dash and check for 12V on one of the leads. If there is, I've got a bad tach. If there isn't then I don't know what. Also there is a sensor on the back of the engine somewhere that I have to check. Fuse #5 & 7 have been replaced.
I need to start somewhere on this chain of problems and am figuring the tach is the place to start.