I've read a ton of posts about having no heat with the problem being the mono valve. Well, I have a related, but not exact issue. I'll be cruising along and the AC will suddenly turn hot. Sometimes, I can fiddle with the temp control to get it to come back to AC, sometimes not. Does that sound like a mono valve issue?

I have tons of heat, just flaky AC.

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85 300D

There is a good AC thread listed in my signature.
You need to put gauges on it...
to see if you have the proper amount of refrigerant in the system first...
If you do not have the proper amount of refrigerant then your compressor may not getting the regular lubrication it needs....
but everything starts with putting the gauges on it...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html