Garrett,

Any problems with the AC and the 850 model have been discussed about a zillion-trillion times on this site because Volvo had a hand in the production of defective parts. Of course, they (Volvo) never claimed it was their fault, leaving the customers with the bill for repair. That's probably why no one has responded to your post. Use the search feature and search for thermostat-AC problems. Or, overheat problems.

You won't get very far with a diagnosis if you don't check for ERROR codes first. Do this before anything else.

Go to volvospeed.com and check out the replacement of the ECT sensor that is connected to the thermostat housing. Maybe this is bad and will not let the AC function correctly. If this sensor did fail (because of heat) then maybe the theremostat needs replacing too.

After you've checked for codes and depending on what you find, here is a site that describes the errors.

http://au.geocities.com/ozbrick850/computercodesvolvo.html#instructions
http://au.geocities.com/ozbrick850/computercodesvolvo.html#motronic

Please post what you find wrong, if anything...