1979 300sd servo heating cooling problem
 

I'm trying to fix up my old 300sd for my daughter. The servo unit was cracked, but I could still get hot air. I picked up a few servo units from junk yards and installed. Now I only get cool air (I tried two different used units). Is it the servo units or some other problem? Could it be the climate control dial or something else? How can I better diagnose the problem? Any help would be appreciated.