Usually one only learns the complexity of MB climate control when its not working right, or at least that my case .

Here are some great troubleshooting articles that explain common faults and go over some basic functions of the system itself. Not W140 specific but a good read.

http://www.peachparts.com/Wikka/AcDiags

http://www.cacavas.com/mb/TTM.html

http://www.peachparts.com/shopforum/showthread.php?t=35373