At min, the climate control unit is suppose to default to maximum cooling, despite the interior or ambient temperatures. Max should be the opposite, maximum heating. Now I will say that last July, when it was in the high nineties (F), I didn't get heat at MAX, even though I had just rebuilt my mono valve and completely resoldiered my climate control unit (and replaced all but two vacuum pods)....this was all performed on my 1987 560SL (Hilda). So, perhaps extreme cold or hot temperature conditions can signal the system not to exaggerate the condition any further...but I really don't know if is just a fluke (I do know for sure that the compressor will not engage if the ambient temperature is below a certain level).
I know a lot about the climate system off the top of my head....how? I have driving either a W107, W123 or W126 for over twenty four years. I like to tinker with it myself and have read a lot over the years. Hence, I would start with the reading and gradually you will know it "like the back of your hand"!
Here is a good place to start:
http://www.peachparts.com/shopforum/showthread.php?t=142408