The heater control wheels don't seem to have any impact on the temperature. It is hot even when turned to the maximum green setting using the wheel controls. Has anybody figured out how to fix this? I also have a 1980 450 SL and this same problem occurred and was fixed with an after market control unit that replaced the original heater control valve in the engine compartment. Living in Alaska you need the heat in the winter, but not in the spring and summer.

 

There could be two possibilities, the heater control valve (Duo valve) may have gone bad, or your climate control unit's electronics are bad. On my car it was a combination of both.
Does the fan speed change when you rotate the dial?
You can send your control unit it to @tighed1 . He can refurbish it and get it back to you quickly. I have tagged him in this post, so hopefully he should respond soon.
I'll post the links to the "How to" posts that I made when I fixed mine. I'll also link to the rebuild kit for the heater control valve that I used. Hopefully they will be helpful.
LINK - Climate control removal
LINK - Climate control installation
LINK - Heater control valve / Duo valve refurbish