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.