I can't offer the answer you are seeking, but I can offer a resolution to your problem once and for all. I am guessing you have a climate control system.

One of the 1st things I did after buying my '77 280E in 1990, was to take out the entire CC system - the worst idea Mercedes has ever had - and replace it with a manual control from a 240D from the same year. This gave me total control of the heater/AC - and it always works! You could do the same with your '81 280CE. You do, however, have to replace the heater box and evaporator along with the control panel, plus change the heater hoses and diverter valve. To keep it simple, I used a thermostat from a 114 car, as suggested by a mechanic. It's a lot of work, but I would do it again in a NY minute!

Good hunting at the junk-yard and happy motoring!