When fixing the whole A/C system, I also had to get a new temp sensor (mounted within the speaker grill on the passenger's side). Without this sensor though, the system "thought" that it must be freezingly cold (since it didn't get any current from that particular sensor) and the heater blower WAS definitely responding my switching blower speed (higher and higher). As soon as I connected the sensor, it never would do that again, making me think that it's either just not cold enough, or that I will just have to make the adjustment on the little white gearwheel you were talking about.
The other thing is, that I really would like to figure out, when and by what impulse the aux. water pump should come on, since opinions seem to differ here.
Steve, you said it's a matter of temperature - could I trick the sensor (where is it? it's not the interior sensor, right?) by cooling it down with a plastic bag full of ice cubes or whatsoever?
The testing machine for the servo your brother built should definitely have turned you guys into millionaires! I wish I could build stuff like that!
T.

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1980 300D, 156K