In all positions of the climate control except economy and off, the AC compressor is running. Heat control is maintained by adding heat to the cold air. If the heater control valve is shut, or the hose is pinched off, as you did, the air can only get colder and colder, as the ac compressor is still running. As to the question of whether the auxiliary pump should always be running, My brother has the factory manual at his vacation home right now, so I cannot give you a definitive answer, but I do suspect it will be running in all positions except economy and off.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C