Thanks, Joel, I'll try them.
Jeff, to clarify my problem, the A/C compressor was engaged on any Climate Control function setting, even 0 (OFF), until I disconnected the A/C pump connector.
I believe that the whirring sound quiets in the summer but not entirely.
I am not sure why it is so audible when the compressor was engaged but could be due to the cold outside temperature and the pump was not cycling on/off so pressure was building up.

I believe that the A/C should only engage on three settings: Defrost, Bi-Level and Normal(Cooling Mode only). Each of these settings has two modes cooling and heating.

Am I right to say that the compressor should not be engaged on OFF "0", Normal (Heating mode) or EC (Ventilation) settings?

I think the cooling mode is switching "on" and "off" properly since the centre vents above the climate control buttons direct air imtermittantly.

The centre vents should only blow air in cooling modes for Bi-Level and Normal settings and during ventilation mode on the EC setting.
This is according to my Owner's Manual.

Does the MAS relay control the heating/cooling modes or is it the Control Panel Module only?
Are there any other sensors/modules that could tell the Control Panel module that should be in cooling mode regardless of setting?
One would think that the 0 (off) setting would disengage the clutch but it doesn't.
Maybe the OFF setting only shuts off the fan, valve and coolant pump.

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Ian Read
90 300TE4MATIC (124)
79 300SD (116)