Three conditions for the A/C to engage (at least on my 1982 300D, in theory...): outside temperature must be above 2 degrees C (34 F), there must be pressure in the A/C circuit (to prevent burning of the compressor) and the climate control brain asks for cool air.

Pushing in the EC button is the only way to prevent it from engaging.

Hope this answers your question.

Nicolas

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1982 300D - http://members.xoom.com/nicodore/