Ford Mercury Villager 1997 Owner's Manual page 66

Heating and air conditioning climate control system panel
The standard heating system and the heating portion of the heating
and air conditioning system are essentially identical. There is
one exception, however; the air conditioning compressor may
automatically engage when the Defrost position is selected to aid in
defogging and defrosting the inside glass. When Defrost is
selected, the A/C compressor will automatically operate when
the outside temperature is above approximately 40° F (4° C),
whether or not the A/C button is illuminated. Manual
selection and illumination of the A/C button in defrost mode
does not override the automatic compressor operation discussed
above. Refer to the heating system operating instructions
covered previously for a more detailed explanation of heating
system operation.
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