Not knowing what year your car is, but most compressors come on when the defroster is on in order to suck out any moisture that may cause the windows to fog.
In the Mix position, some air goes to the defroster, which in turn causes the compressor to engage.
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'89 LX 5.0 - 5 speed. Original Owner
'94 E-150 4.9 Hi-Top Conversion Van
'06 Mustang V-6, 5 speed
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