I have a 2002 S60 with manual climate control. Blower off, Recirculate off. A/C off. Driver & passenger heat setting anywhere. Air distribution to floor. Frequently, the A/C compressor cuts on by itself. I press A/C off. About 30 sec. later, it cuts on again. I press A/C off. It cuts on again, etc. I have this problem about 50% of the time that I drive the car under the conditions described above. Usually, the temperature outside is moderate to chilly. I certainly don't need A/C.

 

Is your vent set to the defrost/defog position? on some cars the AC is used to dry the air going to the windshield at the same time its being heated.

 

No, as mentioned above, air distribution to floor, 180 deg. from full defrost. But you're right, when it's on full defrost, the A/C is always on by design.