I believe all manufacturers keep the compressor from running when the temp is below 40 degrees. I know that GM does. If they didn't the evaporater would freeze over and you wouldn't get any air out of the HVAC system. To prevent this they usually control the compressor with the low pressure switch in the A/C line. The low side pressure equates to the temp that the evaporater will be running at. When it drops below a point in the 35 to 39 degree range the switch opens and the compressor clutch loses its power. The light on the dash may flicker or even be on solidly on but the compressor isn't running. You can verify this by going outdoors and trying to run the A/C or defroster when its 20 or 30 degrees outside. The compressor will not run at all. The clutch will turn but the compressor shaft will not turn.