I've noticed that if it's cold outside (below freezing) that my cars AC system will not turn on.
The light on the AC button is on but the clutch doesn't engage.
I hit the button by accident after installing an aftermarket deck this afternoon and noticed the clutch didn't engage. The engine was running, I know I don't need AC in the winter but found this peculiar.
Is there a sensor or fail safe on the system that prevents the clutch from engaging if the ambient temperature is too low?
I have a 2006 4.0 manual, thanks in advance!

 

The A/C does not operate unless the outside temperature is above approximately 2 degrees Centigrade (35 degrees Fahrenheit). This is from the 2007 Mustang Workshop Manual.

 

Turn on your defroster button and see if it kicks on.

 

When you run your defroster, the compressor will cycle as needed. It's drying the air that is blown on the windshield to prevent it from fogging up. But it's not "running the A/C".