Does anyone know if the a/c compresors don't cycle on & off when it's cold outside? I have a 2006 with the 5.4. I noticed today that the inside of the windows stayed foggy for a long time. I came back home and tried to cycle the heater through the different settings and tried to turn the a/c on. Normally I can hear the compressor 'clicking' on & off regularly, but couldn't as I cycled through the settings. It has been in the 20's here for a few days and thought maybe it stays on or off when it is this cold out?

 

When it's cold enough, the system refrigerant pressure drops (due to contraction) below the trigger point of the cycling switch.

Steve

 

Edit: Sorry, bad info. haha

I think around 20 degrees or so will be about the low pressure switch in the A/C circuit, which equates to roughly 20psi, give or take. You're just out of luck when it gets that cold, the system won't work. It wouldn't even work very well if you bypassed the switch, it just gets too cold for the physics of A/C. The hotter the better! It's all about temp change and change of state, can't really change cold to cold.

 

The evaporator freezes at 32°F so the system is set to turn off before the pressure drops low enough (22 - 25 psi) to cause it to freeze. It turns back on at around 39 - 47psi, roughly 45°F. Obviously at 20° the pressure is below the freeze threshold so it doesn't even turn on.

JMC