From the 2008 Owner Manual, other years similar.

- (Defog): This mode directs approximately half of the
air to the windshield and the side window outlets and half
to the floor outlets. When this mode is selected the
system runs the air conditioning compressor. To defog
the windows faster, turn the temperature control knob
clockwise to the warmest setting.
Recirculation mode is not available in defog mode.

(Defrost): This mode directs most of the air to the
windshield, with some air directed to the side window
outlets and the floor outlets. When this mode is selected
the system runs the air conditioning compressor. To
defrost the windows faster, turn the temperature control
knob clockwise to the warmest setting.
Recirculation mode is not available in defog mode

 

With the temps finally getting above 25 degrees, all the strange issues have seemed to disappeared. Temps (while driving) stay a constant 190-199, and even the occasional Brake Fluid warning has not come up as often. I do check it when it comes up and when I fill with gas, I also check oil and coolant levels but then i am old school and on the other old cars, that is what you did.

Donbrew, at -20 below, there is not a lot of moisture in the air for the A/C to remove, except for maybe when you exhale your breath. I have asked others and they also do not see the A/C compressor come on in freezing temps (below zero) At warmer temps, the A/C does come on. (say above 20 degrees)

 

A final follow-up, temps the last few days were in the 70's and I took a peek at the radiator fan with the a/c on high, the fan started up after a few seconds so I believe it is working correctly. Driving down the highway at 65-75 mph, temps on the dash readout are in the lower 190-196 range. so life is good.

 

Someone once told me GM runs the A/C with the defrost to make sure it get run during the Winter for the benefit of the compressor seals.

 

What happens when you breath on a cold piece of glass? The humidity in your breathe condenses on the cold surface. The AC removes moisture from the air to aid in defrosting. Same thing for recirc.

 

Just a follow-up. temp today was a balmy 93 degrees F and driving to work with the A/C running at 70 MPH, temps varied a bit with the terrain, but stayed between 194 and 199, with a brief touch at 201 for just a minute. Nice and cool inside and happy with what I see. Still have not replaced the radiator fan but still have the replacement one in the box...