A/C & defrost

**slade8200** · 01-29-2007, 01:37 PM

Does anyone know if the A/C comes on automatically when you have the knob on the "defrost" or "lower & defrost" selections?? Or does the A/C come on only when you select the A/C??

**bigrondownhiller** · 01-29-2007, 01:56 PM

it should come on whenever you put on the defrost, so it can take the humidity out of the air.

**Zapper2003** · 01-29-2007, 03:41 PM

Yeah, whenever you turn on the defrost or defrost and floor the A/C compressor will kick on and off. That function is built in by most car makers if not all anymore. I think it had more to do with keeping the compressor free'd up during winter to prevent it from seizing than anything. However, if during the summer you dont want the AC blowing in your face, you CAN run the defrost on the windshield or the windshield/floor and have cold air.

**Steve Crowe** · 01-29-2007, 08:47 PM

Actually the compressor will only kick in if the outside temperature is above some number, usually 40* F on most GM cars.

It does serve a double duty, most important is removing humidity from the air. Ever have your windows fog up after a sudden cool rain on a warm day? That's when blowing dry air on the window is important. You're also correct in that it tends to exercise the cooling system on occasion during the off-cooling but that turned out to be a side benefit, wasn't the real reason for the design.