Chevrolet 2008 TrailBlazer Owner's Manual page 157

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A/C (Air Conditioning): Press this button to
manually turn the air conditioning system on or off.
When the system is on, the system will automatically
begin to cool and dehumidify the air inside of your
vehicle. The air conditioning symbol will appear on the
display when the air conditioning is on and will turn
off when the air conditioning is off.
If you turn the air conditioning off while in front defrost
or defog mode, the air conditioning symbol will turn
off, however, the A/C compressor will remain on to help
de-humidify the air inside the vehicle. If one of the
other modes is selected the compressor will then turn
the A/C off until it is selected again or the AUTO button
is pressed.
To avoid fogging the inside glass on rainy and humid
days at a temperature above freezing, press the
air conditioning button to run the A/C compressor.
Also it is best to avoid the use of the recirculation mode
except when maximum air conditioning performance
is needed or for short times to avoid exterior odors.
You may notice a slight change in engine performance
when the air-conditioning compressor shuts off and
turns on again. This is normal.
Defogging and Defrosting
Fog on the inside of the windows is a result of moisture
condensing on the cool window glass. This can be
reduced if the climate control system is used properly.
You can use either defog or front defrost to clear
fog or frost from your windshield.
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(Defog): Use this setting to clear the windows of
fog or moisture. This setting will deliver air to the
floor and windshield outlets.
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(Front Defrost): Press the front defrost button to
clear the windshield and side windows of frost or
fog quickly. The system will automatically control the fan
speed if you select defrost from AUTO mode. If the
outside temperature is 40°F (4°C) or warmer, your air
conditioning compressor will automatically run to help
dehumidify the air and dry the windshield. Do not drive
the vehicle until all the windows are clear.
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