1997 GMC Safari Owner's Manual - Page 152

If your vehicle has rear air conditioning, setting it on LO may enhance the front A/C performance by allowing trapped refrigerant in the rear lines to circulate. The rear air conditioning system will only send cooled air if the front system is on. It can still be used to circulate air, even if the front system is off. When the air conditioning, DEFROST or BLEND is on, you may notice a slight increase or decrease in engine speed, due to compressor operation. Thisis normal because the system is designed to cycle the compressor on and off to keep the desired temperature. Defrosting Use DEFROST to remove fog or icefrom the windshield quickly in extremely cold conditions. The temperature knob should be in the red area and the fan control toward high. The air conditioning compressor may operate in this setting to dehumidify the air. The BLEND setting is useful for cold weather with a large number of passengers or very humid conditions to help keep the windshield clear. Ventilation System For mild outside temperatures when little heating or cooling is needed, use VENT to direct outside air through your vehicle. Air will flow through the instrument panel vents. Your vehicle's flow-through ventilation system supplies outside air tothe inside of your vehicle when it is moving. With the side windows closed, air will flow into the front air inlet grilles at the base of the windshield, through the vehicle and out the rear air exhaust valves. Outside air will also enter thevehicle when the heater or the air conditioning fan is running. Heating On cold days, use HEATER with the temperature knob in the red area. Outside air will be brought in through the floor outlets. The heater works best if you keep your windows closed while using it. If you use the optional engine coolant heater before starting your engine, your heating system will produce warmer air faster to heat the passenger compartment in cold weather. See "Engine Coolant Heater" in the Index.