1999 GMC Sonoma Owner's Manual - Page 128

yellowblue HEATER: This setting directs most of the warmed air through the heater floor outlets and some air through the windshield defroster outlets. BLEND: Airflow is divided equally between the heater floor outlet and the windshield defroster outlets. DEFROST: This setting directs most air through the windshield defroster outlets and some through the heater outlets. On cool but sunny days, use BI-LEVEL A/C to deliver warm air to the floor and cooler air to the instrument panel outlets. If you have the VORTEC 4300 V6 engine, when the air conditioner is on, you may sometimes notice slight changes in your vehicle's engine speed and power. This is normal because the system is designed to cycle the compressor on and off to keep the desired temperature. Heating The heater works best if you keep your windows closed while using it. On cold days, use HEATER with the temperature knob turned clockwise. BLEND is useful in cool weather when you have fog or ice on the windshield or side windows. 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. Air Conditioning (If Equipped) On hot days, open the windows long enough to let hot inside air escape. This reduces the time it takes for your vehicle to cool down. Then keep your windows closed for the air conditioner to work its best. For quick cool-down on very hot days, use MAX A/C with the temperature knob turned counterclockwise. This setting should be used to keep odors and/or dust from entering the vehicle. For normal cooling on hot days, use A/C with the temperature knob turned counterclockwise. 3-4