Chevrolet 1979 Light Duty Truck Owner's Manual page 58

The Maximum A/C position, with Temperature lever in full
COLD, is used when maximum cooling is required under
conditions of high temperature and humidity.
•
Norm A/C -Outside air is passed through the system and
discharged
recommended for most air conditioning situations because of
reduced blower noise and reduction of cigarette smoke within the
vehicle. Fan speed and tempering may be varied as required.
•
Bi-Level A/C -Outside air is delivered from the heater lower
outlet and upper outlets to provide comfort and keep the
windshield and side glass clear under low fogging conditions.
•
Economy Ranges -These positions, VENT AND HEATER
are recommended for greater economy in vehicle operation. The
A/C compressor will not operate in either of these positions.
•
Vent -Outside air is passed through the system and discharged
from the upper outlets. This position is provided for cool to
moderate weather when refrigeration is not required. Fan speed
and Temperature can be adjusted as required.
•
Heater -Outside air is delivered through the heater outlet and
with some air through the defroster outlets. Temperature may be
adjusted as required. This position is recommended for most
winter driving.
•
Def -Outside air is delivered through the defroster outlets from
the heater, and some air through the floor outlets. Temperature
and blower speeds may be adjusted as required. This position is
recommended for conditions of severe fogging and icing only.
NOTICE: Operate in "Heater" position for 30 seconds before switching
to "DEF". This will remove humid air from the system and minimize rapid
fogging of the glass which can occur if humid air is blown onto a cool
windshield.
ROOF-MOUNTED AIR CONDITIONI NG
The Roof-Mounted air conditioning system, available on Suburban
models, is used with the Four Season Air Conditioning System.
through the
2-28
upper outlets. This position is