and the LED will illuminate until a non-recirculation mode is selected.
Air Delivery
The HVAC control module controls the mode actuator in order to distribute airflow to a desired
outlet. The mode switch provides the vehicle operator with the ability to override the automatic
setting. When the mode door is moved to the defrost position, the A/C compressor clutch engages
and the recirculation actuator will be moved to the outside air position. Regardless of the mode
setting, a small amount of air will be diverted to the defrost ducts to reduce windshield fogging.
Mode Actuator
The mode actuator is an electronic stepper motor with feedback potentiometers. The HVAC
control module sends different signals to the mode door actuator through the mode door control
circuit. Zero volts drives the actuator in one direction while 5 volts moves the actuator in the
opposite direction. When the actuator receives 2.5 volts, the actuator rotation stops. A 5-volt
reference signal is sent out over the 5-volt reference circuit to the mode actuator. When you select
a desired mode setting, logic determines the value of the mode actuator signals. The HVAC
control module's software uses this reference voltage in order to determine the position of the
mode actuator through the mode door position signal circuit. The motor moves the mode door to
the desired position.
Front Defrost
When you select defrost, the A/C compressor is activated. The A/C compressor clutch will engage
when ambient temperatures are above 3°C (38°F). The blower motor will be activated, regardless
of the coolant temperature. The rear window defogger does not affect the HVAC system.
Recirculation Operation
Recirculation Actuator
The HVAC control module controls the air intake through the recirculation actuator.
Recirculation is not available when the mode is in defrost. When the mode is in defog,
Recirculation will only be available for 10 minutes. The operator must activate the blower for
Recirculation operation. The A/C high-pressure recirculation switch can cause the HVAC system
to recirculate air. If the recirculation switch is pressed into the ON position when the mode
switch is in an unavailable mode position, then the recirculation switch LED will flash 3 times.
When the high side pressure reaches 2206-2620 Kpa (320-380 psi), the PCM will place the
HVAC system in recirculation mode. The high side pressure is lowered when the inside air cools
the refrigerant within the A/C evaporator. When the high-side pressure reaches 1447-1861 Kpa
(210-270 psi), the PCM will place the HVAC system out of recirculation mode.
2008 Chevrolet Silverado 1500
2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado