Chevrolet Silverado / GMC Sierra. Manual

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

AIR TEMPERATURE DESCRIPTION AND OPERATION

The air temperature controls are divided into four areas:

HVAC Control Components

Heating and A/C Operation

Engine Coolant

A/C Cycle

HVAC Control Components

HVAC Control Module

The HVAC control module is a class 2 device that interfaces between the operator and the
HVAC system to maintain air temperature and distribution settings. The battery positive voltage
circuit provides power that the control module uses for keep alive memory (KAM). If the battery
positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The
body control module (BCM), which is the vehicle mode master, provides a device on signal. The
control module supports the following features:

Air Temperature Actuator

The air temperature actuators are a 5-wire bi-directional electric motor that incorporates a
feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position
signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt
signal to command the actuator movement. When the actuator is at rest, the control circuit value
is 2.5 volts. A 0 or 5 volt control signal commands the actuator movement in opposite directions.
When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position
signal between 0-5 volts.

The HVAC control module uses a range of 0-255 counts to index the actuator position. The door
position signal voltage is converted to a 0-255 count range. When the module sets a commanded,
or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the
direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft

Feature

Availability

Afterblow

Purge

Personalization

Actuator Calibration

Yes

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

rotates the changing position signal is sent to the module. Once the position signal and the
commanded value are the same, the module changes the control signal to 2.5 volts.

A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt
reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal
can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0
volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts. The PCM
converts the voltage signal to a pressure value.

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively
high pressure condition exists. The PCM disables the compressor clutch if the A/C pressure is
more than 2957 kPa (429 psi). The clutch will be enabled after the pressure decreases to less than
1578 kPa (229 psi).

A/C Low Pressure Switch

The A/C low pressure switch protects the A/C system from a low pressure condition that could
damage the A/C compressor or cause evaporator icing. The HVAC control module applies 5 volts
to the A/C low pressure switch signal circuit. The switch will open when the A/C low side
pressure reaches 138-172 kPa (20-25 psi). This prevents the A/C compressor from operating. The
switch will then close when A/C low pressure side reaches 275-317 kPa (40-46 psi). This enables
the A/C compressor to turn back ON.

Heating and A/C Operation

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of
the vehicle. The A/C system will also remove humidity from the interior and reduce windshield
fogging. Regardless of the temperature setting, the following can effect the rate that the HVAC
system can achieve the desired temperature:

Recirculation actuator setting

Difference between inside and desired temperature

Difference between ambient and desired temperature

Blower motor speed setting

Mode setting

The manual HVAC system is a dual temperature zone system. There are 2 separate air
temperature levers. Moving the air temperature levers to the upward position diverts most of the
airflow through the heater core, which increases the outlet air temperature. Moving the air

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

temperature levers to the most downward position diverts most of the airflow around the heater
core, which decreases the outlet air temperature. The air temperature offset can be as much as
16.7°C (30°F).

Pressing the A/C button enables the HVAC control module to request A/C compressor
engagement and turn on the A/C button LED. The HVAC control module sends a class 2 message
to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C
compressor relay enabling it to close its internal contacts to send battery voltage to the A/C
compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from
the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the
compressor is disengaged. Defrost and Defog mode selections will request A/C operation but not
turn on the A/C LED.

The following conditions must be met in order for the A/C compressor clutch to turn on:

The ambient air temperature is above 4°C (40°F).

The A/C low pressure switch signal circuit is grounded.

The A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi).

The PCM receives an A/C request from the HVAC control module.

The engine coolant temperature (ECT) is less than 121°C (250°F).

The engine RPM is more than 550 RPM.

The throttle position is less than 100%.

The HVAC control module monitors the A/C low pressure switch signal circuit. If the voltage
signal on this circuit has no voltage drop the module will interpret this condition as a low
pressure, disabling the A/C request. The A/C low pressure switch will open its internal contacts at
151 kPa (22 psi). Then close the contacts at 275 kPa (40 psi) to resume A/C operation. This
switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has
a low refrigerant level.

The PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this
circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the
pressure inside the line increases, so does the voltage signal. If the pressure is above 2957 kPa
(429 psi), the A/C compressor output is disabled. When the pressure lowers to 1578 kPa (229
psi), the PCM enables the compressor to operate.

The sensor information is used by the PCM to determine the following:

The A/C high side pressure

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

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Chevrolet Silverado / GMC Sierra. Manual - part 2357