6-8 HVAC - Manual
Description and Operation
Manual HVAC Description and
Operation
The air temperature and the air delivery description and
operation are divided into eight areas:
• HVAC Control Components
• Air Speed
• Air Delivery
• Heating and A/C Operation
• Recirculation Operation
• Engine Coolant and A/C System Refrigerant
HVAC Control Components
HVAC Control
The HVAC control contains all switches, buttons, and
dials which are required to control the functions of the
HVAC system and serve as interface between the
operator and the HVAC control module. The selected
values are passed to the HVAC control module via
LIN-Bus.
HVAC Control Module
The HVAC control module is a GMLAN device that
interfaces between the operator and the HVAC system
to maintain and control desired air temperature and air
distribution settings. The battery positive voltage circuit
provides power that the HVAC control module uses for
keep alive memory. If the battery positive voltage circuit
loses power, all HVAC DTCs and settings will be
erased from keep alive memory. The body control
module (BCM), which is the vehicle mode master,
provides a device ON-Signal. The HVAC control
module provides blower, air delivery mode and air
temperature settings.
Actuators
Doors in the HVAC case assembly are used to control
air flow. The HVAC control module operates the doors
through the use of actuators, with one actuator being
used for each door. The system has the following air
control doors and associated actuators: mode,
temperature, and recirculation.
Each actuator used in the system is a 5-wire
bi-directional electric motor that incorporate a feedback
potentiometer. The five circuits are, low reference, 5 V
reference, actuator position signal, and two control
circuits. The control circuits use either a ground or 12 V
value to coordinate the actuator movement . In order to
move the actuator, the HVAC control module grounds
one of the control circuits while providing the other with
12 V. The HVAC control module reverses the polarity of
the control circuits to move the actuator in the opposite
direction.
When the actuator shaft rotates, the potentiometer’s
sliding contact changes the door position signal
between 0–5 V. The HVAC control module converts the
voltage signal to counts. The total range of the counts
is 0–1024, with an operating range between 20–1000.
The actual operating range of an actuator is determined
during calibration. During calibration, the actuator is
moved though its full range of travel and the module
stores the minimum and maximum value. Based on the
desired system operation, the module sets a
commanded, or targeted, value for the actuators. The
control circuits are operated to move the door to the
required position, and the changing position signal is
sent to the module. Once the actual position signal and
the commanded value are the same, the module
ceases operating the control circuits and the actuator
(and door) remain in the desired position.
Blower Motor
The blower motor speed control signal from the HVAC
Control Module, battery positive and ground circuits
enable the blower motor to operate. The blower motor
control circuitry is integrated within the blower motor
assembly. The HVAC control module provides a low
side pulse width modulation (PWM) signal to the blower
motor to request a specific motor speed. The blower
motor translates the PWM signal and drives the motor
accordingly.
Evaporator Temperature Sensor
The evaporator temperatur e sensor is a 2-wire negative
temperature coefficient thermistor. The sensor operates
within a temperatur e range of −40 to +85°C (−40 to
+185°F). The sensor is installed near the evaporator
core to measure the air temperatur e exiting the core.
Based on vehicle operating conditions and operator
settings, the HVAC software algorithms will determine a
target evaporator air temperature. The operation of the
compressor solenoid will be adjusted as needed to
quickly reach and maintain the targeted temperature.
A/C Refrigerant Pressure Sensor
The A/C refrigerant pressure sensor is a 3-wire
piezoelectric pressure transducer. A 5 V reference
voltage, low reference, and signal circuits enable the
sensor to operate. The A/C pressure signal can be
between 0.2–4.8 V. When the A/C refrigerant pressure
is low, the signal value is near 0 V. When the A/C
refrigerant pressure is high, the signal value is near 5 V.
The engine control module (ECM) converts the voltage
signal to a pressure value. When pressure is too high or
too low, the ECM will not allow the A/C compressor
clutch to engage.
A/C Compressor
The A/C compressor uses a conventional belt driven
magnetic clutch to engage and mechanically turn the
compressor. When the A/C switch is pressed, the
HVAC control module sends an A/C request message
to the ECM via serial data. If specific criteria is met, the
ECM then grounds the A/C compresso r clutch relay
control circuit, which will switch the A/C compressor
clutch relay. With the relay contacts closed, battery
voltage is supplied to the permanently grounded A/C
compressor clutch. The A/C compresso r clutch will then
be activated.
This A/C system utilizes a variable displacement
solenoid valve to alter the amount of displacement
created by the turning of the compressor. The HVAC
control module provides both battery voltage and a
pulse width modulated ground to the variable
displacement solenoid valve. When the A/C switch is
pressed, the HVAC control module grounds the
variable displacement solenoid using a (PWM) signal in
2021 - Silverado 4500HD, 5500HD, and 6500HD Electrical Body Builder Manual
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