Automatic HVAC Description and Operation
The air temperature and the air delivery description and
operation are divided into eight areas:
• |
HVAC Control Components |
• |
Heating and A/C Operation |
• |
Recirculation Operation |
HVAC Control Components
HVAC Controls
The HVAC controls contain all switches that are required to
control the functions of HVAC 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.
The HVAC control module supports the following features:
Feature
|
Availability
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Afterblow
|
Yes
|
Purge
|
Yes
|
Personalisation
|
Yes
|
Actuator Calibration
|
Yes
|
Mode Actuator
The mode actuator is a 5-wire stepper motor. The HVAC control
module supplies a 12 V reference voltage to the stepper motor
and energises the 4 stepper motor coils with a pulsed ground
signal. The stepper motor puts the mode flap into the calculated
position in order to reach the selected position. The null point of
the stepper motor will be calibrated, if the stepper motor is new.
When the stepper motor is calibrated, the HVAC control module can
drive the applicable coil to reach exactly the desired position of
the flap.
Air Temperature Actuators
The interior temperature can be selected separately for the
driver and passenger. For this purpose, two stepper motors are used
that regulates two mixed air flaps independently of each other. The
air temperature actuators are 5-wire stepper motors. The HVAC
control module supplies a 12 V reference voltage to each
stepper motor and energises the 4 stepper motor coils with a
pulsed ground signal. The left air temperature actuator moves the
left mixed air flap into the calculated position, in order to reach
the selected temperature of the left temperature switch. The right
air temperature actuator puts the right mixed air flap into the
calculated position, in order to reach the selected temperature of
the right temperature switch. The null point of the stepper motor
will be calibrated, if the stepper motor is new. When the stepper
motor is calibrated, the HVAC control module can drive the
applicable coil to reach exactly the desired position of the
flap.
Recirculation Actuator
The recirculation actuator is a 5-wire stepper motor. The HVAC
control module supplies a 12 V reference voltage to the
stepper motor and energises the 4 stepper motor coils with a pulsed
ground signal. The stepper motor puts the recirculation flap into
the calculated position in order to reach the desired position. The
null point of the stepper motor will be calibrated, if the stepper
motor is new. When the stepper motor is calibrated, the HVAC
control module can drive the applicable coil to reach exactly the
desired position of the flap.
Air Inlet Actuator
The air inlet actuator is a 5-wire stepper motor. The HVAC
control module supplies a 12 V reference voltage to the
stepper motor and energises the 4 stepper motor coils with a pulsed
ground signal. The stepper motor moves the air inlet flap into the
calculated position, in order to reach the selected position. The
null point of the stepper motor will be calibrated, if the stepper
motor is new. When the stepper motor is calibrated, the HVAC
control module can drive the applicable coil to reach exactly the
desired position of the flap.
Blower Motor Control Processor
The blower motor control processor controls the speed of the
blower motor by increasing or decreasing the voltage drop on the
ground side of the blower motor. The HVAC control module provides a
low side pulse width modulation (PWM) signal to the blower motor
control processor via the blower motor speed control circuit. As
the requested blower speed increases, the HVAC control module
increases the amount of time that the speed signal is modulated to
ground. As the requested blower speed decreases, the HVAC control
module decreases the amount of time that the signal is modulated to
ground.
Air Quality Sensor
The HVAC control module detects exhaust gas by an air quality
sensor. The air quality sensor is a 3-wire sensor with an ignition
voltage circuit, a ground circuit and a signal circuit. The HVAC
control module evaluates the information of the air quality sensor
and closes the recirculation flap, while in automatic mode, as soon
as the concentration of pollutants exceeds a predefined value.
Duct Temperature Sensors
The air temperature sensors are 2-wire negative temperature
co-efficient thermistors. The sensors operate within a temperature
range of -40 to +85°C (-40 to +185°F). The sensors are
installed in the air distribution ducts and measure the temperature
of the air that streams from the ducts. The HVAC control module
uses these values to calculate the mixed air flap position.
Evaporator Temperature Sensor
The evaporator temperature sensor is a 2-wire negative
temperature co-efficient thermistor. The sensor operates within a
temperature range of -40 to +85°C (-40 to +185°F). The
sensor is installed at the evaporator and measures its temperature.
If the temperature drops under 3°C (38°F), the compressor
will be switched off in order to prevent a frozen evaporator.
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 is constantly belt driven. The performance of
the A/C compressor is regulated per a lifting magnet in the A/C
compressor. The HVAC control module supplies battery voltage to the
A/C compressor. When the A/C switch is pressed, the HVAC control
module provides a pulse width modulation (PWM) signal to the
A/C compressor in order to command the performance of the A/C
compressor. The performance of the A/C compressor is regulated
according to adjusted interior temperature on the basis of
characteristic lines. Therefore the HVAC control module grounds the
A/C compressor with the PWM signal.
Windscreen Temperature and Inside Moisture Sensor
The windscreen temperature and inside moisture sensor includes
the relative humidity sensor, windscreen temperature sensor and
humidity sensing element temperature sensor.
This sensor assembly provides information about:
• |
Relative humidity level at windscreen (compartment
side) |
• |
Temperature of the windscreen inside (compartment
side) |
• |
Temperature of the humidity sensor element |
The relative humidity sensor measures the relative humidity of
the compartment side of the windscreen. It also detects the
temperature of the windscreen surface on the passenger compartment
side. Both values are used as control inputs for the HVAC control
module application to calculate the fog risk on windscreen
compartment side and ability to reduce fuel consumption by
decreasing A/C compressor power to a minimum without causing any
fog. The sensor will also enable partial recirculation mode in
order to improve heat-up performance of the passenger compartment
under cold ambient temperature conditions without the risk of mist
build-up on the windscreen. The humidity sensor element temperature
sensor supplies the temperature of the humidity sensor element. It
is only needed if the thermal contact between the humidity sensing
element and the inside windscreen surface is not sufficient.
Ambient Light/Sunload Sensor
The ambient light/sunload sensor includes the sunload sensor and
passenger compartment temperature sensor.
This sensor assembly provides information about:
• |
Passenger compartment temperature |
The sunload sensor is connected to ground and to a 12 V
clocked power supply through the HVAC control module. This clocked
power supply is to power the sensor electronics and to work as a
clock generator to the sunload sensor micro controller. The sensor
uses a pulse signal for data identification and transferring the
sun intensity measurement. At each positive transition from the
clocked supply input, the sunload sensor micro controller will
shift channels enabling new intensity measurement on the signal
output to the HVAC control module. The signal voltage varies
between 0-4 V.
The passenger compartment temperature sensor is a negative
temperature co-efficient thermistor. A signal and low reference
circuit enables the sensor to operate. As the air temperature
increases, the sensor resistance decreases. The sensor signal
varies between 0-5 V.
Bright or high intensity light causes the vehicles interior
temperature to increase. The HVAC system compensates for the
increased temperature by diverting additional cool air into the
vehicle.
Air Speed
The fan control switch is part of the HVAC controls. The
selected value of the blower switch position is sent to the HVAC
control module via LIN-Bus.
The blower motor control module is an interface between HVAC
control module and blower motor. The blower motor control module
regulates supply voltage and ground circuits to blower motor. The
HVAC control module provides a PWM signal to the blower motor
control module in order to command the desired blower motor speed.
The blower motor control module supplies battery voltage to the
blower motor and uses the blower motor ground as a low side control
to adjust the blower motor speed. The voltage amounts between
2-13 V and changes linear to the height of the PWM signal.
Air Delivery
The HVAC control module controls the distribution of air by the
use of recirculation and mode actuator. The modes that may be
selected are:
The desired air distribution mode can be selected with the air
distribution switches at the HVAC controls. The HVAC controls
deliver the values to the HVAC control module via LIN-Bus. The HVAC
control module controls the air distribution actuator so that it
drives the flap to the calculated position. Depending on the
position of the flap, air is distributed through various ducts
leading to the outlets in the dash. Turning the mode flap to the
defrost position, the HVAC control module will move the
recirculation actuator to outside air, reducing window fogging.
When defrost is selected, the blower motor will be activated,
regardless of the coolant temperature. The HVAC control module
enables a high volume of air delivered to the front defrost vents.
A/C is available in all modes.
The rear window demister does not affect the HVAC system.
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 windscreen
fogging. Regardless of the temperature setting, the following can
affect the rate that the HVAC system can achieve the desired
temperature:
• |
Recirculation actuator setting |
• |
Difference between inside and desired
temperature |
• |
Blower motor speed setting |
When the A/C switch or the AUTO switch is pressed, the HVAC
controls send a signal to the HVAC control module via LIN-Bus. The
HVAC control module evaluates this signal and sends an A/C request
signal to the ECM via CAN-Bus. The ECM checks all preconditions
before releasing and if all conditions are met sends a release
signal back to the HVAC control module. The A/C compressor is
activated by the HVAC control module. The HVAC control module
supplies battery voltage to the A/C compressor. When the A/C switch
is pressed, the HVAC control module provides a pulse width
modulation (PWM) signal to the A/C compressor in order to
command the performance of the A/C compressor. The performance of
the A/C compressor is regulated according to adjusted interior
temperature on the basis of characteristic lines. Therefore the
HVAC control module grounds the A/C compressor with the PWM
signal.
The following conditions must be met in order to activate the
A/C compressor:
• |
Battery voltage is between 9-18 V |
• |
Engine coolant temperature is less than 124°C
(255°F) |
• |
Engine speed is greater than 600 RPM |
• |
Engine speed is less than 5 500 RPM |
• |
A/C high side pressure is between
269-2 929 kPa (39-425 PSI) |
• |
Throttle position is less than 100% |
• |
Evaporator temperature is greater than 3°C
(38°F) |
• |
ECM does not detect immoderate torque load |
• |
ECM does not detect insufficient idle quality |
• |
The ambient temperature is above 1°C
(34°F) |
The sensor information is used by the ECM to determine the
following:
• |
The A/C high side pressure |
• |
An A/C system load on the engine |
• |
An immoderate A/C high side pressure |
• |
The heat load at the A/C condenser |
The air streams into the passenger compartment through the
heater core and the evaporator core. The air temperature actuator
drives the mixed air flap to induce the airflow. If the interior
temperature should be increased, the mixed air flap is put into the
position in which more air streams through the heater core. If the
interior temperature should be decreased, the mixed air flap is put
into the position in which more air streams through the evaporator
core.
Recirculation Operation
The recirculation switch is integrated into the HVAC controls.
The selected recirculation switch position is sent to the HVAC
control module via LIN-Bus. The HVAC control module controls the
air intake through the air inlet actuator and recirculation
actuator. In the recirculation mode the air inlet flap closes and
the recirculation flap opens in order to circulate the air within
the vehicle. In fresh air mode the air inlet flap opens and the
recirculation flap is closed again in order to route outside air
into the vehicle. Recirculation is only available if the defrost
mode is not active. When the defrost mode is active, the
recirculation actuator opens the recirculation flap and the air
inlet actuator opens the air inlet flap and outside air is
circulated to the windscreen to reduce fogging.
In automatic mode the values of the windshield temperature and
inside moisture sensor are used as control inputs for the HVAC
control module application to calculate the fog risk on passenger
compartment side of the windshield compartment side. The A/C
compressor and the defrost mode are activated to prevent or remove
fog on the passenger compartment side of the windscreen.
The HVAC control module evaluates the information of the air
quality sensor and closes the recirculation flap while in the
automatic mode as soon as the concentration of pollutants exceeds a
predefined value.
Automatic Operation
In automatic operation, the HVAC control module maintains the
comfort level inside of the vehicle by controlling the A/C
compressor clutch, the blower motor, the air temperature actuators,
mode actuator and recirculation actuator.
To put the HVAC system in automatic mode, the following is
required:
- The auto switch must be activated.
- The air temperature switch must be in any other
position than full hot or full cold position.
Once the desired temperature is reached, the blower motor, mode,
recirculation and temperature actuators automatically adjust to
maintain the temperature selected. The HVAC control module performs
the following functions to maintain the desired air
temperature:
• |
Monitors the following sensors: |
- |
Ambient air temperature sensor |
- |
Lower left air temperature sensor |
- |
Lower right air temperature sensor |
- |
Upper left air temperature sensor |
- |
Upper right air temperature sensor |
- |
Windscreen temperature and inside moisture
sensor |
- |
Ambient light/sunload sensor |
• |
Regulate blower motor speed |
• |
Position the air temperature actuators |
• |
Position the mode actuator |
• |
Position the recirculation actuator |
• |
Position the air inlet actuator |
• |
Control of the A/C compressor |
When the warmest position is selected in automatic operation the
blower speed will increase gradually until the vehicle reaches
normal operating temperature. When normal operating temperature is
reached the blower stays on high speed and the air temperature
actuators stays in the full heat position.
When the coldest position is selected in automatic operation the
blower stays on high and the air temperature actuators stay in full
cold position. The mode actuator remains in the panel position and
the recirculation actuator will remain in the recirculation
position.
Under cold ambient temperatures, the automatic HVAC system
provides heat in the most efficient manner. The operator can select
an extreme temperature setting but the system will not warm the
vehicle any faster. Under warm ambient temperatures, the automatic
HVAC system also provides air conditioning in the most efficient
manner. Selecting an extreme cool temperature will not cool the
vehicle any faster.
In automatic mode the values of the windscreen temperature and
inside moisture sensor are used as control inputs for the HVAC
control module application to calculate the fog risk on the
passenger compartment side of the windscreen and ability to reduce
fuel consumption by decreasing A/C compressor power to a minimum
without causing any fog. The A/C compressor and the defrost mode
are activated to prevent or remove fog on the passenger compartment
side of the windscreen. The sensor will also enable partial
recirculation mode in order to improve heat-up performance of the
passenger compartment under cold ambient temperature conditions
without the risk of mist build-up on the windscreen.
The HVAC control module evaluates the information of the air
quality sensor and closes the recirculation flap while in the
automatic mode, as soon as the concentration of pollutants exceeds
a predefined value.
Engine Coolant
Engine coolant is the essential element of the heating system.
The thermostat controls the normal engine operating coolant
temperature. The thermostat also creates a restriction for the
cooling system that promotes a positive coolant flow and helps
prevent cavitation.
Coolant enters the heater core through the inlet heater hose, in
a pressurised state. The heater core is located inside the HVAC
module. The ambient air drawn through the HVAC module absorbs the
heat of the coolant flowing through the heater core. Heated air is
distributed to the passenger compartment, through the HVAC module,
for passenger comfort. Opening or closing the air temperature flap
controls the amount of heat delivered to the passenger compartment.
The coolant exits the heater core through the return heater hose
and recirculates back to the engine cooling system.
A/C Cycle
Refrigerant is the key element in an air conditioning system.
R-134a is presently the only Environmental Protection Agency
approved refrigerant for automotive use. R-134a is a very low
temperature gas that can transfer the undesirable heat and moisture
from the passenger compartment to the outside air.
The compressor builds pressure on the vapour refrigerant.
Compressing the refrigerant also adds heat to the refrigerant. The
refrigerant is discharged from the compressor, through the
discharge hose, and forced to flow to the condenser and then
through the balance of the A/C system. The A/C system is
mechanically protected with the use of a high pressure relief
valve. If the A/C refrigerant pressure sensor fails or if the
refrigerant system becomes restricted and refrigerant pressure
continued to rise, the high pressure relief will pop open and
release refrigerant from the system.
Compressed refrigerant enters the condenser in a high
temperature, high pressure vapour state. As the refrigerant flows
through the condenser, the heat of the refrigerant is transferred
to the ambient air passing through the condenser. Cooling the
refrigerant causes the refrigerant to condense and change from a
vapour to a liquid state.
The condenser is located in front of the radiator for maximum
heat transfer. The condenser is made of aluminium tubing and
aluminium cooling fins, which allows rapid heat transfer for the
refrigerant. The semi-cooled liquid refrigerant exits the condenser
and flows to the Receiver/Dehydrator(R/D).
The R/D contains desiccant that absorbs moisture that may be in
the refrigerant system. The R/D also acts as a storage vessel to
ensure that a steady flow of liquid reaches the thermal expansion
valve. The refrigerant exits the R/D and flows through the liquid
line to the thermal expansion valve.
The thermal expansion valve is located at the front of dash and
attaches to the evaporator inlet and outlet pipes. The thermal
expansion valve is the dividing point for the high and the low
pressure sides of the A/C system. As the refrigerant passes through
the thermal expansion valve, the pressure on the refrigerant is
lowered. The thermal expansion valve also meters the amount of
liquid refrigerant that can flow into the evaporator.
Refrigerant exiting the thermal expansion valve flows into the
evaporator core in a low pressure, liquid state. Ambient air is
drawn through the HVAC module and passes through the evaporator
core. Warm and moist air will cause the liquid refrigerant boil
inside of the evaporator core. The boiling refrigerant absorbs heat
from the ambient air and draws moisture onto the evaporator. The
refrigerant exits the evaporator through the suction line and back
to the A/C compressor, in a vapour state, and completing the A/C
cycle of heat removal. At the A/C compressor, the refrigerant is
compressed again and the cycle of heat removal is repeated.
The conditioned air is distributed through the HVAC module for
passenger comfort. The heat and moisture removed from the passenger
compartment will also change form, or condense, and is discharged
from the HVAC module as water.
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