Manual HVAC Description and Operation
The air temperature and the air delivery description and
operation are divided into seven 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
|
Afterblow
|
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 Actuator
The air temperature 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 mixed air flap into the
calculated position, in order to reach the selected temperature.
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.
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.
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.
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 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 part of 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 recirculation actuator. The recirculation switch closes
the recirculation flap in order to circulate the air within the
vehicle. Through renewed selection of the recirculation switch, the
recirculation flap is opened 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 outside air is circulated to the
windscreen to reduce fogging.
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.
|