The air temperature controls are divided into 5 areas:
• | HVAC Control Components |
• | Heating and A/C Operation |
HVAC Control Components
HVAC Control Module
The HVAC control module is a class 2 device that interfaces between the
vehicle 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:
Feature
| Availability
|
Afterblow
| No
|
Purge
| No
|
Personalization
| Yes
|
Actuator Calibration
| Yes
|
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 potentiometers 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 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.
Air Temperature Sensors
The air temperature sensors are a 2-wire negative temperature co-efficient thermistor.
The vehicle uses the following air temperature sensors:
• | Ambient air temperature sensor |
• | Inside air temperature sensor assembly |
• | Upper left air temperature sensor |
• | Upper right air temperature sensor |
• | Lower left air temperature sensor |
• | Lower right air temperature sensor |
A signal and low reference circuit enables the sensor to operate. As the air
temperature surrounding the sensor increases, the sensor resistance decreases. The
sensor signal voltage decreases as the resistance decreases. The sensor operates within
a temperature range between -40°C (-40°F) to +101°C (+215°F).
The sensor signal varies between 0-5 volts.
The input of the duct air temperature sensors are different from the ambient
and inside sensors. The HVAC control module converts the signal to a range between
0-255 counts. As the air temperature increases the count value will decrease.
If the HVAC control module detects a malfunctioning sensor, then the control
module software will use a defaulted air temperature value. The default value for
the ambient and inside air temperature sensors will be displayed on the scan tool.
The default value for the duct air temperature sensors will not be displayed on the
scan tool. The scan tool parameter for the duct air temperature sensors are the actual
state of the signal circuit. The default action ensures that the HVAC system can adjust
the inside air temperature near the desired temperature until the condition is corrected.
Sunload Sensor Assembly
The sunload sensor is a 2-wire photo diode. The vehicle uses left and right
sunload sensors. The two sensors are integrated into the sunload sensor assembly.
Low reference and signal circuits enable the sensor to operate. As the light shining
upon the sensor gets brighter, the sensor resistance increases. The sensor signal
decreases as the resistance increases. The sensor operates within an intensity range
between completely dark and bright. The sensor signal varies between 0-5 volts.
The BCM converts the signal to a range between 0-255 counts. The BCM
sends a class 2 serial data message to the HVAC control module for automatic
temperature calculations.
The sunload sensor provides the BCM a measurement of the amount of light shining
on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature
to increase. The HVAC system compensates for the increased temperature by diverting
additional cool air into the vehicle.
If the BCM detects a malfunctioning sensor, then the control module software
will use a defaulted sunload value. This value will not be displayed on the scan tool.
The default action ensures that the HVAC system can adjust the inside air temperature
near the desired temperature until the condition is fixed. The scan tool parameter
for the sunload sensor is the actual state of the signal circuit.
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)
as measured at the switch/accumulator. 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) as measured at the switch/accumulator. This enables the A/C
compressor to turn back ON.
The low pressure switch uses refrigerant pressure to open and close a set of
electrical contacts. When A/C request is authorized the switch is closed and shows
normal status. During this state the switch will show 0 volts on the A/C low
pressure sensor signal circuit. When A/C request is denied due to a low pressure condition
the switch will be open. During this state the switch will show 5 volts on
the A/C low pressure sensor signal circuit.
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. The vehicle operator can determine the passenger
compartment temperature by adjusting the air temperature switch. The vehicle passenger
can offset the passenger temperature as much as 16.7°C (30°F). 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 |
The control module makes the following actions when automatic operation is not
selected, and an air temperature setting is selected:
• | When the air temperature switch is placed in the warmest position, the
control module commands the air temperature door to divert maximum air past the heater
core. |
• | When the air temperature switch is placed in the coldest position, the
control module commands the air temperature door to direct air to bypass the heater
core. |
• | When the air temperature switch is placed between the warmest and coldest
positions, the control module monitors the following sensor inputs to determine the
air temperature door position that diverts the appropriate amount of air past the
heater core in order to achieve the desired temperature: |
The A/C System can be engaged by either pressing the A/C switch or during automatic
operation. The HVAC control module sends a class 2 message to the powertrain
control module (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.
The following conditions must be met in order for the A/C compressor clutch
to turn on:
• | The ambient air temperature 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 |
• | An A/C system load on the engine |
• | An excessive A/C high side pressure |
• | The heat load at the A/C condenser |
Once engaged, the compressor clutch will be disengaged for the following conditions:
• | The ambient air temperature is less than 4°C (40°F) |
• | The throttle position (TP) is 100 percent |
• | The A/C low pressure switch is open |
• | The A/C high side pressure is more than 2957 kPa (429 psi) |
• | The A/C low side pressure is less than 151 kPa (22 psi) |
• | The engine coolant temperature (ECT) is more than 121°C (250°F) |
• | The engine speed is more than 5,500 rpm |
• | The PCM detects excessive torque load |
• | The PCM detects insufficient idle quality |
• | The PCM detects a hard launch condition |
Automatic Operation
In automatic operation, the HVAC control module will maintain the comfort level
inside of the vehicle by controlling the A/C compressor clutch, the blower motor,
the air temperature actuators, the mode actuator and the recirculation actuator.
To place 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 other than full
hot or full cold position |
Once the desired temperature is reached, the blower motor, mode, recirculation
and temperature actuators will automatically be adjusted to maintain the temperature
selected. The HVAC control module performs the following functions to maintain the
desired air temperature:
• | Monitors the following sensors: |
- | The inside air temperature sensor |
- | The ambient air temperature sensor |
- | The lower left air temperature sensor |
- | The lower right air temperature sensor |
- | The upper left air temperature sensor |
- | The upper right air temperature sensor |
• | Regulates blower motor speed |
• | Positions the air temperature actuators |
• | Positions the mode actuator |
• | Positions the recirculation actuator |
• | Requests the A/C operation |
A/C Cycle
Refrigerant is the key element in an air conditioning system. R-134a is presently
the only EPA approved refrigerant for automotive use. R-134a is an very low temperature
gas that can transfer the undesirable heat and moisture from the passenger compartment
to the outside air.
The A/C compressor is belt driven and operates when the magnetic clutch is engaged.
The compressor builds pressure on the vapor 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 were to fail
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
vapor 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 vapor to a liquid state.
The condenser is located in front of the radiator for maximum heat transfer.
The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid
heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser
and flows through the liquid line, to the orifice tube.
The orifice tube is located in the liquid line between the condenser and the
evaporator. The orifice tube is the dividing point for the high and the low pressure
sides of the A/C system. As the refrigerant passes through the orifice tube, the pressure
on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant,
the refrigerant will begin to vaporize at the orifice tube. The orifice tube also
meters the amount of liquid refrigerant that can flow into the evaporator.
Refrigerant exiting the orifice tube 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 compressor, in a vapor state, and completing the
A/C cycle of heat removal. At the 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.