The air temperature controls are divided into six areas
• | HVAC Control Components |
• | Heating and A/C Operation |
• | Steering Wheel Controls |
HVAC CONTROL COMPONENTS
HVAC Control Module
The HVAC control module is a device that interfaces between the operator and
the HVAC system to maintain air temperature and distribution settings. The control
module sends switch input data to the instrument panel module (IPM), and receives
display data from the IPM through signal and clock circuits. The ignition 3
voltage circuit provides a device on signal. The control module does not retain
any HVAC DTCs or settings.
Instrument Panel Integration Module
A function of the IPM operation is to process HVAC system inputs and outputs.
Also, the IPM acts as the HVAC control module's Class 2 interface. The
battery positive voltage circuit provides power that the IPM uses for keep alive
memory (KAM). If the battery positive voltage circuit loses power, then all HVAC
DTCs and settings will be erased from KAM. The ignition 3 voltage circuit
provides a device on signal. The IPM supports the following features:
Feature
| Availability
|
Afterblow
| Optional
|
Purge
| Yes
|
Personalization
| Optional
|
Actuator Calibration
| Yes
|
Air Temperature Actuator
The flatpack actuator is 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 IPM 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 IPM 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 IPM. Once the position signal and
the commanded value are the same, the IPM changes the control signal to 2.5 volts.
Air Temperature Sensors
The ambient temperature sensor is a 2 wire negative temperature co-efficient
thermistor. The vehicle uses the following air temperature sensors:
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 decreases as the resistance decreases. The sensor operates
within a temperature range between -40° to 150°C (-40°
to 300°F). The sensor signal varies between 0-5 volts. The IPM converts
the signal to a range between 0-255 counts.
The IPM uses the sensor signals to determine the air temperature door position.
Left and right temperature sensor inputs in a dual zone HVAC system are used to adjust
the corresponding left and right air temperature door. In HVAC systems with upper
and lower duct temperature sensors, the IPM uses the following sensor inputs with
the indicated mode positions:
Temperature Sensor
| Mode Position
|
Upper
| Panel, Bi-level
|
Lower
| Defrost, Floor, Defog
|
If the IPM detects a malfunctioning sensor, then the control module software
will use a defaulted air temperature value. The default action ensures that
the HVAC system can adjust the inside air temperature near the desired temperature
until the condition is corrected.
The scan tool has the ability to update the displayed ambient air temperature.
The ambient air temperature value is displayed or updated under the following
conditions:
Conditions
| Display
|
• | Engine coolant temperature is less than 10°C (18°F) above the
ambient air temperature reading. |
• | Engine has not been started in 2 hours |
| Displays real-time temperature
|
Engine coolant temperature is more than 10°C (18°F) above the sensor reading.
| Displays last stored temperature
|
• | Vehicle speed is more than 16 km/h (10 mph) |
• | 300 second time delay to allow ambient air to cool the sensor.
Time delay begins when vehicle speed reaches or maintains at least 16 km/h
(10 mph). |
| Temperature is updated every second
|
Sunload Sensor
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 conductance increases.
The sensor signal decreases as the conductance increases. The sensor operates
within an intensity range between completely dark and bright. The sensor signal
varies between 0-5 volts. The IPM converts the signal to a range between
0-255 counts.
The sunload sensor provides the IPM 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 IPM detects a malfunctioning sensor, then the IPM software will use
a defaulted sunload value. The default action ensures that the HVAC system can
adjust the inside air temperature near the desired temperature until the condition
is fixed.
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 A/C refrigerant pressure sensor protects the A/C system from operating
when an excessively high or low pressure condition exists. The PCM disables
the compressor clutch under the following conditions:
• | A/C pressure is more than 2968 kPa (430 psi). |
• | A/C pressure is less than 255 kPa (35 psi). |
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.
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 IPM makes the following actions when an air temperature setting is selected:
• | When the air temperature switch is placed in the warmest position,
the IPM 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 IPM commands the air temperature door to direct air to bypass the heater
core. The blower motor, recirculation actuator, and mode actuator are set to the
following positions: |
- | Blower Motor -- High speed |
- | Recirculation actuator -- Recirculation |
• | When the air temperature switch is placed between the warmest and coldest
positions, the IPM 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 IPM receives an input from the HVAC control module.
In order for the powertrain control module (PCM) to internally ground the A/C
clutch relay control circuit, the body control module (BCM) and the PCM must
communicate to each other over the Class 2 serial data circuits.
- | A/C refrigerant line pressure |
- | Engine coolant temperature |
- | A/C refrigerant line pressure |
- | Engine coolant temperature |
- | A/C request from the IPM |
The BCM will request A/C operation from the PCM if these parameters are within
normal operating limits and the ambient temperature is more than 3°C (38°F).
Once engaged, the compressor clutch will be disengaged for the following conditions:
• | Throttle position is 100% |
• | A/C Pressure is more than 2968 kPa (430 psi) |
• | A/C Pressure is less than 255 kPa (35 psi) |
• | Engine coolant temperature (ECT) is more than 121°C (250°F) |
• | Engine speed is more than 5000 RPM |
• | PCM detects excessive torque load |
• | PCM detects insufficient idle quality |
• | PCM detects a hard launch condition |
When the compressor clutch disengages, the compressor clutch diode protects
the electrical system from a voltage spike.
Dual Zone Operation
The right air temperature switch allows the passenger to offset air discharge
temperatures on the right side of the vehicle by 3°C (5°F). To activate
the dual zone, the passenger slides the switch to the desired offset. The switch
assembly has LED's that will illuminate in order to inform the passenger of
the temperature offset. The temperature offset is allowed as long as the driver's
set temperature is not in the maximum hot or cold settings. The display will
not show the passenger temperature selection.
The IPM will position the right air temperature actuator, located on the right
side of the HVAC module to a position to divert sufficient air past the heater
core to achieve the desired passenger temperature.
Automatic Operation
In automatic operation, the IPM will maintain the comfort level inside of
the vehicle by controlling the A/C compressor clutch, the blower, the air temperature
actuators, mode actuator and recirculation.
To place the HVAC system in Automatic mode, the following is required:
• | The blower switch must be in the AUTO position. |
• | The air temperature switch must be in any other position other than
60 or 90 degrees. |
• | The mode switch must be in the Auto position. |
Once the desired temperature is reached, the blower motor, recirculation and
temperature actuators will automatically be adjusted to maintain the temperature
selected. The IPM performs the following functions to maintain the desired air
temperature:
• | Monitor the following sensors: |
- | Inside air temperature sensor |
- | Ambient air temperature sensor |
- | Upper air temperature sensor if cool air is required |
- | Lower air temperature sensor if warm air is required |
• | Regulate blower motor speed |
• | Position the air temperature actuator |
• | Position the mode actuator |
• | Position the recirculation actuator |
Steering Wheel Controls
An additional temperature switch is mounted on the steering wheel in order
to allow the driver to adjust the HVAC temperature. If the driver adjusts the
temperature using the steering wheel temperature control switch, then voltage
is sent through a series of resistors. That varied voltage is sent back through
the inflatable restraint steering wheel module coil to the radio through the
remote radio control signal circuit. Once the radio receives the varied voltage
signal, the information is sent out over the entertainment and comfort serial
data circuit, to one of the IP fuse block connectors and on to the IPM where
the air temperature actuator is adjusted.
Engine Coolant
Engine coolant is the key element of the heating system. The thermostat controls
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 pressurized state.
The heater core is located inside the HVAC module. The heat of the
coolant flowing through the heater core is absorbed by the ambient air drawn
through the HVAC module. Heated air is distributed to the passenger compartment,
through the HVAC module, for passenger comfort.
The amount of heat delivered to the passenger compartment is controlled
by opening or closing the HVAC module air temperature door. The coolant exits
the heater core through the return heater hose and recirculated back through
the engine cooling system.
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 a very low temperature
gas that can transfer the undesirable heat and moisture from the passenger compartment
to the outside air.
The A/C system used on this vehicle is a non-cycling system. Non-cycling A/C
systems use a high pressure switch to protect the A/C system from excessive pressure.
The high pressure switch will OPEN the electrical signal to the compressor clutch,
if the refrigerant pressure becomes excessive. After the high and the low sides
of the A/C system pressure equalize, the high pressure switch will CLOSE. This completes
the electrical circuit to the compressor clutch. The A/C system is also mechanically
protected with the use of a high pressure relief valve. If the high pressure switch
were to fail or if the refrigerant system becomes restricted and refrigerant pressure
continues to rise, the high pressure relief will pop open and release refrigerant
from the system.
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. The refrigerant is discharged from the compressor through
the discharge hose, and forced through the condenser and then through the balance
of the A/C system.
Compressed refrigerant enters the condenser at a high-temperature, high-pressure
vapor state. As the refrigerant flows through the condenser, the heat is transferred
to the ambient air passing through the condenser. Cooling 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, causing the refrigerant to vaporize at the
orifice tube. The orifice tube also measures 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 to boil inside
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 flows back to the compressor in a vapor state, 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 condenses, and discharges
from the HVAC module as water.