Re: AC question
Interesting...looking at the diagrams for the ac. Each side (driver/pass) reads off the ambient light sensors to factor in sunload in the climate control.
I gotta ask why did you replace so many components? If the shop said all that was needed, I'd go someplace else and get a second opinion on your ACs current operation.
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AIR TEMPERATURE DESCRIPTION AND OPERATION
The air temperature controls are divided into 5 areas:
- HVAC Control Components
- Heating and A/C Operation
- Automatic Operation
- Engine Coolant
- A/C Cycle
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 shown features:
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
124 kPa (18 psi). This prevents the A/C compressor from operating. The switch will then close when A/C low pressure side reaches 275 kPa (40
psi). This enables the A/C compressor to turn back ON.
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
- Mode 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:
- Sunload
- Duct temperatures
- Ambient temperature
- Inside temperature
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