HVAC Control Components
HVAC CONTROL MODULEAir Temperature Description And Operation:
The HVAC control module is a class 2 device that interfaces between the 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 features:
AIR TEMPERATURE ACTUATOR
The air temperature actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground and control circuits enable the actuator to operate. The control circuit uses a 0-12 volt linear-ramped signal to command the actuator movement. The 0 and 12 volt control values represent the opposite limits of the actuator range of motion. The values in between 0 and 12 volts correspond to the positions between the limits.
When the HVAC control module sets a commanded, or targeted, value, the control signal sets to a value between 0-12 volts. Actuator shaft rotation is the results of a commanded position input by the vehicle operator. With a new actuator commanded value action, the maintained actuator control value is changed.
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 2979 kPa (432 psi). The clutch will be enabled after the pressure decreases to less than 1510 kPa (219 psi).
^ A/C pressure is less than 186 kPa (27 psi). The clutch will be enabled after the pressure increases to more than 207 kPa (30 psi).
EVAPORATOR TEMPERATURE SENSOR
The evaporator temperature sensor protects the A/C system. The sensor located on the evaporator core provides the HVAC control module with the surface temperature of the evaporator core. If the evaporator temperature sensor reads a temperature of 0°C (32°F), the HVAC control module will turn off the A/C compressor clutch until evaporator temperatures reach 2°C (36°F).
AMBIENT AIR TEMPERATURE SENSOR
The ambient air temperature sensor is a 2 wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:
^ Ambient
^ Inside
A 5 volt reference and signal circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases.
The sensor operates within a temperature range of -40°C (-40°F) to 85°C (185°F). The HVAC control module receives a class 2 message from the Drivers Information Center (DIC). The ambient air temperature sensor is mounted in the forward engine area of the vehicle. In this position, it is exposed to the airflow through the grill before it reaches the radiators. The sensor signal varies between 0 to 5 volts. The HVAC control module converts the signal to a working temperature range.
If the HVAC control module detects a faulty class 2 signal, 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 system is fixed.
SUNLOAD SENSOR
The sunload sensor is a 2 wire photo diode. 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 HVAC control module converts the signal to a range between 0-255 counts.
The sunload sensor provides the HVAC control module 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 HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted sunload valve. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed. A resistance check of the sunload sensor should not be performed as it will damage the sensor.