Ambient Temperature Sensor / Switch HVAC: Description and Operation
AIR TEMPERATURE SENSORSThe air temperature sensors are 2 wire negative temperature co-efficient thermistors. The vehicle uses the following air temperature sensors:
- Ambient
- Inside
- Upper right
- Upper left
- Lower right
- Lower left
The upper and lower duct sensors are divided into left and right zone operation. The left side upper and lower duct sensors will only effect the operation of the left air temperature actuator and the right side upper and lower duct sensors will only effect the operation of the right air temperature actuator. The duct sensors are used to measure the temperature of the air exiting the ducts. The sensors operate within a temperature range between -6.5°C (20.3°F) to 57.5°C (135.5°F). When temperature increases the sensor signal decreases. The HVAC control module converts the 0-5 volt sensor signal to a range between 0-255 counts. If the HVAC control module (IPM) detects that one of the duct temperatures must change, the HVAC control module (IPM) will adjust the appropriate air temperature actuator. The following list shows the duct sensors that are monitored by the HVAC control module in each mode position.
If the HVAC control module (IPM) detects a malfunctioning sensor, the HVAC system will only malfunction when the sensor is monitored.
The inside temperature sensor operates within a temperature range between -6.5°C (20.3°F) to 57.5°C (135.5°F). If the sensor is shorted to ground, the system will operate in the maximum cooling mode. If the sensor is shorted to voltage or open, the system will operate in the maximum heating mode. The ambient sensor operates within a temperature range between -30°C(-22°F) to 51°C (123.8°F). If the HVAC control module (IPM) has determined that the ambient air temperature sensor has failed, the IPM shall display "- -" in place of the ambient air temperature. If the sensor is shorted to ground, the system will operate in the maximum cooling mode. If the sensor is shorted to voltage or open, the system will operate in the maximum heating mode. If the engine coolant temperature is not more than 28°C (50°F) above the sensor reading, or if the engine has not been started in two hours, then the actual ambient air temperature sensor reading is displayed. The ambient air temperature displayed may be allowed to increase after 80 seconds as long as the vehicle speed is greater than 32 km/h (20 mph), but only at a slow, filtered rate. If the sensor reading is ever less than the displayed value or if the vehicle speed is 72 km/h (45 mph) or greater, then the ambient air temperature changes are displayed as rapidly as possible. The (IPM) displays the ambient air temperature.
A/C REFRIGERANT LOW TEMPERATURE SENSOR
The dash integration module (DIM) monitors the sensor to determine low side pressure based on the pressure to temperature relationship of the R134a. This sensor is located on the low side line. The sensor is used to cycle the A/C compressor ON and OFF to prevent the evaporator core from freezing. A thermistor inside the sensor varies its resistance to monitor the temperature of the Freon. The DIM monitors the voltage drop across the thermistor when supplied with a 5-volt reference signal. The DIM will send a class 2 message to the PCM to stop requesting the A/C compressor clutch operation if the temperature drops below 2°C (28°F). Vehicles operating above 32 km/h (20 mph) will be controlled to a minimum ON time of 46 seconds before the DIM will allow the clutch to disengage. The sensor must be above 10°C (50°F) before the DIM will request A/C compressor clutch operation again.
The sensor operates within a temperature range between -40°C (-40°F) to 215°C (-355°F). If the DIM detects an open in the A/C refrigerant low temperature sensor or circuit, the class 2 message sent to the PCM will not allow A/C operation. The HVAC control module (IPM) will then send a request to the Radio for display of the "SERVICE A/C SYSTEM" that will be displayed on the DIC. The HVAC control module (IPM) will also display A/C OFF on the module as long as the condition is present.
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 prevents 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 2850 kPa (413 psi). The clutch will be enabled after the pressure decreases to less than 1750 kPa (254 psi).
- A/C pressure is less than 204 kPa (30 psi). The clutch will be enabled after the pressure increases to more than 220 kPa (32 psi).
If the PCM detects a failure in the A/C refrigerant pressure sensor or circuit, the class 2 message sent to the HVAC control module (IPM) will be invalid. The HVAC control module (IPM) will then send a request to the Radio for display of the "SERVICE A/C SYSTEM" that will be displayed on the DIC. The HVAC control module will also display A/C OFF on the module as long as the condition is present.