Ambient Temperature Sensor / Switch HVAC: Description and Operation
AIR TEMPERATUREAir Temperature Sensors
The air temperature sensor is a 2 wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:
- Ambient
- Inside
- Duct
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 operates within a temperature range of -40°C (-40°F) to 150°C (300°F). The sensor signal decreases as the resistance decreases. 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 air 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 shown sensor inputs with the indicated mode positions.
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 shown conditions.
A/C Compressor Temperature Switch
The A/C compressor is protected from overheating by the temperature switch, which is mounted at the rear of the A/C compressor. The switch is wired internally to the compressor and is not serviceable. The switch opens and disengages the compressor clutch when temperatures reach 155°C (311°F). The switch closes when the compressor temperature cools to 125°C (257°F).
If the compressor temperatures get to high, then the switch opens the ground circuit to the compressor clutch coil, disengaging the A/C compressor clutch. There is no feedback to the PCM that this has occurred. The time frame the switch remains open, depends on how the vehicle is being operated. Conditions when the A/C compressor temperature switch may open:
- High ambient temperatures
- Low refrigerant charge
- Towing a trailer on a long steep grade
- Descending a grade at high speed in a low gear