Duct Air Temperature Sensors

WARNING: This page is about a different car, the 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, 2002 Chevrolet Silverado, and 2002 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The HVAC control module receives inputs for the air duct outlet temperature from the upper and lower air temperature sensors. The HVAC control module uses these inputs to position the air temperature actuator to achieve and maintain the set temperature on the HVAC control module.

The upper and lower air temperature sensors are Negative Temperature Coefficient (NTC) thermistors, when the temperature of the sensor changes so does the resistance across the thermistor. When the air temperature is warm, the sensor resistance and signal voltage is low. When the air temperature is cool, the sensor resistance and signal voltage is high. Inside the HVAC control module 5 volts is supplied to the air temperature signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the air temperature. The ground for the upper and lower air temperature sensor is provided by the low reference circuit.

The HVAC control module will use a default value for the upper and lower air temperature signal if there is a fault with the input. The HVAC control module will use this default to ensure HVAC operation is still performed. The scan tool value will be the actual reading of the signal circuit. This means if signal circuit is shorted to a ground then the scan tool will read 0 Counts. If the signal circuit is more than 5 volts than the scan tool will read 255 Counts.