Auxiliary Duct Air Temperature Sensors

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

The upper and lower auxiliary 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 rear auxiliary HVAC control module 5 volts is supplied to the auxiliary air temperature signal circuit through a fixed resistance. The fixed resistance inside the rear auxiliary 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 rear auxiliary HVAC control module. The rear auxiliary 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 auxiliary air temperature sensors is provided by the low reference circuit.

The rear auxiliary HVAC control module will use a default value for the upper and lower auxiliary air temperature signal if there is a fault with the input. The rear auxiliary HVAC control module will use this default to ensure auxiliary 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 it will read 255 Counts.