Air Temperature Sensors

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The air temperature sensors are a 2-wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:

Ambient Air Temperature Sensor
Inside Air Temperature Sensor Assembly
Inside Air Temperature Sensor Assembly-Auxiliary
Upper Left Air Temperature Sensor
Upper Right Air Temperature Sensor
Lower Left Air Temperature Sensor
Lower Right Air Temperature Sensor
Upper Air Temperature Sensor-Auxiliary
Lower Air Temperature Sensor-Auxiliary

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 signal voltage decreases as the resistance decreases. The sensor operates within a temperature range between -40° to +101°C (-40° to +215°F). The sensor signal varies between 0-5 volts.

The input of the duct air temperature sensors are different from the ambient and inside air temperature sensors. The HVAC control module converts the signal to a range between 0-255 counts. As the air temperature increases the count value will decrease.

If the HVAC control module or auxiliary HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted air temperature value. The default value for the ambient and inside air temperature sensors will be displayed on the scan tool. The default value for the duct air temperature sensors will not be displayed on the scan tool. The scan tool parameter for the duct air temperature sensors are the actual state of the signal circuit. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is corrected.

The ambient air temperature sensor mounts underhood and can be affected by city traffic, by idling, and by restarting a hot engine. Therefore, the HVAC control module filters the value of the ambient air temperature sensor for temperature display.

The ambient air temperature value is updated under the following conditions:

Fig 1: Air Temperature Sensor Condition Table

Courtesy of GENERAL MOTORS CORP.

The scan tool has the ability to update the displayed ambient air temperature. To update the ambient air temperature display on the HVAC control module, perform the following procedure:

Turn ON the ignition.
Simultaneously press the MODE, FRONT DEFROST and REAR DEFROST switches.