Temperature Sensors

The automatic system uses multiple sensors to achieve and maintain the desired temperature. The inside air temperature sensor provides the HVAC control processor software with the temperature of the air drawn through the aspirator. A 5-volt reference signal is sent from the HVAC control processor to the inside air temperature sensor over the inside air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. As temperature increases, resistance decreases. That varied voltage provides a signal to the software inside the HVAC control processor. Remaining voltage from the inside air temperature sensor is sent back to the HVAC control processor ground through the low reference circuit.

The ambient air temperature sensor provides the HVAC control processor software with the temperature of the air outside the vehicle and displays that temperature on the HVAC control module. As temperature increases, resistance decreases. A 5-volt reference signal is sent from the HVAC control processor to the ambient air temperature sensor over the ambient air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control processor. Remaining voltage from the ambient air temperature sensor is sent back to the HVAC control processor ground through the low reference circuit.

Since the sensor is mounted underhood it can be affected by city traffic, idling, and hot engine restarts. A temperature memory feature is used by the HVAC control processor ambient air temperature programming to help provide greater accuracy under engine restart conditions. If the engine coolant temperature is less than 10°C (50°F) above the ambient air temperature sensor reading, or if the engine has not been started in two hours, then the actual ambient air temperature sensor reading is displayed. However, if the engine coolant is more than 10°C (50°F) above the sensor reading, the memorized ambient air temperature is displayed. This is the last displayed temperature sensed when the vehicle was operating. At vehicle speeds greater than 32 kmH (20 mph), the ambient air temperature displayed may be allowed to increase, but only after a built-in 80-second time delay which allows for ambient air to cool the sensor. The time delay starts when vehicle speed reaches or maintains at least 32 kmH (20 mph). If the sensor reading is ever less than the displayed value or if the vehicle speed is 72 kmH (45 mph) or greater, then the ambient air temperature changes are displayed as rapidly as possible.