Theory Of Operation

The In-Car (Cabin) Temperature Sensor is a negative temperature coefficient thermistor type sensor. As the sensor gets colder, the resistance is higher. As the temperature increases, the resistance is lower. The HVAC Module applies a low current, 5-volt signal to the signal circuit and measures the voltage drop across the thermistor. The lower the temperature, the higher the voltage drop.

There are four circuits between the In-Car Temperature Sensor and the HVAC Module:

• 12 Volt Ignition
• Temperature Sensor Signal
• Sensor Ground
• Ground

The 12-volt and ground circuits operate a fan that pushes air across the sensor from the cabin compartment. These circuits are not connected to the HVAC Module. Power is supplied directly from the power distribution center. The fan will run when the ignition is turned on. Changes in cabin temperature are monitored by the HVAC Module which will make adjustments as necessary to keep the cabin at the operator set temperature. If the customer is concerned that the cabin temperature is not maintained properly, and no DTCs are set, make sure the fan is operating correctly.

The HVAC Module monitors the temperature sensor signal circuit for voltages that are out of normal operating range. If a problem is detected, a DTC will be set.