Cabin Temperature Sensor / Switch: Description and Operation

Cabin Temperature Sensor







The cabin temperature sensor is fitted in the roof panel by the rear-view mirror and is the NTC type. The sensor is supplied with 5 V from K1 pin 16 via a 10 kohm resistor built into the ACC unit and is grounded from K1 pin 22. This way the voltage over the sensor is inversely proportional to the temperature (the voltage drops when the temperature rises). The value is used to calculate the current temperature at head height for front seat passenger and driver. In addition, the following are used as input values:
- outside temperature
- time since engine last ran
- time since start
- blended-air temperatures
- fan speed
- solar irradiance

The cabin temperature sensor's value is only included in the calculation at the start for the ACC unit's regulation. The calculated temperatures are then compared with those the driver and passenger set with the ACC unit's temperature knob. The set temperature is corrected slightly with respect to the outside temperature so that the perceived temperature should equal the set temperature.

As soon as a difference occurs, the blended-air temperature must be increased or decreased. The control module turns the air-blending flap in question until the required blended-air temperature is achieved.







Diagnostics
In the event of open circuit or short circuit diagnostic trouble code B1350 is generated.

Outside temperature when ignition is switched on is used as substitute value followed by mean value of the calculated temperatures at head height for front seat passenger and driver.