Switching off after expiry of max. heating time, function

Function sequence

The rear window defroster has a maximum switch-on time if it is not switched off manually by the rear window defroster switch (S21s9). The switch-on time depends on the outside temperature and the vehicle speed.

The outside temperature is sensed by the temperature sensor (B14) and transmitted directly to the instrument cluster (A1). The instrument cluster (A1) stores the outside temperature value on the engine compartment CAN bus whereby it is available to the extended activity module (EAM) (N10/1).

The vehicle speed is determined by the traction systems control module (N47) on the basis of the wheel speed and transmitted to the engine compartment CAN bus.

The switch-on time for the rear window defroster is determined in relation to the conditions listed above at the time the rear window defroster is switched on.

The following conditions and switch-on times are distinguished:

 At outdoor temperatures above 5 °C

Switch-on time: 10 minutes

 At outdoor temperatures below 5 °C and speeds below
80 km/h:
Switch-on time: 15 minutes

 At outdoor temperatures below 5 °C and speeds above
80 km/h
Switch-on time: 18 minutes


If the voltage is too low for 2 seconds while switching on, the heating current from the extended activity module (EAM) (N10/1) is interrupted and the rear window defroster indicator lamp (S21e2) starts flashing. The switch-on time continues to expire. If the voltage is too low for more than 30 seconds the rear window defroster switches off completely and must be switched on again with the rear window defroster switch (S21s9).