Overview
This document contains the following information:
General
Function requirement
Windshield heating
General
The windshield heater is integrated in the windshield as heatable film.
The windshield heater is designed for a voltage of 42 V. When a voltage is applied, the windshield heater heats up because of the electrical resistance. The heat is used, for example, to defrost ice or snow from the windshield or counteract a fogging up of the windshield. In case of strong solar radiation, the windshield heater reflects part of the sun's rays, thereby preventing the vehicle interior from heating up quickly. The windshield heater is controlled by the windshield heater control unit.
If the heated windshield is operated at over 600 W in vehicles with combustion engine, the engine management increase the idle speed of the combustion engine.
Function requirement
Engine running or drivetrain operational
Windshield heating
One of two operating modes can be specified via the variant coding of the vehicle. In operating mode "Defrost", the function is operated at 100% output and is activated when the function "Defrost windshield" is switched on. In operating mode "AUTOMATIC", the windshield heater is activated in the case of an imminent fogging on the inside of the windshield and operated in an output-controlled manner.
The windshield heater is activated and regulated according to the programed operating mode depending on the following factors:
Status of function "Defrost windshield"
Outside temperature
Fogging tendency (operating mode AUTOMATIC)
The fogging tendency is referenced via the dew point temperature. By comparing the surface temperature of the windshield with the dew point temperature, an imminent fogging can be detected.
Coolant temperature
The evaluation of the coolant temperature allows for conclusions on the vehicle non-operational time and thus the differentiation between a short-term and long-term stop.
Vehicle speed
Vehicle position (operating mode AUTOMATIC, vehicles with navigation system)
Operating mode "Defrost"
The windshield heater can be activated/deactivated via the defrost button on the air conditioning control panel. At an outside temperature below 10 °C, the windshield heater is actuated at full output for 30 s. Depending on the temperature at the inside of the windshield, the coolant temperature and vehicle speed, the operating duration is extended to max. 8 min.
Operating mode "AUTOMATIC"
In the case of an imminent fogging on the inside of the windshield, the windshield heater is actuated for 30 s at full output. The system then regulates the windshield heater output depending on the fogging tendency and the vehicle speed between 35 and 100 %. From a speed of 120 km/h, the windshield heater is deactivated.
Vehicles with navigation system
At tunnel entrances and for increasing outside temperatures, the windshield heater is actuated at full output. A fogging on the outside of the windshield is thus prevented. Once the temperature of the windshield is 2 °C above the outside temperature, the windshield heater is deactivated again.
Consumer reduction
The windshield heater is reduced in its output or deactivated by the energy management via the consumer reduction in the case of an undervoltage of the on-board electrical system.