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Deactivating heated windscreen > < Heated windscreen

A4 Mk2

Heating
Heated windscreen
Operation of heated windscreen
 
A87-0392
  • ◆ → The heated windscreen control unit -J505 -B- is actuated by the operating and display unit for air conditioner/Climatronic -E87 (heating) (=>Reading measured value block, display group 015, Page 01-114):
  • ‒ Whenever cold engine is started if ambient temperature is less than + 5° C. Temperature at -E87 (heating) must be set such that heating system is in heating mode and none of the vent temperature senders (-G150, -G151, -G261 and -G262) may measure more than + 35°C. Depending on ambient temperature, max. heating period is 6 minutes (at -40 °).
  • ‒ When "Defrost" mode is selected on -E87 (heating) if ambient temperature is less than + 5° C. Depending on ambient temperature, heating period is between 2 and 6 minutes. Lamp in "Defrost button" flashes as long as heated windscreen is switched on. Actuation only takes place with engine running.
 
A87-0392
  • ◆ → The control unit -J505 -B- switches on the heated windscreen if the following conditions are satisfied:
  • ‒ Operating and display unit for air conditioner/Climatronic -E87 (heating) actuates -J505.
  • ‒ Voltage at control unit -J505 is greater than 12.70 V.
  • ‒ Control unit -J505 is not overheated.
  • ◆ The control unit -J505 switches approx. battery voltage through to -E87 (heating), thus indicating to -E87 (heating) that the control unit -J505 is ready to switch on the heated windscreen. If this voltage is halved by -E87 (heating) to approx. 5 - 8 V, -J505 switches on the heater.
  • ◆ The resistance of the metal foil in the heated windscreen -Z2 is approx. 2.3 ohms. A voltage higher than the electrical system voltage is required to attain the maximum possible heating power at the windscreen of approx. 1000 W. The heated windscreen control unit -J505 converts the electrical system voltage into a variable voltage of between 25 and 48 V depending on the system voltage level.
  • ◆ So as not to overload the vehicle electrical system, -J505 regulates the heating power of the heated windscreen (approx. 200 W with an electrical system voltage of 12.70 V and infinite regulation to 1000 W at 13.10 V). To stop the electrical system voltage collapsing when the heated windscreen is switched on, the maximum increase in power output permitted by -J505 is 200 W per second.
  • ◆ Actuation of the heated windscreen -Z2 in the event of a short circuit or an open circuit in the wiring is prevented by the control unit -J505 interrogating the resistance of the heated windscreen -Z2. If the measured resistance is less than approx. 1.7 ω (short circuit) or greater than approx. 3.3 ω (open circuit or contact resistance), -J505 does not switch on the heater or switches it off.

Deactivating heated windscreen > < Heated windscreen