Rear Defogger: Description and Operation

Windshield Defroster

Windshield Defroster, Function







- The Heated Windshield Control Module -J505- is activated by the A/C control module (see Read Measuring Value Block, display group 015): Measuring Value Blocks (Function 08)
- After each cold start of the engine if the outside temperature is smaller than + 5 degrees C and "Auto" mode has been selected on the A/C control head -E87-. The temperature of the A/C control head -E87- has to bet set so that the air conditioning is in heating mode and none of the vent temperature sensors (-G150-, -G151-, -G261- and -G262-) should measure more than + 35 degrees C. Depending on the outside temperature, the heating period is maximum 6 minutes (at -4 degrees C).
- When selecting "Defrost" mode on the A/C control head -E87- and if outside temperature is smaller than + 5 degrees C. Depending on the outside temperature the heating period is between 2 and 6 minutes. As long as the windshield defroster is switched on, the indicator control light in the "Defrost" button flashes. The activation occurs only with engine running.







- The Heated Windshield Control Module -J505- switches on the windshield defroster, if the following conditions are met.
- The A/C Control Head -E87- controls the Heated Windshield Control Module -J505-.
- The voltage on Heated Windshield Control Module -J505- is greater than 12.70 volts.
- Heated Windshield Control Module -J505- is not overheated.
- The Heated Windshield Control Module -J505- directs approx. batter voltage to the A/C control head -E87-, with this signal the -E87- recognizes, that -J505- is ready to switch on the windshield defroster. If this voltage is cut in half (to approx. 5 to 8 volt) the Heated Windshield Control Module -J505- switches the windshield defroster on.
- The electrical resistance of the metal foil in windshield defroster -Z2- is approx. 2.3 Ohms. In order to attain a maximum heating performance of approx. 1000 W (Watts) on the front windshield, it is necessary that the voltage is greater than the vehicle electrical system. The Heated Windshield Control Module -J505- changes the vehicle electrical voltage into a variable voltage, this variable voltage is between 25 and 48 volts depending on the vehicle electrical voltage.
- To avoid an overload on the vehicle electrical system, the -J505- regulates the heating output of the windshield defroster, at vehicle voltage of 12.70 the output is approx. 200 W and is gradually increased to 1000 W at 13.10 volts. To avoid the vehicle electrical system from collapsing when the windshield defroster is switched on, the Heated Windshield Control Module -J505 allows for an output increase of 200 W per second.
- To avoid the Windshield defroster -Z2- from being activated if there is a short circuit or an open circuit, the Heated Windshield Control Module -J505- checks the resistance of the windshield defroster -Z2-. If the measured resistance is smaller than approx. 1.7 ohms (short circuit) or greater than approx. 3.3 ohms (open circuit or contact resistance) the Heated Windshield Control Module -J505- does not switch on or off the heater.