Windshield Defogger Functionality
The Windshield Defogger Control Module -J505- control module -A- is activated by the Comfort System Central Control Module -J393- following a request from the front Air Conditioning (A/C) display control head, the Climatronic Control Module -J255-, via connector -D-, refer to Vehicle Diagnostic Tester in the "Guided Fault Finding " function and Wiring Diagram.
The request to activate the windshield defogger of the Climatronic Control Module -J255- is made under the following conditions (for an exact description, refer to the Operating Instructions and see "Infotainment/MMI" operating instructions).
- After every cold start of the engine if the outside temperature is less than 5 °C (+ 41 °F), "Auto" mode is set on the front A/C display control head, the Climatronic Control Module -J255-, and the function is not switched off via the MMI (Multi Media Interface) Operating Instructions and see Operating Instructions "Infotainment / MMI". The temperature must be set on the Climatronic Control Module -J255- such that the A/C system is in heating mode and the temperature measured by the sensors (Left Vent Temperature Sensor -G150-, Right Vent Temperature Sensor -G151-) must not exceed + 35 °C (+ 95 °F). Depending on the outside temperature, heating period is a maximum of six minutes (at -40°).
- When selecting "defrost" mode on the Climatronic Control Module -J255-, the outside temperature is less than 5 °C (+ 41 °F) and the function was not switched off via the MMI (Multi Media Interface). Depending on the exterior temperature, the heating period is between two and six minutes. As long as the activation of the Windshield Defogger -Z2- is switched on, the activation indicator lamp flashes in the Defrost . Actuation only takes place with engine running.
The Windshield Defogger Control Module -J505- -A- activates the heater of the Windshield Defogger -Z2- if the following basic conditions are met.
- The front A/C display control head, the Climatronic Control Module -J255-, transmits an activation request to the Comfort System Central Control Module -J393-, and the Comfort System Central Control Module -J393- relays this request to the Windshield Defogger Control Module -J505-.
- The voltage at the Windshield Defogger Control Module -J505- is greater than 12.7 V.
- The Windshield Defogger Control Module -J505- is not overheated.
The activation of the Windshield Defogger -Z2- can be switched off via a function in the MMI (Multi Media Interface) see Operating Instructions and see Operating Instructions "Infotainment / MMI".
The Windshield Defogger Control Module -J505- is activated by the Comfort System Central Control Module -J393- via a data line. If there is a condition in the Windshield Defogger Control Module -J505- that does not allow activation of the windshield defogger, this is communicated to the Comfort System Central Control Module -J393- via this data line. The Comfort System Central Control Module -J393- then relays this information that the windshield defogger cannot be activated, to the front A/C display control head, Climatronic Control Module -J255- see Vehicle Diagnostic Tester in the "Guided Fault Finding " function.
The electrical resistance of the metal foil in -1- in the Windshield Defogger -Z2- is approximately 1.9 Ohm. A voltage greater than the electrical system voltage is needed in order to reach the maximum possible heat output of approximately 1000 Watt (W) on the front window. The Windshield Defogger Control Module -J505- converts the vehicle electrical system voltage into a variable voltage of between 25 and 48 V, depending on vehicle electrical system voltage. If a voltage at the Windshield Defogger Control Module -J505- is greater than 13.1 V, the windshield defogger -Z2- is activated with a voltage greater than 41 V (up to max. 48 V), refer to Vehicle Diagnostic Tester in the "Guided Fault Finding " function.
So as not to overload the vehicle electrical system, Windshield Defogger Control Module -J505- regulates the heating output of the heated windshield (approximately 200 W with an electrical system voltage of 12.70 V and infinite regulation to 1000 W at 13.10 V). To prevent the vehicle electrical system voltage from collapsing when the windshield defogger is switched on, the Windshield Defogger Control Module -J505- allows for an output increase of maximum 200 W per second.
To prevent activation of the Windshield Defogger -Z2- in the event of a short circuit or open circuit, the Windshield Defogger Control Module -J505- checks the resistance of the -Z2-. If the measured resistance is less than approximately 1.5 ohms (short circuit) or greater than approximately 3.3 ohms (open circuit or contact resistance), the Windshield Defogger Control Module -J505- does not switch the heater on or off, refer to Vehicle Diagnostic Tester in the "Guided Fault Finding " function.
The Windshield Defogger -Z2- is only activated in the output diagnostic test mode up to a temperature of 40 °C (104 °F) in the vehicle interior. If the temperature is higher, "Function unknown" or "Function currently unavailable" is shown in the fault read out device display see Vehicle Diagnostic Tester in the "Guided Fault Finding " function.
The activation function of the Windshield Defogger Control Module -J505- can be checked with the Analog/Digital Multimeter:FLU83III for example. Place the Analog/Digital Multimeter:FLU83III current clamp over the Windshield Defogger Control Module -J505- positive wire to the Windshield Defogger -Z2- and select the "Measure current with current clamp" function on the Analog/Digital Multimeter:FLU83III . The display on the Analog/Digital Multimeter:FLU83III changes from approximately 0 A to greater than 7 A => Wiring Diagram.