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| The heated windscreen control unit -J505--B- is actuated by the operating and display unit, Climatronic control unit -J255- (reading measured value block, display group “015”) → Chapter: |
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| After starting cold engine if ambient temperature is less than + 5° C and operating and display unit, Climatronic control unit -J255- is set to “Auto” or “Defrost” mode. The temperature setting on the operating and display unit, Climatronic control unit -J255- must be such that the air conditioner is in heating mode and the temperature measured by the senders (left vent temperature sender -G150-, right vent temperature sender -G151-, left footwell vent temperature sender -G261- and right footwell vent temperature sender -G262-) must not be in excess of +35 ° C. The heating period is governed by ambient temperature (max. 6 minutes at -40 °). |
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| On selecting “Defrost” mode on the operating and display unit, Climatronic control unit -J255- if the ambient temperature is less than + 5° C. The heating period is between 2 and 6 minutes depending on ambient temperature. As long as the heated windscreen is activated, the indicator lamp in the Defrost button flashes (only if, on pressing the Def button, the heated windscreen had not already been activated by the operating and display unit, Climatronic control unit -J255-). Actuation is only effected if the engine is running and the activation conditions described above are fulfilled. |
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| The heated windscreen control unit -J505--B- switches on the heated windscreen if the following conditions are fulfilled: |
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| Operating and display unit, Climatronic control unit -J255- actuating heated windscreen control unit -J505- |
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| Voltage at heated windscreen control unit -J505- greater than 12.70 V |
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| Heated windscreen control unit -J505- not overheated |
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| The heated windscreen control unit -J505- switches approx. battery voltage to the operating and display unit, Climatronic control unit -J255-. This informs the operating and display unit, Climatronic control unit -J255- that the heated windscreen control unit -J505- is ready to switch on the heated windscreen. If this voltage is halved by the operating and display unit, Climatronic control unit -J255- (to approx. 5 to 8 V), the heated windscreen control unit -J505- switches on the heating. |
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| The resistance of the metal foil in the heated windscreen -Z2- is approx. 1.9 Ω. A voltage greater than the electrical system voltage is required to attain the maximum possible heat output of approx. 1000 W (watts) at the windscreen. The heated windscreen control unit -J505- converts the electrical system voltage into a variable voltage of between 25 and 48 V depending on electrical system voltage. If the voltage at the heated windscreen control unit -J505- is greater than 13.10 V, the heated windscreen -Z2- is actuated with a voltage greater than 41 V (up to max. 48 V). In the event of problems relating to poor heated windscreen -Z2- heat output, check the voltage at the two connection points of the two wires at the windscreen → Current flow diagrams, Electrical fault finding and Fitting locations. To do so, actuate the heated windscreen -Z2- for example by way of the “Final control diagnosis” function with the engine running and the battery fully charged (electrical system voltage must be greater than 13.1 V). |
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| To prevent electrical system overload, the heated windscreen control unit -J505- regulates the heat output of the heated windscreen. With an electrical system voltage of 12.70 V this is approx. 200 W and is infinitely regulated to 1000 W at 13.10 V. The heated windscreen control unit -J505- permits an increase in power output of max. 200 W per second to guard against voltage dips in the electrical system on switching on the heated windscreen. |
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| To prevent actuation of the heated windscreen -Z2- in the event of a short circuit or an open circuit in the wiring, the heated windscreen control unit -J505- interrogates the resistance of the heated windscreen -Z2-. If the measured resistance is less than approx. 1.5 Ω (short circuit) or greater than approx. 3.3 Ω (open circuit or contact resistance), the heated windscreen control unit -J505- does not switch on the heating or switches it off. Check the wiring between the heated windscreen control unit -J505- and the heated windscreen -Z2- for contact resistance or short circuit. The resistance measured between the two wires from the heated windscreen control unit -J505- (to the heated windscreen -Z2-) must be less than 2.5 Ω and greater than 1.4 Ω → Current flow diagrams, Electrical fault finding and Fitting locations |
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