Rear Window Defogger
If there is a condition where the rear window defogger cannot be switched on (short circuit in the connection to the Rear Window Defogger Relay -J9-, interruption in the power supply to the Comfort System Central Control Module -J393-, etc.), this is stored as a malfunction in the Comfort System Central Control Module -J393- and information is sent via the data bus to the Front A/C Display Control Head -E87- -A and B- that indicates that the DTC memory in the Comfort System Central Control Module -J393- must be read out. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function. The activation of the Rear Window Defogger -Z1- is displayed in the measured value of the Comfort System Central Control Module -J393-. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
If the measured voltage on the Vehicle Electrical System Control Module -J519- at terminal "30" falls below a value stored either in the Vehicle Electrical System Control Module -J519- or the Front A/C Display Control Head -E87-, the rear window defogger is switched off completely (or the output is reduced) so that the load on the Generator -C- reduced. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
If the activation of the Rear Window Defogger -Z1- must be switched off due to insufficient voltage, the indicator lamp in the rear window defogger button in the Front A/C Display Control Head -E87- remains switched on. If the defogger is switched off for longer than approximately 100 seconds, the Front A/C Display Control Head -E87- switches off the indicator lamp.
The "Read measured values" function of Guided Fault Finding for the Front A/C Display Control Head -E87- indicates that the rear window defogger is switched on or why there is no activation even though there is a request. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
The function test for the rear window defogger activation is described in Guided Fault Finding for the Comfort System Central Control Module -J393-. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
The activation duration of the Rear Window Defogger -Z1- is determined by various factors (coding for the rear window version in the Front A/C Display Control Head -E87-, the measured exterior temperature, etc.). If, for example, there is an exterior temperature greater than +5 °C (41 °F) and a certain rear window type is installed, it can be switched off after only three minutes. If the exterior temperature is less than -20 °C (-4 °F) (and a different rear window type is installed, the maximum activation duration can be up to 20 minutes. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
The activation of the Rear Window Defogger -Z1- can also be continually switched on (until the ignition is switched off) by pressing and holding the "rear window defogger" button on the Front A/C Display Control Head -E87-. The necessity time that the button needs to be pressed can be adjusted using the "Adaptation" function in the Front A/C Display Control Head -E87-. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function and refer to the Owner's Manual.
If the exterior temperature changes while driving (it becomes greater than 0 °C (32 °F)), the Rear Window Defogger -Z1- is automatically switched off after the operating time stored in the Front A/C Display Control Head -E87- has elapsed. Refer to the Owner's Manual.
A Start/Stop System is available for certain engines for this vehicle. For vehicles with a Start/Stop System, the rear window defogger can be switched off for the time in which the Stop function is active (to protect the Battery -A-). The setting for activating the Rear Window Defogger -Z1- remains stored however in the Front A/C Display Control Head -E87-. After the engine was restarted using the Start function, the Rear Window Defogger -Z1- is switched on again. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.