Rear window defroster (RWD), function

Function prerequisites:

 C. 15: ON

 Battery voltage okay

Switching on the rear window defroster (at outdoor temperatures above -10 °C):

 Press rear window defroster button on A/C pushbutton control module (automatic A/C, N22)!

 The A/C pushbutton control module (automatic A/C, N22) sends the message to switch on the rear window defroster to the CAN bus (passenger compartment). The function indicator LED on the A/C pushbutton control module (automatic A/C, N22) comes on.

 The rear SAM control module (N10/8) receives this message and switches on the rear window defroster element (R1) with the rear window defroster relay (K40/5kU). The relay closes circuit 30 to the rear window defroster element (R1).

 The rear SAM control module (N10/8) sends the confirmation message "Rear window defroster: ON" to the

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A/C pushbutton control module automatic A/C (automatic A/C, N22).
If this message is not received for longer than one second, the function indicator LED in the A/C pushbutton control module (automatic A/C, N22) goes out.

Switching on the rear window defroster (at outdoor temperatures below -10 °C):

 Press rear window defroster button on A/C pushbutton control module (automatic A/C, N22)!

 The A/C pushbutton control module (automatic A/C, N22) sends the message to switch on the rear window defroster to the CAN bus (passenger compartment). The function indicator LED on the A/C pushbutton control module (automatic A/C, N22) comes on.

 The rear SAM control module (N10/8) receives this message and immediately sends the confirmation signal "Rear window defroster: ON" to the A/C pushbutton control module (automatic A/C, N22).
However, the rear window defroster (R1) is switched on by

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the SAM only when the passenger compartment temperature is at least 5 °C.

Manually switching off the rear window defroster:

The rear window defroster can be switched off at any time by pressing the button on the A/C pushbutton control module (automatic A/C, N22) again.

Automatic rear window defroster switch-off:

Normally (at outdoor temperatures of at least 5 °C) the rear window defroster element (R1) is switched off automatically after it has been switched on for 6 minutes.

At outdoor temperatures below 5 °C the switch-on time is increased:

 Outdoor temperature between approx. 0 °C and 5 °C
If the outdoor temperature is between approx. 0 °C and 5 °C, the switch-on time per °C less than 5° is increased by
30 seconds. The maximum increase is 6 minutes corresponding to an outdoor temperature of - 7 °C.
Example: At an outdoor temperature of 2 °C the increase in the switch-on time is 3 * 30 seconds. The rear window defroster is therefore switched off after 7.5 minutes.

 Outdoor temperature below 0 °C
If the outdoor temperature is less than 0 °C a speed-dependent increase in the switch-on time is added to the temperature-dependent increase (see table).

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Example: Outdoor temperature -3 °C, speed 80 km/h.
The temperature-dependent increase is 8*30 seconds, the speed-dependent increase 3 minutes. The rear window defroster therefore remains switched on for 13 minutes.

 If the speed changes during the time the rear window defroster is switched on, the increase in the switch-on time is calculated using the highest speed achieved during this time.

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Speed in km/h	Speed-dependent increase in minutes
60	1
70	2
80	3
90	4
above 100	5

Data required for function:

 The outdoor temperature is measured by the outdoor temperature sensor (B14). The value is read in the left front SAM control module (N10/6) and transferred to the rear SAM control module (N10/8) on the CAN bus (passenger compartment).

 The interior temperature is measured by the passenger compartment temperature sensor (N70b1) in the overhead control panel control module (N70). The values are transferred to the rear SAM control module (N10/8) on the CAN bus (passenger compartment).

 The speed is determined in the rear SAM control module (N10/8) using the wheel speed signal from the left front axle VSS sensor (L6/1). The left front axle VSS sensor (L6/1) is read in by the traction system control module (N47). The traction system control module (N47) transfers the speed signal as a message on the CAN bus (engine). The electronic ignition/starter switch control module (N73) serves as the interface (gateway) between the CAN bus (engine) and CAN bus (passenger compartment), transferring the wheel speed to the CAN bus (passenger compartment).
The rear SAM control module (N10/8) reads the message from the CAN bus (passenger compartment).