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Home >> Mercedes Benz >> 2012 >> CL 550 4MATIC (216.394) V8-4.6L Turbo (278.920) >> Repair and Diagnosis >> Windows and Glass >> Heated Glass Element >> Description and Operation >> GF67.29-P-0003LE Heated Rear Window, Function

GF67.29-P-0003LE Heated Rear Window, Function



GF67.29-P-0003LE Heated Rear Window, Function

Function requirements, general
- Circuit 15 On
- No undervoltage or overvoltage
- Consumer shutoff not active

Circuit status c. 15 is transmitted by the EIS control unit (N73) to the rear SAM control unit with fuse and relay module (N10/2) via the interior CAN.

Rear window heater function (HHS), general
The HHS system removes ice and fog resulting from moisture on the rear window so that the driver has a clear view to the rear.

When the heated rear window (R1) is switched on, the heated outside mirror is also switched on.
To reach the operating conditions of the particulate filter in the vehicles with diesel engine and code (474) more quickly, the CDI control unit (N3/9) (on model 221 with diesel engine) can switch on the rear window heater to increase the idle speed. This function is not indicated to the driver.

The HHS system comprises the following subfunctions:
^ Manual on/off function sequence
^ Function sequence for determining operating time
^ Function sequence, automatic shutoff

Manual on/off function sequence
When the button on the rear window heater (N72/1s39) is actuated on the upper control panel control unit (N72/1) the rear window heater is requested to switch on.
The upper control panel control unit reads in the status of the rear window heater button and transmits the switch-on request to the rear SAM control unit via the interior CAN. The rear SAM control unit actuates the rear window heater via the rear window heater relay (N10/2kP).
After switching on, the rear SAM control unit transmits a corresponding signal via the interior CAN to the upper control panel control unit, where the LED in the rear window heater button is switched on.
When the rear window heater button is pressed again, actuation of the rear window heater is terminated and the LED in the rear window heater button goes out

Function sequence for determining operating time
The operating time of the rear window heater is calculated by the rear SAM control unit depending on the outside temperature and the vehicle speed.
The outside temperature is determined directly by the front SAM control unit with fuse and relay module (N10/1) via the outside temperature indicator temperature sensor (B14) and is transmitted to the rear SAM control unit to the interior CAN.
Information on vehicle speed is made available by the instrument cluster (A1) on the central CAN.
The central gateway control unit (N93) transmits this information to the rear SAM control unit via the interior CAN.
The normal operating time is t = 6 min. This period of time is increased depending on the outside temperature and the vehicle speed.
- At outside temperatures of T < 5 °C, the duty cycle is extended by t = 30 s per 1 °C drop in temperature up to a maximum of t = 6 min. (corresponding to a temperature of T = - 7 °C).
- At outside temperatures of T < 0 °C a speed-dependent time factor is also taken into consideration.
If, during the heating phase, the vehicle speed is greater than
- v > 60 km/h t = 1 min.
- v > 70 km/h t = 2 min.
- v > 80 km/h t = 3 min.
- v > 90 km/h t = 4 min.
- v > 100 km/h t = 5 min. (maximum value) is added to the operating time.

When the outside temperature is low, switch-on of the rear window heater is delayed until the vehicle interior is slight heated. The interior temperature is determined directly by the AAC control unit (N22/1) with the in-car temperature sensor (B10/4) and transmitted to the rear SAM control unit to the interior CAN. However, the LED in the rear window heater is switched on immediately.

Function sequence, automatic shutoff
The front SAM control unit monitors and controls the power to all electrical consumers to match the requirements and situation thus ensuring that the vehicle can always be started. The on-board power supply voltage is monitored permanently to ensure that the on-board power supply battery (G1) is not discharged. The status of the on-board vehicle system battery is determined by the battery sensor (B95).
It calculates the parameters serving as the basis for the on-board electrical system by measuring the voltage, power and temperature and transmits these values to the front SAM control unit via the on-board LIN.
If the on-board power supply voltage drops to U < 10.5 V for t = 5 s or when the on-board power supply voltage increases to U > 17 V for t = 0.7 s, the front SAM control unit transmits a corresponding signal to shut off the consumers to the interior CAN. The rear SAM control unit receives this signal, switches off the rear window heater and transmits the status "Rear window heater deactivated" to the upper control panel control unit via the interior CAN.

The upper control panel control unit receives the signal to shut off the consumers and the status "Rear window heater deactivated" and actuates the LED in the rear window heater button at a frequency of H = 1.5 Hz.
If the period during which the voltage is too high or too low lasts for longer than 30 s, the operation is deleted completely and is then necessary to switch it on again with the heated rear window button on the upper control panel control unit.
Unless the on-board power supply voltage increases to U > 11.5 V or decreases to U < 16 V within t = 30 s, the rear window heater is switched off again.
The LED in the heated rear window button is continuously actuated.





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