Heated rear window, function - GF67.29-P-0003CE

MODEL 204.0/2 as of model year 2009 /YoM 08

MODEL 204.3

MODEL 204.9

Fig 1: Heated Rear Window Block Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements, general

Circuit 15 ON
No undervoltage or overvoltage
Consumer shutoff not active

The electronic ignition lock control unit sends the status of circuit 15 via the interior CAN to the rear SAM control unit.

Rear window heater (HHS), function

The HHS system keeps the rear window free of ice and smog caused by moisture and ensures that the driver has a clear view to the rear.

When the rear window heater is switched on, the heated outside mirror is also switched on.

To rapidly achieve the operating conditions of the engine, the CDI control unit or ME-SFI control unit can request switching on high-load consumers (e.g. rear window heater). The CDI control unit or ME-SFI control unit transmit the signal to switch on additional current consumers to the front SAM control unit via chassis CAN. The front SAM control unit transmits the signal to the rear SAM control unit via the interior CAN.

The HHS system comprises the following partial functions:

Function sequence for manual on/off
Function sequence for determining switch-on duration
Function sequence for traffic light stop
Function sequence, automatic shutoff

When adjustments affecting the temperature, air distribution and air quantity are made on the automatic air conditioning control and operating unit with the rear window heater switched on, the symbol for the rear window heater and the corresponding settings appear momentarily on the Audio/COMAND display (A40/8).

Manual on/off function sequence

When the rear window heater button on the automatic air conditioning control and operating unit is actuated, the rear window heater is requested to switch on.

The automatic air conditioning control and operating unit reads in the status of 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. On model 204.2 the rear window antenna amplifier 1 is integrated between the rear window heater relay and rear window heater. After the rear window heater is switched on, the rear SAM control unit transmits a corresponding signal to the automatic air conditioning control and operating unit via the interior CAN, whereby the LED in the rear window heater button is switched on. When the rear window heater button is actuated again, actuation of the rear window heater is terminated and the LED in the rear window heater button goes out.

Function sequence for determining switch-on duration

The operating time of the rear window heater is calculated by the rear SAM control unit depending on the outside temperature and vehicle speed.

The outside temperature is recorded by the outside temperature sensor. The front SAM control unit directly reads in the signal from the outside temperature sensor, calculates the outside temperature and transmits it to rear SAM control unit via the interior CAN.

The instrument cluster sends the vehicle speed to the rear SAM control unit over the interior CAN.

The preset duty cycle of the rear window heater is t = 15 min.

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, the following value is added to the duty cycle depending on the vehicle speed:

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)

At outside temperatures of T > 50 °C, the operating time of the rear window heater is limited to t = 2 min. to protect the rear window.

Function sequence for traffic light stop

If the engine is switched off with the rear window heater active (e.g. traffic stop light, function requirements, c. 15R On), the load signal for the rear window heater is switched off, however, the switch-on time does not continue to run. If, within t = 5 min, c. 15 is not switched back on, the rear window heater is reactivated.

After expiration of the traffic light stop time without switching on c. 15, the rear window is switched off completely.

During engine start, the rear window heater is switched off momentarily. The rear window heater is switched back on t = 10 s after the engine is started.

The traffic stop light time can be parameterized with the diagnosis tester in the rear SAM control unit with range t = 0 min. to t = 10 min.

Function sequence, automatic shutoff

The automatic switch-off function comprises the following subfunctions:

D Function sequence for automatic switch-off by on-board electrical management

Function sequence for automatic switch-off wit activated PRE-SAFE function (with code (299) PRE-SAFE)

Function sequence for automatic switch-off by on-board electrical management

In order to prevent the battery from discharging in all driving cycle phases, the state of charge is continuously evaluated by the rear SAM control unit. If required, heavy power consumer such as the rear window heater are then switched off in a designed order until the specified minimum on-board electrical system voltage is reached again.

If the rear SAM control unit detects for t = 1 s an undervoltage of U ≤ 10.0 V or for t = 1 s an overvoltage of U18 V, the rear SAM control unit switches off the rear window heater.

The duty cycle does not continue to elapse during the shutoff period. If the rear SAM control unit switches off the rear window heater, the LED in the rear window heater button remains on for the time being. If the on-board electrical system reaches its normal voltage within the next t = 5 min., the rear window heater is automatically reactivated. However, if the consumer shutoff lasts longer than t = 5 min., the LED in the rear window heater switch is also switched off.

If switch-off continues and the function is requested again with the rear window heater button, the LED comes on for t = 1.5 s and then goes out.

The rear window heater function can only be requested by driver, when the on-board electrical system management has canceled the switch-off.

Function sequence for automatic switch-off with activated PRE-SAFE function

If the seat belts in the vehicle have to be tensioned as part of the PRE-SAFE function, the rear window heater is switched off if it was previously switched on.

For this reason the electronic stability program control unit transmits the request to actuate PRE-SAFE via chassis CAN to the front SAM control unit which relays the message to the rear SAM control unit via the interior CAN. The on-board electrical system management then switches off the rear window heater.

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