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 

Function requirements, general 

• Circuit 15 ON
• No undervoltage or overvoltage
• Consumer shutoff not active

The circuit status c. 15 is transmitted by the electronic ignition lock control unit (N73) via the interior CAN (CAN B) to the rear SAM control unit with fuse and relay module (N10/2).

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 (R1) is switched on, the heated outside mirror is also switched on.

The rear window heater may be switched on by the CDI control unit (N3/9) to increase the idle speed and thereby reach the operating conditions of the particulate filter in vehicles with diesel engine and code (474) Particulate filter more quickly. The driver is not advised of this function.

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 

Manual on/off function sequence 

When the rear window heater button (N22/7s1) on the automatic air conditioning control and operating unit (N22/7) 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 (N10/2kC).

After switch 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 determined directly by the front SAM control unit with fuse and relay module (N10/1) via the outside temperature sensor (B14) and is transmitted to the rear SAM control unit on the interior CAN.

Information on vehicle speed is made available by the instrument cluster (A1) over the interior CAN.

The normal operating time is t = 15 min. This time is increased depending on the outside temperature and 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 is driven faster 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.

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 shutoff comprises the following subfunctions:

• 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 U ≥ 18 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 (N30/4) transmits a signal via the chassis CAN (CAN E) 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.

Function schematic for electrical rear window heater

PE67.29-P-2051-97FAB