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

MODEL 212 

Fig 1: Heated Rear Window Block Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Terminal 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, general 

If the rear window heater is active, the heated outside mirror is always active as well. The rear window heater comprises the following partial functions:

• Manual on/off function sequence 
• Function sequence for automatic switch-on 
• Function sequence for determining operating time 
• Function sequence for traffic light stop 
• Consumer shutoff function sequence 
• Function sequence for switching off rear window heater with PRE-Safe "activated" 

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 

The rear window heater is switched on and off with the rear window heater button in the automatic air conditioning control and operating unit. The automatic air conditioning control and operating unit directly reads in the status of rear window heater button and transmits it to the rear SAM control unit via the interior CAN. The rear SAM control unit directly actuates the rear window heater via the rear window heater relay. The rear SAM control unit transmits the "Rear window heater active" status to the automatic air conditioning control and operating unit via the interior CAN. The automatic air conditioning control and operating unit directly actuates the light-emitting diode (LED) in the rear window heater button on to confirm the function.

The rear window heater function is terminated when the rear window heater button is reactuated. The LED goes out simultaneously.

Function sequence for automatic switch-on 

To reach the operating temperature of the engine, the CDI control unit or ME-SFI control unit can switch on the rear window heater to increase the engine load for max. t = 15 minutes. In this case the LED in the rear window heater button does not confirm by flashing.

The CDI control unit or ME-SFI control unit transmits the request "Additional consumer ON" on vehicles (up to 28.2.13) via chassis CAN, front SAM control unit and interior CAN to the rear SAM control unit and on vehicles (as of 1.3.13) via the chassis CAN 1, front SAM control unit and interior CAN to the rear SAM control unit. The rear SAM control unit directly actuates the rear window heater via the rear window heater relay.

Function sequence for determining operating time 

The information whether and how long the heated rear window is switched on, is stored in the rear SAM control unit. The timer control (t = 2 min to t = 26 min) depends on the outside temperature, the interior temperature and the vehicle speed. The outside temperature is recorded by the outside temperature sensor. The front SAM control unit directly reads in the signal from the sensor, calculates the outside temperature and sends it to the rear SAM control unit over the interior CAN. The automatic air conditioning control unit transmits the information on the interior temperature to the 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 duty cycle is as follows:

• The normal duty cycle 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, 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 when the rear window heater is active (e.g. at traffic lights, function requirements circuit 15R ON), then the rear window heater is switched off, the operating time does not continue to run here. If, within t = 5 min. circuit 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.

Consumer shutoff function sequence 

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. Heavy power consumers such as the rear window heater are switched off as required in a defined order until the specified on 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 onboard electrical system reaches its normal voltage within the next t = 5 min, the rear window heater function is automatically reactivated.

However, if the consumer shutoff lasts longer than t = 5 min., the LED of the rear window heater button 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, when the on-board electrical system management has canceled the switch-off.

Function sequence for switching off rear window heater with PRE-Safe "activated" 

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 purpose the electronic stability program control unit or regenerative braking system transmit a corresponding signal on vehicles (up to 28.2.13) over the chassis CAN, front SAM control unit and interior CAN to the rear SAM control unit or on vehicles (as of 1.3.13) via the chassis CAN 1, front SAM control unit and interior CAN to the rear SAM control unit. The on-board electrical system management then switches off the rear window heater.