Rear window heater, function - GF67.29-P-3002RF

Model 205, 253

Fig 1: Rear Window Heater Function 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 over 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 function 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

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 signals from the rear window heater button are read in over the air conditioning control LIN by the climate control unit. The climate control unit sends the signals over the interior CAN to the rear SAM control unit. This switches on the rear window heater over the rear window heater relay in the rear fuse and relay module. To confirm the function, the rear SAM control unit transits the status of the rear window heater via the interior CAN, climate control unit and climate control LIN to the front climate control unit, which actuates the LED in the rear window heater button.

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

Function sequence for automatic switch-on

To reach the operating temperature of the engine more quickly, the rear window heater is switched on to max. t = 15 min. to increase the engine load. In this case the LED in the rear window heater button does not confirm by flashing.

The CDI control unit (diesel engine) or the ME-SFI [ME] control unit (gasoline engine) sends the switch-on request via engine CAN, powertrain control unit, chassis FlexRay, electronic ignition lock 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.

The rear window heater function can be switched on and off in addition by the preclimate control function (model 205 (except 205.012/047/147/212/247) with CODE 577 (Remove activation of heater and ventilation systems) or model 205.012/047/147/212/247).

Function sequence for determining operating time

The switch-on duration of the rear window heater is stored in the rear SAM control unit. Depending on the outside temperature, inside temperature and vehicle speed, it is t = 2 to t = 26 min.

The outside temperature is recorded by the outside temperature sensor. The front SAM control unit reads in the sensor signal directly, uses it to calculate the outside temperature, and sends it via the interior CAN, electronic ignition lock control unit and user interface CAN to the instrument cluster. The instrument cluster sends the outside temperature displayed via the user interface CAN, electronic ignition lock control unit and interior CAN to the rear SAM control unit. The interior temperature is recorded by the interior temperature sensor with integrated fan in the overhead control panel control unit. The overhead control panel control unit transmits the interior temperature to the front SAM control unit via the roof LIN. The front SAM control unit sends this via the interior CAN to the climate control unit. The climate control unit sends the value over the interior CAN to the rear SAM control unit.

The instrument cluster transmits the vehicle speed via user interface CAN, electronic ignition lock control unit and interior CAN to the rear SAM control unit.

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 (this corresponds to an outside temperature of T = -7 °C).
At outside temperatures of T < 0 °C, a speed-dependent time factor is also taken into consideration.

The following times are added up for the operating time during the heating phase dependent on the vehicle speed:

Vehicle speed v > 60 km/h: t = 1 min
Vehicle speed v > 70 km/h: t = 2 min
Vehicle speed v > 80 km/h: t = 3 min
Vehicle speed v > 90 km/h: t = 4 min
Vehicle speed v > 100 km/h: t = 5 min (maximum value)

At outside temperatures of T > 50 °C, the switch-on 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 activated (e.g. traffic light stop, function requirement c. 15R ON), the rear window heater is switched off. The switch on time is not decreased here. 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 circuit 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 lights stop time can be parameterized using the diagnostics tester in the rear SAM control unit in a range from t = 0 min to t = 10 min.

Consumer shutoff function sequence

To ensure vehicle onboard electrical network stability during all driving cycle phases, the front SAM control unit continuously evaluates the voltage in the on-board electrical system. In event of overload situations, the rear SAM control unit switches back or off various comfort functions such as rear window heater (energy management). The operating time is not decreased. The LED in the rear window heater remains switched on.

If the on-board electrical system voltage is stabilized within t = 5 minutes, the rear SAM control unit switches the rear window heater back on. The rear window heater is then operated for the remaining operating time.

However, if the consumer shutoff lasts longer than t = 5 min., the LED of the rear window heater button is also switched off. For this purpose the front SAM control unit sends the status of the consumer shutoff via interior CAN to the rear SAM control unit.

If the consumer shutoff continues and the rear window heater button is pressed again, the LED in the rear window heater button comes on for t = 1.5 s and then goes out.

The rear window heater can only be reactivated, when the energy management for the consumer shutoff has canceled the switch-off.

Detailed information about the consumer shutoff can be found in the "Energy Management Function" function description.

Vehicles with CODE 299 (PRE-SAFE® system):

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 transmits a signal to the front SAM control unit via chassis FlexRay, electronic ignition lock control unit and interior CAN. The energy management then switches off the rear window heater function.

	Function schematic for electrical rear window heater		PE67.29-P-2051-97FBA
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