Rear window heater, function

P67.29-2150-79

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 switching on and off:

 Switching on automatically

 Determination of operation time

 Traffic light stop (except model 253.99)

 Consumer shutoff

Manual switching on and off:

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 control unit. The climate control 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 transmits the status of the rear window heater via the interior CAN, climate control control unit and air conditioning operating LIN to the air conditioning control panel. The air conditioning control panel 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.

Switching on automatically

Except model 253.99:

To reach the operating temperature of the engine more quickly, the rear window heater is switched on for max. 15 minutes 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 or the ME-SFI [ME] control unit transmits a switch-on request to the rear SAM control unit via the engine CAN, powertrain control unit, suspension FlexRay, electronic ignition lock control unit and interior CAN. The rear SAM control unit directly actuates the rear window heater via the rear window heater relay.

 Model 205 (except 205.012/013/047/053/147/212/213/247/253)
with code 577 (Remote activation of heating and ventilation systems)
Model 205.012/013/047/053/147/212/213/247/253, 253.354/954/99
The rear window heater function can also be switched on and off via the pre-entry climate control function.

Determination of operation time

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

The outside temperature is recorded by the outside temperature sensor. The front SAM control unit reads in the signal from the outside temperature sensor directly and calculates the outside temperature. The front SAM control unit then transmits the outside temperature 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. In vehicles up to model year 2019, the overhead control panel control unit transmits the vehicle interior temperature via the roof LIN to the front SAM control unit. The front SAM control unit sends this via the interior CAN to the climate control control unit. In vehicles from model year 2019, the overhead control panel control unit transmits the vehicle interior temperature via the interior CAN to the climate control control unit.

The climate control 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 15 min.

 At outside temperatures below 5 °C, the operating time is extended by 30 s for each 1 °C that the outside temperature is lower. However, it is not extended beyond a maximum of 6 min (this corresponds to an outside temperature of ­7 °C).

 At outside temperatures below 0 °C, a speed-dependent time factor is also added.

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

 Vehicle speed above 60 km/h: 1 min

 Vehicle speed above 70 km/h: 2 min

 Vehicle speed above 80 km/h: 3 min

 Vehicle speed above 90 km/h: 4 min

 Vehicle speed above 100 km/h: 5 min (maximum value)

 At outside temperatures above 50 °C, the operating time of the rear window heater is limited to 2 min to protect the rear window.

Traffic light stop (except model 253.99)

If the engine is switched off when the rear window heater is activated (e. . traffic light stop, function requirement c. 15R ON), the rear window heater is switched off. The switch on time is not decreased here. If circuit 15 is switched back on within 5 min, 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 after 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 0 min to 10 min.

Consumer shutoff

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 various convenience functions, e.g. rear window heater), to a lower level or off (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 5 min, the rear SAM control unit switches the rear window heater back on. The rear window heater is then operated for the remaining operating time.

If consumer shutoff lasts longer than 5 min, the LED in 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 1.5 s. Then it goes out again.

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.