Rear window heater, function - GF67.29-P-3002GR
MODEL 166 up to model year 2016
Function requirements, general
- Terminal 15 ON
- No undervoltage or overvoltage
- Consumer shutoff not active
The electronic ignition lock control unit transmits the status of circuit 15 over the interior CAN to the automatic air conditioning control and operating unit.
Rear window heater, general
If the rear window heater is active, the mirror heater is always active too.
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 automatic air conditioning control and operating unit directly reads in the status of the rear window heater button and then actuates the rear window heater via the rear window heater relay. The automatic air conditioning control and operating unit directly 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.
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 (A408).
Function sequence for automatic switch-on
To reach the operating conditions of the engine more quickly, the CDI control unit or ME-SFI [ME] 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 transmit the request "Switch on additional current consumers" to the automatic climate control and operating unit via the chassis CAN 1, electronic ignition lock control unit and interior CAN. The automatic climate control control and operating unit then 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 automatic air conditioning control and operating. The timer control is dependent on the outside temperature and the vehicle speed. The outside temperature is recorded by the outside temperature sensor. The SAM control unit reads in the sensor signal directly, calculates the outside temperature and sends this over the interior CAN to the instrument cluster. The instrument cluster transmits the outside temperature indicated via the interior CAN to the automatic air conditioning control and operating unit. The instrument cluster transmits the vehicle speed via the interior CAN to the automatic air conditioning control and operating 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 (corresponding 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 > 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 when the rear window heater is activated (e.g. traffic light stop, function requirement circuit 15R ON), the rear window heater is also 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.
The traffic stop light time can be parameterized with the diagnosis tester in the automatic air conditioning control and operating with range t = 0 min to t = 10 min. 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.
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 SAM control unit. Heavy power consumers such as the rear window heater are switched off as required in a defined order until the specified minimum on-board electrical system voltage is reached again.
If the SAM control unit detects for t = 1 s an undervoltage of U ≤ 10 V or for t = 1 s an overvoltage of U ≥ 18 V, a request for consumer shutoff is transmitted via the interior CAN to the automatic air conditioning control and operating unit which then switches off the rear window heater. The duty cycle does not continue to elapse during the shutoff period.
If the automatic air conditioning control and operating unit switches off the rear window heater, the LED in the rear window heater button remains on initially. If the on-board electrical system stabilizes its normal voltage within the next t = 5 min, the automatic air conditioning control and operating unit starts the rear window heater again.
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 again. The rear window heater function can only be requested, when the energy management has canceled the switch-off.
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 sends the "PRE-SAFE® status" over the chassis CAN 1, electronic ignition lock and the interior CAN to the SAM control unit. Then SAM control unit then starts consumer shut-off.
Detailed information on consumer shutoff can be found in the separate function description "Energy Management Function" function description.
| Function schematic for electrical rear window heater | PE67.29-P-2051-97NAA |
| Overview of automatic climate control system components | GF83.40-P-9995GR |