Rear Window Defogger

Function Requirements, General

Ignition switch ON
No undervoltage or overvoltage
Consumer shutoff not active

NOTE: The electronic ignition switch control unit transmits the status of ignition switch status signal to the A/C auto amp. via body CAN communication 1.

Rear Window Defogger, General

If the rear window defogger is active, the door mirror defogger is always active as well.

The rear window defogger function comprises the following partial functions:

Manual on/off function
Automatic switch-on function
Determining switch-on duration function
Stop light shutoff function
Consumer shutoff function

Manual On/off Function

The rear window defogger is switched on and off with the rear window defogger switch in the A/C control. The A/C auto amp. directly reads in the status of the rear window defogger switch and actuates the rear window defogger directly via rear window defogger relay.

The A/C auto amp. directly actuates the indicator lamp in the rear window defogger switch.

The rear window defogger function is terminated when the rear window defogger switch is pressed again. The indicator lamp goes out simultaneously.

When adjustments affecting the setting temperature, air distribution and air flow are made on the A/C control with the rear window defogger switched on, the symbol for the rear window defogger and the corresponding settings appear momentarily on the Audio/Navigation display.

Automatic Switch-on Function

To reach the engine operating state more quickly, the ECM transmits switch on additional current consumers request signal via chassis CAN communication, electronic ignition switch control unit and body CAN communication 1 to the A/C auto amp.

The A/C auto amp. then switches on the rear window defogger to increase the engine load for max. 15 minutes. In this case the indicator lamp in the rear window defogger switch does not confirm by flashing.

Determining Switch-on Duration Function

The information whether and how long the rear window defogger is switched on, is stored in the A/C auto amp. The timer control (2 minutes to 26 minutes) depends on the outside temperature and the vehicle speed. The outside temperature is recorded by the ambient sensor. The BCM directly reads in the sensor signals, calculates the outside temperature and transmits it to the combination meter over the body CAN communication 1. The combination meter transmits the displayed outside temperature signal and vehicle speed signal via body CAN communication 1 to the A/C auto amp.

The duty cycle is as follows:

The normal duty cycle is 15 minutes.
At outside temperatures of less than 5 deg.C (41 deg.F), the duty cycle is extended by 30 seconds per 1 deg.C (34 deg.F) drop in temperature, up to a maximum of 6 minutes [This corresponds to an outside temperature of -7 deg.C (19 deg.F)].
At outside temperatures of less than 0 deg.C (32 deg.F), a speed-dependent time factor is also taken into consideration.

If, during the heating phase, the vehicle is driven faster than:

60 km/h (37 MPH) or more, 1 minute is added to the switch-on time.
70 km/h (43 MPH) or more, 2 minutes is added to the switch-on time.
80 km/h (50 MPH) or more, 3 minutes is added to the switch-on time.
100 km/h (62 MPH) or more, 5 minutes is added to the switch-on time (maximum value).

NOTE: At outside temperatures of 50 deg.C (122 deg.F) or more, the switch-on time of the rear window defogger is limited to 2 minutes to protect the rear window.

Stop Light Shutoff Function

If the engine is switched off when the rear window defogger is activated (e.g. traffic light stop, function requirement ignition switch ON), the rear window defogger is also switched off. The switch on time is not decreased. If, within 5 minutes, ignition switch is not switched back on, the rear window defogger is reactivated.

After expiration of the traffic light stop time without switching on ignition switch ON, the rear window is switched off completely.

NOTE: During engine start, the rear window defogger is switched off momentarily. The rear window defogger is switched back on 10 seconds after the engine is started. The traffic stop light time can be parameterized with the CONSULT in the A/C control (A/C auto amp.) with range 0 minute to 10 minutes.

Consumer Shutoff Function

To prevent battery discharge in all driving cycle phases, the state of charge is continuously evaluated by the BCM. Heavy power consumers such as the rear window defogger are switched off as required in a defined order until the specified minimum on-board electrical system voltage is reached again.

If the BCM detects for 1 second an undervoltage of 10 V or less or for 1 second an overvoltage of 18 V or more, switch on additional current consumers request signal is transmitted via body CAN communication 1 to the A/C auto amp. which then switches off the rear window defogger. The duty cycle does not continue to elapse during the shutoff period.

If the A/C auto amp. switches off the rear window defogger, the indicator lamp in the rear window defogger switch remains on for the time being. If the on-board electrical system reaches its normal voltage within the next 5 minutes, the A/C auto amp. starts the rear window defogger function.

However, if the consumer shutoff lasts longer than 5 minutes, the indicator lamp of the rear window defogger switch is also switched off.

If switch-off continues and the function is requested again with the rear window defogger switch, the indicator lamp comes on for 1.5 seconds and then goes out.

The rear window defogger function can only be requested, when the energy management has cancelled the switch-off.