Rear Window Defroster

Using the E38 as an example for this test, the rear window defroster operating strategy depends upon ambient temperature.

• If ambient temperature is < 5°F (-15°C), pressing the rear window defroster but ton (ignition switch in "Run"), requests the control module to activate the defroster for 17 minutes, and then switch it off.
• If the ambient temperature is > 5°F (-15°C), the control module activates the defroster for only 10 minutes before switching it off.
• Reactivating the defroster by pressing the button again will give an additional 5 minutes of defrosting, regardless of ambient temperature.
• If the ignition is switched "OFF" during defroster operation, the timer keeps operating and the defroster is deactivated. If the ignition is switched "ON" again with time remaining, the defroster is automatically reactivated until the timer runs out.
• If the battery voltage drops bellow 11.4 volts, the defroster automatically switched "OFF", and will resume operation when the voltage rises above 12.2 volts.
  Fig 1: Rear Window Defroster Circuit Diagram

  Courtesy of BMW OF NORTH AMERICA, INC.