BMW

 
Battery disconnecting procedure on vihecles with MK3, MK4 navigation computer.
BMW GPS MK4/ MKIV DVD
 
    The MK3, MK4 Navigation computer powers down in approximately 60 seconds after the ignition has been turned OFF (red LED on the computer goes OFF). If the power supply to the MK3, MK4 unit is interrupted (by removing/disconnecting the computer from the vehicle, or by disconnecting the vehicle's battery) in the time period of 55 to 57 seconds after the ignition has been turned OFF, the power-down procedure ("erase and save data in Non Volatile RAM") can be corrupted. This will result in a total failure of the MK3 computer (red LED off, CD eject function inoperative, board monitor screen permanently blank, and no sound from the audio system). In such a case, the navigation computer needs repair.
    On vehicles with MK3, MK4 navigation system, prior to disconnecting the battery or removing (disconnecting) the Nav computer, make sure the MK3 Nav computer is in the power-down mode (red LED on the unit is OFF).