Known fault patterns for the electrical swing-out trailer coupling

• The trailer tow hitch is inoperative, a Check Control fault message (trailer tow hitch not locked) occasionally appears while driving. Affected vehicles: E90 and E91. 
  • Description 

    The customer complains that the electrical swing-out trailer tow hitch is inoperative.

  • Cause 

    Water ingress in the control unit for unlocking the trailer tow hitch, caused by obstructions in the panorama glass roof water drain hose, leaks in the fuel filler flap lock, Touring side window, for example.

• Electrical swing-out trailer coupling without function 
  • Description 

    The customer complains that the electrical swing-out trailer tow hitch is inoperative. The trailer coupling cannot be released electrically and does not swing out. The fault memory contains no fault for the trailer coupling release. However, other faults can be stored. The indicator light in the button for the trailer coupling release flashes slowly red (2 Hz).

Fig 1: Identifying Drive Mechanism

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 2: Identifying Drive Mechanism Micro-Switch

Courtesy of BMW OF NORTH AMERICA, INC.

• Cause 

A possible cause for this fault pattern can be that water has entered through a faulty (squashed) sealing ring into the drive mechanism of the trailer coupling. Water can also enter the drive mechanism at the cable entry point Another cause can be a failure of the micro-switch. This micro-switch detects the end position of the ball end. If this microswitch is defective, the trailer module does not detect the position of the ball end. The ball end can then only be swung out with emergency activation. A short circuit caused by water entry can be the cause of this fault.

Fig 3: Identifying Electrical Swing-Out Trailer Coupling Components

Courtesy of BMW OF NORTH AMERICA, INC.