Mercedes-Benz M-Class Owner's Manual page 235

Towing a trailer
232
Problems with trailer towing
Problem
The released ball cou-
pling cannot be
engaged.
The ball coupling does
not unlock, even
though:
the vehicle is station-
R
ary.
no trailer cable is
R
connected.
the release knob to
R
unlock the ball cou-
pling has been pulled
for more than a sec-
ond.
Possible causes/consequences and M Solutions
Visit a qualified specialist workshop.
X
The on-board voltage is insufficient.
Start the engine.
X
If the ball coupling still does not unlock:
Visit a qualified specialist workshop.
X