I have a problem on my 850. There is a plastic crank connected to the damper motor. The crank is also connected to the flap that shuts/opens the ventilation duct. The problem is that this crank often comes off from the damper motor shaft. I have to put it back once a year at least. I noticed that the amount of shaft rotation does not match the amount of movement of the flap. In other words, when the flap is at "shut" position, the motor still tries to rotate further, and when the flap is at "open" postion, the motor also tries to rotate further, and this additional force seemingly makes the crank come off from the shaft. Is there a way to adjust the amount of rotation of the damper motor?