Many times the dash vent mechanisms seems to self-destruct, breaking the attaching point on the flap, and/or "hunting" for its proper position. Inside the motors for the flaps are limiter gears or cams that eventually break, letting the motors move the flap to, and past, it's breaking point.
Here's the shrapnel from decaying plastic:
Here's the center door flap after being abused by the motor:
Here's approximately what the cams are supposed to look like. This gear was made from nylon stock.
In the picture below, I made the cam on the left from a piece of aluminum. The one on the right is made from nylon. No casting material, such as Alumilite, is strong enough for the force.
The actuator on the left is for the center door; that gear is made from aluminum stock. The one on the right, made from nylon, is for the left and right doors. This one has teeth on the cam for an additional gear-driven sensor; that gear is circled in red.
