Non-working technic system Recently I've been working on a car which when going forward is fast, and when going backward automatically goes slowly. 
Here are two photos of it:



(Just replace the text by "input") As you can see, it uses a switch to change of speed.



Above view.

To make the system work, the motor not only turns the most visible circular gears but also the worm gears. And then these worm gears move the switch and the speed is automatically changed.

Here is my problem: Once the 8-tooth gear between the worm gears  gets to one end, it won't go back. Plus, when it is going from left to right, the motor has a big difficulty turning the worm gears. I want to avoid using a rubber band, so can someone help me?
I would suggest another solution. On the 9396 helicopter the blades can only rotate in one direction, because when a backwards rotation is input, a gear in the transmission line is lifted away from its neighbor automatically. In the instructions it's here (the yellow L-beam lifts up):



Where the gear is lifted away, place another gear for it to interact with there, which would be the transmission for your backwards drive.