http://www.accutrak.ca/bins/content_page.asp?cid=3-99

We have used it on a 9400 tractor in the past with  JD gps system. Works good. Now we use it on our Macdon swather,with JD gps. It ties into the wheel motors and "steals" a little bit of oil that goes to the wheels,and returns it to the hydraulic tank,through a "pop pop" valve. Thats how it steers and it works good.  Except if you have fairly good hills, it will tend to free wheel down the hill and not steer too good.