Outback S -- JD9400T

Pictured is the steering unit on a 9000T series John Deere. We removed the unit and upon partial disassembly, found the steering potenitometers (pots). These are variable resitors similar to the volume knob on your basic stereo. (For an explanation of how the pots work, click here.) What we intended to do was mechanically turn the pots from the bottom. When you turn the steering wheel in the cab, the armatures of the pots turn. If the body of the pot is turned and the armature stays put. (The top portion of the steering unit is a spring detent canister, which keeps the steering wheel spring-loaded in the center postion.) So, if the pot bodies turn, the tractor turns.

We removed the pin holding the pots in postion. Without that pin, the pot stack turn freely on their armatures. The stack is glued together so they all move as one and don't move out of calibration from each other. The stack is also sealed at the top and bottom with weather sealant foam.

Now that the pots spun freely, we could turn them using a servo. We built an aluminum mount for the servo, then connected the pots to the servo with a linkage.

During operation, the PA reads an imbalance of light between it's two sensors and commands the servo to rotate the pots. The tractor then turns right or left, depending on which steering indicator LED is illuminated. If the GPS unit does not illuminate a right or left LED, the PA automatically centers the servo therefore, the steering pots.

There are 3 pots in the stack, two giving identical readings, the third giving an opposite reading. As two pots increase in signal, the third decreases as the steering wheel turned in one direction. (The signals react opposite in the other direction, respectively.) The steering computer (the SSU) processes these signals and causes the tractor to turn. We tried to intercept these signals and influence them but the system just gave us errors.