Is the Terragator steering valve (often called a motor) stock?  Standard valves are non-load reaction which means that the steering wheel does not move (or try to break your arm)  when you hit a big bump off center with the front wheel.  Non-load reaction steering systems do not need counter balance valves and the wheel will not turn when a auto steer system activates.

The only Ag Chem chassis that I know of with a load reaction steering valve is the Ag Gator 1004.  That was used to allow it follow rows.  It would do exactly what you are describing.