I have a Viper Pro and Smartrax on a CIH tractor.  It suddenly stopped disengaging the autosteer by turning the steering wheel.  I can still disengage by pressing the foot switch or by touching the "steering wheel" on the screen.  It has the disengage valve with the hand knob with a number scale from about 0-100 or so.  It has operated flawlessly for many years and stopped working all at once.  Is this a hardware problem with the valve, or a setting/calibration issue?  Where do I start?