A guidance and vehicle control system for automatically steering a vehicle, such as an agricultural vehicle or a tractor, through a field. The system includes a GNSS receiver and antenna for determining the vehicle's instantaneous position, a guidance CPU, and an automatic steering subsystem integrated with the vehicle's electrical power system. The automatic steering subsystem can be interfaced with the steering column of the vehicle, and mechanically activates the steering column, thereby steering the vehicle according to instructions received from the CPU based upon the vehicle's position and a predetermined path. An interrupt element, such as a wheel movement sensor or a slip gear, may be interfaced with the automatic steering subsystem to allow for manual steering override of the automatic steering control.
