Abstract: A steering control hookup for use in connection with a mobile assembly comprised of a conventional tractor in which the normal, steerable front wheel and axle structure is in a raised, non-ground engaging disposition, and a grader blade attachment provided with a ground-engaging front wheel and axle unit having a steering apparatus. The steering hookup includes an arrangement of selectively shiftable hydraulic valves interposed in a power steering system of the tractor in order to render the steering apparatus of the attachment responsive to actuation of the power steering system while at the same time rendering a steering mechanism associated with the tractor front wheel end axle structure inoperative, thus making the turning movements of the mobile assembly responsive solely to the operation of the steering apparatus of the attachment.