Steering gearbox mechanism for automotive vehicles that allows feeding the hydraulic fluid control mechanism of said steering mechanism with pressurized hydraulic fluid, depending on the vehicle's working conditions. Thus, when the vehicle is stopped or running at a very low speed, the hydraulic fluid is sent to the steering mechanism for assistance. At this condition, the user may carry out the driving maneuvers effortlessly. Moreover, when the vehicle is running at high speeds, the assistance is disconnected, by means of deviating the pressurized fluid back to the fluid reservoir, allowing the engine to continue impelling the pump but without assisting the steering mechanism since at those speeds the user can carry out any maneuver without any effort.