In hydraulic power assist steering systems, a hydraulic pump provides hydraulic fluid for assisting steering action. The pump is directly driven by an engine or an electric motor in the case of an electro-hydraulic power assist system. In these systems, the steering system can be turned to such a degree that the steering rack reaches its travel limits, also referred to in the art as its end stops. If an operator holds the steering system "against" the end stops, especially with force, considerable power can be consumed by the pump, while causing no change in the vehicle steering.
In systems employing engine driven hydraulic pumps, relief valves have been used to divert hydraulic flow to prevent overloading the hydraulic pump. However, due to their considerable expense and complexity, coupled with their lack of complete effectiveness reducing power loss, other solutions would appear to be desirable.
It is therefore a recognized desire among those skilled in the art to provide a hydraulic power assist steering control system in which efficiency is significantly improved by limiting or reducing hydraulic pumping losses otherwise incurred when the system is operated near or against the steering end stops.