a result of the reduced pump displacement due to the forcing of lead screw 36 u2032 to the left the opposite result can be achieved by interchanging the positions shown in fig4 of cam positioning motor arrangement 36 and return force arrangement 47 of fig2 and 3 with respect to cam ring 44 this arrangement is shown in a schematic representation of a pump control system 30 u2032 u2033 in the cross section view of fig5 with vane pump 31 and the positionally interchanged cam positioning motor 36 and return force arrangement 47 here if return force arrangement 47 cannot force lead screw 36 u2032 through cam ring 44 to the right in a system failure that leaves drive shaft 33 continuing to rotate pump rotor 41 at the same angular rate such a failure again will result in no change in the flow rate of fuel pumped by pump 31 to outlet 34 thereof if alternatively return force arrangement 47 can force lead screw 36 u2032 to the right in a system failure that leaves drive shaft 33 continuing to rotate pump rotor 41 at the same angular rate such a failure will nevertheless result in a significantly increased rate of flow of fuel pumped by pump 31 to outlet 34 thereof as a result of the increased pump displacement due to the forcing of lead screw 36 u2032 to the right.