Control system of hydraulic construction machinery

A control system of hydraulic construction machinery comprising a prime mover, revolution number control means for controlling number of revolutions of the prime mover, at least one variable-displacement hydraulic pump driven by the prime mover, displacement volume control means for controlling displacement volume of the hydraulic pump, at least one actuator driven by pressurized fluid delivered by the hydraulic pump, maximum revolution number altering means associated with the revolution number control means for altering the maximum number of revolutions of the prime mover between at least a first maximum value and a second maximum value greater than the first maximum value, maximum displacement volume altering means associated with the displacement volume control means for altering the maximum displacement volume of the hydraulic pump between at least a first maximum value and a second maximum value geater than the first maximum value and information provider means for providing information on the operation mode of the actuator, so that the maximum revolution number altering means and maximum displacement volume altering means are controlled by an output signal of the information provider means to provide a maximum number of revolutions and a maximum displacement volume which suit the operation mode indicated by the output signal, characterized in that the system further comprises control means for controlling said maximum revolutions number altering means and said maximum displacement volume altering means such that a product of the maximum displacement volume and number of revolutions becomes at a constant always at the same time when the maximum displacement volume of said hydraulic pump is varied in response to an output signal of said information provider means, whereby a change in the quantity of pressurized fluid delivered by said hydraulic pump can be essentially avoided when the maximum displacement volume of the hydraulic pump is altered.