SYSTEM / Controller Valve Control (Solenoid Valve Unit) Pressure Increase Selection Control When Trav- eling, Power Digging Control MC drives solenoid valve unit (SG) temporarily, supplies back-pressure to the main relief valve and increases main relief pressure. Auto-Power Lift Control When operating boom raise, MC drives solenoid valve unit (SG) temporarily, supplies back-pressure to the main relief valve and in- creases main relief pressure. Travel Motor Swash Angle Control When shifting the travel mode switch, MC drives solenoid valve unit (SI) and selects the travel mode. Boom Mode Selector Control MC drives solenoid valve unit (SC) and selects the overload relief control valve.
Therefore, as re- lief set-pressure of the overload relief valve at the boom lower side decreases, vibration of the ma- chine is reduced during digging and grading work. Boom Flow Rate Control Valve Control During combined operation of boom lower and arm or boom lower and bucket, MC drives sole- noid valve unit (SF). Therefore, the switch valve of boom flow rate control valve is shifted and oil through the boom cylinder is restricted. When the boom is lowered due to own weight with the regenerative circuit and pressure oil from the pump is used for other actuator, operating speed of actuator increases. T2-1-8