The main working conditions of hydraulic excavator are excavation and loading. For these two working conditions, the lifting speed of the boom is the main factor determining the working cycle time. The faster the lifting speed of the boom, the shorter the cycle time and the higher the working efficiency of the machine. In order to improve the lifting speed of the boom, the excavator not only designs the boom lifting, stick extension and stick retraction actions to be completed by the confluence of two pumps, but also cancels the confluence function of stick retraction when the combined action of the machine requires the rapid lifting of the boom, so as to further accelerate the lifting speed of the boom. This control of the boom is called boom priority mode.
When the excavator is loading or trenching, the boom lifting and stick retraction are generally carried out at the same time. Since the boom and stick have the function of double pump confluence, the two pumps will supply oil to the boom cylinder and stick cylinder at the same time. Under the condition that the engine power allows, whichever action needs more oil, give it more oil. When the boom is lifted quickly, the engine is close to full load. At this time, if you want to lift the boom faster, you have to reduce the oil supply to the stick cylinder, that is, the "boom priority" working mode. However, the "boom priority" working mode does not mean that the boom is always in the "priority state", but only when the boom needs to be lifted quickly, the boom is in the "priority state".
The symptom of "boom priority" function failure is generally that the boom lifting speed becomes slow, and other actions of the machine are still normal at this time. This shows that there is no problem with the common part of the hydraulic system, and the fault causes can be reduced to the main hydraulic system, pilot control part of the boom cylinder and the electronic control part of "boom priority". This article will explain the reasons for the failure of "boom priority" function caused by the electronic control part of "boom priority", which is for reference only.
1. There is a problem with the monitoring board
Press the "boom priority" button on the monitoring board to confirm whether the "boom priority" indicator is on. If the indicator light is not on or stable, replace the monitoring board.
2. The pressure switch cannot send the correct signal to the computer
The computer can display the working state of some switching elements, so as to check the switch. The multimeter can also be used to check whether there is short circuit or open circuit between the pressure switch and the computer, and whether there is a problem with the switch itself. If there is a problem, it should be eliminated. If the multimeter and pressure gauge are used together, the pressure switch can be accurately judged. The closing pressure of the pressure switch shall be 2646 ± 196kpa and the return pressure shall be 2156 ± 196kpa.
3. The fine control solenoid valve 9 does not operate normally
① Check the solenoid circuit. Start the engine and select the "boom priority" working mode. When the boom control handle is in the middle position, the solenoid valve loses power. When the handle is pulled to the boom full speed lifting position, the solenoid valve is energized. If the test results are opposite, indicating that there is a problem with the computer, replace the computer. If the power is always off, the circuit between the solenoid valve and the computer is open or grounded. If there is always power, the circuit is short circuited to the power supply. Use a multimeter to find out the cause of short circuit or open circuit and eliminate it.
② Check the solenoid valve. Check the output pressure of the solenoid valve with a pressure gauge, that is, keep the bucket retracted and lift the boom at full speed. At this time, the output oil pressure of the solenoid valve should be greater than 3727kpa. When the boom handle is in the middle position, the output oil pressure of the solenoid valve shall be less than 294kpa, that is, the return oil pressure of the hydraulic system. If not, check whether the solenoid valve core is stuck or worn and whether the solenoid coil meets the requirements.
4. The action of travel speed solenoid valve is abnormal
The function of this valve is to control the fast and slow states of the travel motor. When power is lost, the travel motor is in low speed. When there is power, the motor is at high speed. However, when it is energized, the output pressure oil will push the limit valve core of the boom confluence valve at the same time, so that the boom confluence valve is in the neutral position, and the function of boom double pump confluence will disappear. In tamping mode, the computer also powers it up to slow down the boom. Therefore, in the "boom priority" state, the travel speed solenoid valve should be dead. Check and troubleshoot the travel speed solenoid valve by comparing the method of checking the fine control solenoid valve.
