The engine of a Hitachi EX200-2 hydraulic excavator was overhauled after 9850h of operation. After the overhaul, the engine operated well during the separate commissioning, but the following symptoms occurred during the commissioning after the main engine was installed: the engine emits black smoke when idling, and it seems that the engine has been loaded; When the engine is running at high speed, the black smoke is thicker, and as long as the operating handle on the engine is slightly pulled, the engine will shut down immediately.
The analysis shows that the engine runs well when it is tested separately after overhaul, indicating that the repair quality is guaranteed. Therefore, the hydraulic pump connected to the engine was checked: according to the fault phenomenon of "the engine will shut down immediately as long as the operating handle is slightly pulled when the engine is running at high speed", we infer that the swashplate or plunger of the hydraulic pump is stuck, so that the flow of the hydraulic pump can not change with the change of external load, resulting in the engine being stalled. Then, we disassembled and inspected the hydraulic pump (the engine was started after the hydraulic pump was disassembled, and the engine operated normally, which proved once again that the engine itself had no fault). After the disassembly and inspection, no abnormal conditions were found, the swashplate and plunger moved freely, and the wear was within the specified range, so the cause of the fault caused by the hydraulic pump was eliminated.
Since there are no abnormalities in the engine and hydraulic pump, we infer that the cause of the fault should be in its control part, and the problem should be in the electrical control part. The electrical system of the machine includes main circuit, monitoring circuit and control circuit. The main circuit includes engine starting, battery charging and light on operation; The monitoring circuit includes sensors, switches, controllers and monitors, which can transmit all working states of the machine; The operation of the machine controlled on the control circuit is completed by sensors, switches, controllers and actuators. On the EX200-2 hydraulic excavator, the whole machine is equipped with 6 microcomputers, which are respectively installed on the monitor controller, EC (engine controller), PVC (pump valve controller), the first switch panel, the second switch panel and engine accessories to monitor the corresponding actions of each part of the machine.
EC and PVC can control engine speed, hydraulic pump flow and the movement of control valve compensation valve. EC and PVC are connected by circuit and exchange signals with each other to change the control state of the device and improve the working efficiency of the machine. PVC receives signals such as engine speed, pump pressure, pump displacement angle and pilot pressure. After FVC analysis, all signals will send control signals to the pump displacement solenoid valve, so as to adjust the pump flow and make the machine give full play to the best performance. Therefore, according to the fault symptoms, the circuit parts of PVC and EC were checked: during the inspection, it was found that the red light of PVC diode was not on (it should flash during normal operation), and it was preliminarily concluded that PVC was not working normally. When inspecting the PVC related circuit, it is found that the circuit is not normally connected; When checked with a multimeter, it was found that a fuse in the electrical cabinet of the circuit had been burned. When the engine is started again after replacement, the PVC red light still does not flash, so it is concluded that the PVC has been damaged. Restart the engine after replacement, and the fault has been eliminated.
Afterwards, we analyzed the cause of the fault again, and considered that the engine was not protected for some disconnected electrical circuit connectors when it was lifted from the main engine for overhaul, resulting in some oil and water entering the connector, but did not cause attention when installing the engine wiring, resulting in circuit short circuit and burning the insurance: after the insurance was burned, The maintenance personnel did not replace the fuse of the same model as required (mistakenly replaced the fuse of 1A with the fuse of 10a), so that the insurance did not play a protective role, thus burning out the PVC computer board. Through the treatment of this fault, we have paid enough attention to the replacement of fuse, and basically put an end to the wrong practice of replacing fuse according to specifications and using copper wire instead.
