A Hitachi Ex200-2 hydraulic excavator underwent engine overhaul after working for 9850h. After overhaul, the engine worked well when tested alone, but the following symptoms appeared when the engine was tested after being replaced: the engine emits black smoke when idling, and the sound seems to sound The engine is already under load; the black smoke emitted from the engine at high speed is thicker, and as long as the operating handle on the machine is slightly turned, the engine will immediately stop.
Analysis believes that the engine works well when 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 immediately stop when the engine is turned at a slight speed when the engine is running at high speed", we infer that the swash plate or plunger of the hydraulic pump is stuck. Therefore, the flow of the hydraulic pump cannot be changed with the change of the external load, which leads to the engine being stalled. Immediately, we dismantled the hydraulic pump (the engine was operated after the hydraulic pump was removed, and the engine was operating normally, once again proved that the engine itself was fault-free). After the dismantling, no abnormal conditions were found, and the swash plate and plunger were free to move and worn The conditions are all within the specified range, thus eliminating the cause of the failure caused by the hydraulic pump.
Because neither the engine nor the hydraulic pump has any abnormal conditions, 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 has a main circuit, a monitoring circuit and a control circuit. The main circuit includes the operation of engine starting, battery charging and lighting; the monitoring circuit includes sensors, switches, controllers and monitors, which can transmit the full working state of the machine; The operation of controlling the machine on the control loop is accomplished by sensors, switches, controllers and actuators. On the Ex200-2 hydraulic excavator, the whole machine is equipped with a total of 6 microcomputers, which are respectively installed in the monitor controller, EC (engine controller), PVC (pump valve controller), the first switch panel, the second switch panel and On the engine attachment, it is used to monitor the corresponding action of each part of the machine.
EC and PVC can control the engine speed, hydraulic pump flow and control valve compensation valve movement, EC and PVC are connected by a circuit, 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 all signals are analyzed by FVC, the control signal is sent to the pump displacement solenoid valve in order to adjust the flow of the pump and make the machine play its best. performance. Therefore, according to the symptoms of the fault, the circuit parts of PVC and EC were investigated: during the inspection, it was found that the red light of the PVC diode did not light (should flash during normal operation), and it was preliminarily concluded that the PVC was not working properly. When checking the PVC related circuit, it was found that the circuit was not properly connected; when using a multimeter to check, it was found that a fuse in the electrical cabinet of the circuit was burnt out. When the engine was started after replacement, the red PVC lamp did not flash, so it was concluded that the PVC was damaged. Restart the engine after a new one, the fault has been eliminated.
Afterwards, we analyzed the cause of the failure again, and believed that the engine was not protected from the partially disconnected electrical circuit connector when the engine was hung from the host for overhaul, which caused some oil and water to enter the connector and the engine was installed. No attention was paid to the connection, which caused a short circuit in the circuit and burned the insurance: after the insurance was burned, the maintenance personnel did not replace the fuse of the same type as the regulations (the 1A fuse was mistakenly replaced by the 10A fuse), so that the insurance did not work Protection, so as to burn out the PVC computer board. Through the treatment of this fault, we have paid enough attention to the replacement of the fuse, basically eliminating the wrong practice of not replacing the fuse and replacing it with copper wire when replacing the fuse.
