Troubleshooting of Slow Movement of CAT 320B Excavator
CAT 320B excavator is used at an altitude of 4850m, with a high and cold climate, low oxygen, and abnormal changes. During the construction process, the machine suddenly showed that the whole machine moved abnormally slowly (such as 13 seconds for one circle without load), while the engine speed was normal and the smoke color was normal when it was running at full load.
After analyzing the fault phenomenon, the cause from the engine can basically be ruled out. It is determined that the fault comes from the hydraulic system. It may be due to a mechanical failure of the hydraulic pump itself causing serious internal leakage, or it may be due to insufficient system pressure. In addition, it may be that the power mode is set incorrectly. The 320B hydraulic excavator is designed with 3 different power modes: heavy load mode, normal mode, and fine mode. The engine power absorbed by the hydraulic system is 100%, 90% and 75% respectively. If the power mode is set to fine mode, it may cause the machine to move slowly.
The pilot pump provides pressure oil for the pilot system. One of the functions of the pilot system is to control the main oil circuit so that it can complete the corresponding action. If the system fails, the pilot system pressure is too low, so that the pilot oil cannot control the main control valve accordingly or the main control valve spool stroke is too small, resulting in insufficient flow in the main oil circuit, which may also cause the machine to move too slowly.
If the main pump electronic control system fails, it may also cause the above-mentioned machine failure phenomenon. For this, you can check it according to the attached figure.
The electronic control system controls the power absorbed by the main hydraulic pump from the engine through the main controller, so that the engine output power matches the power absorbed by the hydraulic pump to adapt to different working environments. The main functions of the electronic control system are: when the machine is heavily loaded, the system makes the hydraulic pump have a larger displacement so that it can output the maximum power that matches the engine; according to the load conditions, the system has 3 different power modes to control the output power of the hydraulic pump in the best state and reduce fuel consumption; when it is unloaded or the load is very small, the system automatically reduces the engine speed and minimizes the main pump displacement, which can achieve energy saving, consumption reduction and noise reduction.
Working process of the electronic control system: The main component of the electronic control system-the main controller receives input signals from various aspects (such as the position of the throttle knob, the power mode set by the monitor, the pressure signal sent back by the main pump pressure sensor, the pressure signal sent back by the position of the throttle motor, the position sensor signal of the throttle motor, and the engine speed sensor signal, etc.). After processing these input signals, the main controller outputs instructions to the throttle motor of the engine and the main control component of the main hydraulic pump-the proportional pressure reducing valve. The corresponding action of the throttle motor can change the engine speed, and the action of the proportional pressure reducing valve can adjust the pump displacement. Under the control of the main controller, the two can achieve the best match between the engine and the hydraulic system. Therefore, as long as there is a problem in any part of the electronic control system, the machine may not work properly.
The hydraulic pump's own adjustment system has a fault, which may be caused by a fault in the hydraulic pump regulator or the control signal oil pressure.
The method to troubleshoot the problem is to conduct corresponding inspections in sequence according to the above analysis. As we all know, if the hydraulic system has a fault, the engine cannot be started easily, because it may cause further damage to some hydraulic components, thereby polluting the entire hydraulic system. Therefore, the oil level of the hydraulic oil tank was checked first, and the oil level was found to be normal; then the main return oil filter element, the housing oil drain filter element and the pilot oil filter element in the hydraulic oil tank were taken out, and they were cut open one by one to check whether there were foreign objects such as copper chips. The inspection result showed that all three filters were normal. Therefore, a new filter element was installed, the engine was started and operated at idle speed, and the sound of the hydraulic pump was listened to. As a result, there was no abnormal sound from the hydraulic pump; then the system pressure was measured, and the overflow pressure of the main safety valve was 34.3MPa, that is, the system pressure was normal. Therefore, the conclusion that the hydraulic pump was mechanically damaged can be ruled out.
Reset the power mode to heavy load mode, test the machine, the action is still slow, so the possible cause of the failure due to incorrect power mode setting can be ruled out.
Test the pilot pressure, the value is 4.1 MPa, which is normal. Therefore, the possibility of failure of the pilot system can be ruled out.
Then, verify whether the electronic control system is faulty. The judgment method of this fault is relatively simple, because the system is equipped with a manual backup switch. When the main controller or monitor has a problem, the backup switch is turned to the manual position to allow the machine to continue working. Operate the machine according to the above method, and the action is still slow. Therefore, it can be preliminarily considered that the main controller is normal; then turn the switch back to the automatic position, and then check the three light-emitting diodes on the main controller. All three diodes are green, which proves that the main controller and the communication line between the main controller and the monitor are normal.
The monitoring system on Caterpillar's excavator has a strong self-test and data memory function. Therefore, we can understand the status and data of other electronic components through the monitor to determine whether they are working properly. After entering the password from the monitor, enter the maintenance mode of Caterpillar, first check the status of various switches, the results show that all switches are in normal position; then check the rated speed of the engine and the pressure of the pump when the main safety valve is discharged. The monitor shows that the data is 1800r/min and 34.3 MPa, both of which are normal values; check the pressure change range of the proportional deceleration valve. During normal operation, the monitor shows that the pressure change range of the proportional pressure reducing valve is 3.2~0.76 MPa, that is, the pressure change of the proportional pressure reducing valve is normal. Then check the actual fault code and the previously recorded fault code. The monitor does not show any fault code. So far, it can be confirmed that the problem is not in the electronic control system.
Finally, check the hydraulic pump's own adjustment system. The regulators of the upper and lower pumps are separate. There are problems when testing the left and right single-sided travel. Since the possibility of failure of both pump regulators is small, we will put it aside and check from the outside. There are two external control signals for the hydraulic pump regulator: one is the pressure PN from the central oil channel of the main control valve, one for each upper and lower pumps; the other is the pressure PS from the proportional pressure reducing valve that controls the upper and lower pumps at the same time. First, measure the pressure PN values of the upper and lower pumps respectively, and get the pressure of 4.07 MPa when the main control valve is in the middle position. No matter what action is taken, PN drops to 0.35 MPa, and the PN value is normal. Then measure the PS value. Although the variation range of the PS value has been checked through the monitor, and its pressure variation value is normal, this is not necessarily accurate, because there is no pressure sensor on the proportional pressure reducing valve to directly input the signal to the main controller. The pressure value displayed on the monitor is converted by the main controller through the analysis of the output current signal of the proportional pressure reducing valve, that is, this pressure value corresponds to the strength of the output current of the proportional pressure reducing valve by the main controller. For this reason, connect the pressure gauge to the PS pressure side point on the pump body, start the engine and operate normally, and the pointer of the pressure gauge is always around 1.2 MPa. This indicates that the valve core of the proportional pressure reducing valve has been stuck and cannot move according to the output signal of the main controller, so it means that the fault is on the proportional pressure reducing valve. Replace the new proportional pressure reducing valve, calibrate the new valve on the monitor, and retest the machine. The result is normal. Check the stuck proportional pressure reducing valve. It is found that colloid has been precipitated in the hydraulic shaft oil, proving that the hydraulic oil has deteriorated and cannot be used. The reason for the premature deterioration of the hydraulic oil is the poor working environment of the machine, high cold and lack of oxygen, and rapid temperature rise.
From the troubleshooting of this fault, we can get some inspiration: the electronic control system of the excavator also has limitations. When using the machine's own repair mode for fault diagnosis, it is necessary to conduct a comprehensive analysis from multiple aspects and angles, and make comprehensive use of various detection methods. The front and back are mutually verified, and it is best not to use a single point for the whole. Source: Second-hand excavator https://www.lh-usedexcavator.com/
