After 2104 hours of operation, an SK210-8 crawler excavator suddenly exhibited jitter in all actions. The jitter was obvious at medium and low speeds, weakened above 1800 r/min, and basically disappeared at maximum speed.

 

　　Analysis showed that the jitter was mainly caused by two factors: first, pulsation in the output flow of the hydraulic pump; second, unstable diesel engine speed.

 

　　1. Troubleshooting of hydraulic pump flow pulsation

 

　　Pulsation in the output flow of the hydraulic pump is usually caused by severe wear of individual plunger pairs or a fault in their flow control mechanism.

 

　　(1) Check the hydraulic pump

 

　　First, the hydraulic oil return filter element was checked, and then the oil drain port at the bottom of the hydraulic pump body was opened. No metal impurities were found, and the hydraulic oil color showed no obvious deterioration or contamination, indicating that the hydraulic pump had not suffered significant wear.

 

　　During testing of the machine's actions, it was found that when the right travel pump or the left travel pump supplied oil independently, jitter occurred at medium and low travel speeds. Analysis suggested that it was almost impossible for both hydraulic pumps to be damaged simultaneously and to the same degree. In addition, since the machine had been in continuous use without worsening of the jitter (generally, severe wear between the plunger and cylinder block would noticeably worsen symptoms within a short time), the possibility of severe wear between the plunger and cylinder block was ruled out.

 

　　(2) Check the hydraulic pump flow control mechanism

 

　　The output flow of the hydraulic pump is controlled by the ITCS system, which involves components such as high-pressure and low-pressure sensors, the CPU central processing unit, pump proportional valves, and pump regulators.

 

　　First, the self-diagnostic system of the ITCS was used for testing. When the excavator performed any action, the pressure values detected by the high- and low-pressure sensors were normal and stable, ruling out the possibility of jitter caused by the high- and low-pressure sensors. Then the proportional solenoid valves were tested, and no abnormalities were found.

 

　　When the excavator's jitter persisted during operation, the "powershift" data in the ITCS self-diagnostic system was found to be abnormal. The "powershift" data indicates that the CPU central processing unit actively adjusts the hydraulic system flow reduction to match the decrease in diesel engine output power. Therefore, it was preliminarily judged that the cause of the jitter was a decrease in diesel engine output power, which caused speed fluctuation during the "powershift" adjustment process, thereby causing pulsation in the hydraulic pump output flow.

 

　　2. Troubleshooting of diesel engine output power decrease

 

　　The decrease in diesel engine output power may be caused by a fault in its fuel supply system or in its external load detection device.

 

　　(1) Check the fuel supply system

 

　　The diesel engine configured for this excavator is a turbocharged diesel engine with an electronic injection ECU (electronic control unit). The inspection of the fuel supply system focused on the low-pressure fuel circuit and the electronic control part.

 

　　First, the low-pressure fuel supply line was checked for leaks. After replacing the diesel filter element, the machine was tested, but the jitter did not improve.

　

　　Second, the electronic control part of the diesel engine was comprehensively tested using the DX data cable. The result showed that the fuel injection quantity was low, only 90 mL (normal injection quantity should be 100–110 mL), indicating that the diesel engine's power output had not reached the normal value. Analysis suggested two possible reasons: first, some parts of the diesel engine had faults (such as insufficient turbocharger boost or abnormal water temperature), causing the ECU to actively reduce fuel injection, thereby reducing output power; second, the ECU detected that the diesel engine's external load (hydraulic system load) was too small.

 

　　Finally, actual inspection of the diesel engine components and analysis of the data detected by the DX data cable ruled out the possibility of a fault in the diesel engine itself. The ECU detecting a small external load was the main reason for insufficient diesel engine output power.

 

(2) Check the external load detection device

 

　　The external load of the diesel engine is detected by the hydraulic oil high- and low-pressure sensors and the diesel engine speed sensor (the CPU central processing unit and the ECU each have their own dedicated speed sensor, both installed on the flywheel). The detection results are then input to the CPU central processing unit, which performs matching calculations between diesel engine power and hydraulic system load, and then sends command signals to the proportional solenoid valve to adjust the hydraulic pump flow.

 

　　Since the high- and low-pressure sensors and proportional solenoid valves had been previously tested and found fault-free, only the speed sensor or the CPU central processing unit could be faulty. To distinguish whether the fault was in the hydraulic components or the electro-hydraulic control components, the pump proportional solenoid valve connector was unplugged. At this time, the entire hydraulic system flow control entered a fully hydraulic control state, i.e., the CPU central processing unit no longer controlled the hydraulic pump flow changes. In this state, the machine did not exhibit jitter, indicating that the fault was related to the CPU central processing unit's control of the hydraulic pump. The CPU central processing unit was replaced, but the fault remained after testing.

 

　　The ITCS self-diagnostic system was used to check whether the speed sensor was faulty. It was found that the indicated value of the CPU central processing unit's speed sensor decreased and fluctuated when the action jitter occurred, and this value was lower than the speed value displayed by the ECU's speed sensor, i.e., there was a difference between the detection results of the two speed sensors, as shown in Figure 1. To eliminate the influence of the speed sensor, the CPU central processing unit's speed sensor was replaced, but the fault persisted after testing.

 

　　When the excavator's emergency throttle knob was used instead of the conventional throttle, the jitter disappeared. Analysis suggested that when the emergency throttle is used, the CPU central processing unit no longer uses its dedicated speed sensor signal but instead uses the signal detected by the ECU's speed sensor, i.e., the signal is transmitted through the communication line between the ECU and the CPU central processing unit. Combined with the slight difference between the two speed sensors, it was determined that the signal from the CPU central processing unit's dedicated speed sensor was abnormal. Since this speed sensor had been replaced previously and should not fail within a short period, the signal acquisition point was suspected to be faulty.

 

　　The hydraulic pump was removed from the diesel engine flywheel housing, and one tooth of the flywheel ring gear was found to be broken. After replacing the flywheel ring gear, the jitter disappeared.

 

　　Analysis of the cause: because one tooth of the diesel engine flywheel ring gear was broken, the feedback signal from the CPU central processing unit's dedicated speed sensor produced periodic changes. Based on this signal, the CPU central processing unit mistakenly believed that the actual diesel engine speed was decreasing and immediately reduced the hydraulic pump flow. This periodic flow adjustment caused the excavator's action jitter.