For technicians engaged in construction machinery maintenance and after-sales service for excavator foreign trade, issues such as weak movement, stuttering and abnormal noise of excavators caused by hydraulic failures are frequently encountered. Based on professional maintenance manuals, this article systematically sorts out the classification of excavator hydraulic faults, diagnostic logic, root causes of faults under different scenarios and standardized troubleshooting procedures. It can be directly used by novice maintenance technicians.

1. Five Core Categories of Excavator Hydraulic Faults

According to the operating characteristics of hydraulic transmission systems, all excavator hydraulic failures fall into the following five categories, and all fault phenomena can be classified into these scopes:

1. Insufficient total system flow
   The hydraulic pump supplies insufficient total oil, resulting in slow overall machine movements and insufficient power.
2. Low system working pressure
   Actuators lack thrust, leading to weak movements or complete immobilization; the machine will stall under loaded conditions.
3. Internal system leakage
   Damaged internal seals of hydraulic pumps, various valves, cylinders and hydraulic motors cause internal leakage, which consumes oil pressure and flow and leads to sluggish operation.
4. External system leakage
   Damaged oil pipe joints, pump bodies, valve blocks and cylinder oil seals cause external hydraulic oil leakage, contaminating equipment and depleting hydraulic oil.
5. Vibration and abnormal noise
   Vibration of pipelines and mechanical noise occur during equipment operation, accompanied by unstable flow or oil pressure. Prolonged operation will damage hydraulic components.

2. Standard General Troubleshooting Procedure for Hydraulic Faults

Fault inspection must strictly follow the principle: from outside to inside, from simple to complex. The standardized maintenance steps are listed below:

1. Collect fault information: Learn about equipment working conditions and operations before and after the fault, and sort out oil circuits against the hydraulic schematic diagram of the machine.
2. Instrument testing: Connect external pressure gauges and flowmeters to measure key data including system pressure, flow rate, engine speed and oil temperature.
3. Visual external inspection: Check hydraulic oil level and cleanliness, inspect oil pipes for breakage and leakage, and confirm whether filter elements are clogged.
4. Test run to reproduce faults: Operate machine movements and record working conditions and scenarios where faults appear.
5. Disassembly and maintenance: Disassemble, repair or replace spare parts after locating faulty components.
6. Post-repair commissioning: Carry out no-load and loaded test runs after installation to verify fault elimination.
7. Record and summarize faults: Document root causes and maintenance solutions for quick troubleshooting of similar faults in the future.

Key Reminder for Priority Inspection
Pilot control systems, overflow valves, overload valves, main hydraulic pumps and filters are high-failure components of hydraulic systems and should be inspected first.

3. In-depth Analysis of Three Typical Excavator Faults

(1) Faults of All Machine Movements: Weak or Complete Loss of All Actions

Fault Logic: If all movements malfunction simultaneously, the fault lies in the common oil circuit shared by all movements (sections before the control valve). Common root causes are as follows:

1. Faults of oil supply circuit
   Insufficient hydraulic oil, clogged oil suction pipeline (blocked oil suction filter element) or air intake in the oil circuit prevents the hydraulic pump from absorbing adequate oil. The pump output flow and pressure drop sharply, leading to slow and weak movements of the whole machine.
2. Pilot oil circuit faults
   This fault only occurs on pilot-controlled excavators. Insufficient pressure in the pilot oil circuit prevents normal reversing of control spools, resulting in failed machine operation.
   Inspection priorities: Pilot pump, pilot filter element, pilot overflow valve and leakage of pilot oil pipes.
3. Damaged transmission structure between engine and hydraulic pump
   Worn or broken transmission parts connecting the engine and main hydraulic pumps stop power transmission from the engine to hydraulic pumps. No oil pressure is generated by pumps, and the machine loses all movements.
4. Severe wear or damage of dual main hydraulic pumps
   Severe wear of valve plates and plungers inside front and rear main pumps causes massive internal leakage. The system cannot build up pressure, and all machine movements lack power.
5. Malfunction of hydraulic pump power adjustment system
   Stuck or damaged pump power adjustment mechanisms prevent the pump from outputting rated flow and pressure, resulting in drastically slowed movements under loaded conditions.

Step-by-step Field Troubleshooting for Whole-machine Faults

1. External inspection: Check hydraulic oil level; verify oil suction pipes for breakage and air intake; check if oil suction filter is clogged. If a large amount of metal powder is found in the return oil filter, the hydraulic pump suffers internal wear and requires disassembly and overhaul.
2. Pilot oil circuit testing: If the pilot circuit works normally, further inspect the transmission structure between the engine and hydraulic pump.
3. Troubleshooting under different working conditions:
   • No movement of the whole machine and no load feedback during operation: Focus on the transmission structure between the pump and engine.
   • Slow movements with insufficient power under load: Inspect the power adjustment mechanism and internal leakage of the hydraulic pump.

(2) Faults of Movements Controlled by One Control Valve Group

Fault Logic: If only movements controlled by one valve group fail while movements of the other group work perfectly, the common oil circuit is intact, and the fault exists in the independent sub-system of this valve group. Three major root causes are listed below:

1. Damaged main overflow valve of the faulty sub-system
   Worn valve spools, broken springs or clogged damping holes of overflow valves prevent the sub-system from reaching standard operating pressure, leading to weak movements and poor load capacity.
   Quick Diagnosis Method: Swap Comparison Test
   Swap the overflow valve of the faulty sub-system with the one from the normal sub-system for a test run. If the fault transfers along with the overflow valve, the valve is defective. If the fault remains in the original circuit, the overflow valve works properly.
2. Worn or damaged main hydraulic pump of the faulty sub-system
   Worn plungers and valve plates inside a single pump cause severe internal leakage, resulting in insufficient flow and pressure for the corresponding sub-system.
   Quick Diagnosis Method: Swap the output pipelines of the two main pumps for testing. If the fault transfers along with the pipelines, this hydraulic pump is damaged.
3. Malfunction of hydraulic pump flow adjustment mechanism
   Stuck or worn flow adjustment plungers. The machine works normally when cold; as oil temperature rises, plungers expand thermally and get stuck. The pump cannot increase flow, and corresponding movements slow down. The fault temporarily disappears after cooling down.

Real Maintenance Case Reference

An excavator suffered slow swing movement; boom and arm movements driven by the same pump also operated sluggishly, while travel movements on the other pump circuit worked normally. After swapping output pipelines of the two main pumps, the fault transferred to the other side. Disassembly of the faulty hydraulic pump revealed metal peeling notches on the cylinder valve plate, triggering pressure pulsation and insufficient pump output pressure. The fault was completely eliminated after replacing pump accessories.

(3) Single-action Faults: Weak, Erratic or Unresponsive Individual Movement

Fault Logic: If only one single movement malfunctions while all other movements operate normally, faults in common oil circuits and main pumps can be ruled out. The fault is limited to the independent branch of this movement (pilot handle, pilot oil circuit, single control valve spool, overload valve, hydraulic cylinder or hydraulic motor).

① Slow or completely unresponsive single movement

Follow the inspection sequence: from simple to difficult: pilot oil circuit → sectional overload valve → control valve spool → hydraulic cylinder/hydraulic motor.

Case: The swing movement of an excavator suddenly stalled during operation and failed to restart after shutdown, while all other movements worked normally.
Troubleshooting Process:
① Inspection confirmed no faults of the swing motor overflow valve and charge valve.
② Vibration and oil pressure were detected on the oil inlet pipe of the swing motor when operating the handle, proving normal oil supply of pilot and main circuits.
③ Fault component confirmed: Damaged meshing gear teeth inside the swing reducer interrupted power transmission. Swing function recovered after replacing reducer accessories.

② Erratic automatic movements: Machine moves without operation

The machine automatically executes travel or boom movements without manipulating control handles. The essence is uncontrolled pressure oil entering actuators.
Root causes: Leakage of pilot pull rods or pilot oil pipes corresponding to the movement, worn control valve spools, incorrect spool assembly, aging oil seals. Continuous pressure oil pushes the spool away from neutral position.

Case: A mechanically rod-controlled excavator automatically traveled on the left track after working for a period of time. Disassembly of the control valve discovered aging oil seals inside the spool bore. Oil pressure continuously pushed the spool away from neutral and kept the oil circuit connected. The problem was permanently solved after installing brand-new oil seals.

4. Core Principles for Judging and Eliminating Hydraulic Faults

1. Segmented isolation and troubleshooting by circuit
   Against the hydraulic schematic diagram, divide the whole machine oil circuit into independent branch circuits for segmented isolation testing. Simple single faults can be quickly located. For complex composite faults, list all potential causes and eliminate them one by one from outside to inside.
2. Targeted maintenance for different machine models
   Hydraulic systems vary in complexity:
   Electro-hydraulic proportional control system > variable displacement pump control system > fixed displacement pump control system
   Pilot-operated hydraulic system > mechanical rod-operated hydraulic system
   Maintenance solutions cannot be copied indiscriminately. Always carry out inspection according to the specific model and hydraulic structure of the equipment.
3. Summarize and archive after maintenance
   After resolving each hydraulic fault, fully record fault phenomena, root causes, replaced spare parts and commissioning schemes. Long-term accumulation will greatly improve troubleshooting efficiency for similar failures.