Hydraulics power almost every working function on an excavator. The main pumps supply oil to the control valve, which directs flow to the boom, arm, bucket, swing and travel circuits. Many modern machines also use a separate pilot circuit to control the main valve.
Excavator hydraulic pressures, pump-control settings, relief pressures, pilot pressures and test ports vary greatly by make, model and serial range. Use the correct workshop manual for exact specifications and test procedures.
1. Main Hydraulic Components
A typical excavator hydraulic system may include:
- Hydraulic tank
- Main hydraulic pumps
- Pilot pump
- Main control valve
- Main relief valve
- Port relief valves
- Boom, arm and bucket cylinders
- Swing motor
- Travel motors
- Oil cooler
- Filters and strainers
2. Main Hydraulic Pumps
Most modern excavators use one or more variable-displacement piston pumps.
The pump output changes according to:
- Control demand
- Engine load
- Pressure
- Electronic or hydraulic pump-control signals
3. Fixed vs Variable Displacement
Older and smaller machines may use fixed-displacement pumps.
Variable-displacement pumps can reduce or increase flow depending on demand, improving efficiency and control.
4. Pump Regulation
Variable pumps may use:
- Hydraulic regulator
- Electronic solenoid control
- Load-sensing signal
- Power-control system
A pump-control fault can cause weak or slow hydraulics even when the pump itself is mechanically sound.
5. Main Control Valve
The main control valve contains spool sections that direct oil to each function.
Typical Sections
- Boom
- Arm
- Bucket
- Swing
- Left travel
- Right travel
- Auxiliary circuit
6. Pilot Hydraulic System
Many excavators use low-pressure pilot oil to move the main control-valve spools.
The pilot circuit may include:
- Pilot pump
- Pilot filter
- Joystick valves
- Safety lock lever
- Pilot accumulator
7. Low Pilot Pressure Symptoms
Low pilot pressure can cause:
- All controls weak or dead
- Slow spool response
- Functions working intermittently
- Travel/swing problems
Do not condemn the main pumps until pilot pressure is checked where applicable.
If all functions are weak or unavailable, check the pilot system, safety lock circuit and controller inputs before replacing expensive main hydraulic components.
8. Main Relief Valve
The main relief valve limits maximum system pressure.
If set too low or leaking, symptoms may include:
- Weak digging force
- Poor lifting performance
- Functions stopping under load
9. Never Increase Relief Pressure by Guesswork
Excessive pressure can damage:
- Hoses
- Cylinders
- Pumps
- Control valve
- Machine structure
Adjust only to the exact manufacturer specification and only after the cause of poor performance has been properly diagnosed.
10. Port Relief Valves
Individual circuits may use port relief or shock-relief valves.
These protect specific functions from pressure spikes caused by external loads.
11. Boom Cylinder Circuit
The boom circuit must handle high loads.
Problems may involve:
- Internal cylinder leakage
- Control-valve leakage
- Relief-valve fault
- Load-holding valve
- Pump flow or pressure
12. Boom Drifts Down
Possible causes include:
- Cylinder piston seal bypass
- Control-valve spool leakage
- Load-holding/check valve leakage
- External hose leakage
13. Arm / Dipper Circuit
The arm circuit may use regeneration to combine return oil with pump flow for faster movement.
A regeneration-valve fault can cause:
- Slow arm movement
- Jerky operation
- Heat
14. Bucket Circuit
Bucket faults can be caused by:
- Cylinder leakage
- Control-valve fault
- Port relief valve
- Mechanical pin/bush binding
15. Hydraulic Cylinders
Inspect cylinder rods for:
- Scoring
- Pitting
- Rust
- Bending
- Oil leakage
Damaged rods quickly damage seals.
16. Internal Cylinder Leakage
A cylinder can leak internally without visible external oil.
Symptoms may include:
- Drift
- Weakness under load
- Uneven movement
17. Swing Hydraulic Circuit
The swing circuit includes:
- Swing motor
- Swing brake
- Relief valves
- Control-valve spool
Some systems also include anti-reaction or cushioning valves.
18. Swing Weak or Slow
Possible causes include:
- Low main pressure
- Swing motor leakage
- Brake not releasing
- Control valve fault
- Pilot pressure fault
19. Travel Hydraulic Circuit
Travel motors receive oil through:
- Main control valve
- Centre swivel / rotary joint
- Travel hoses
A centre-swivel leak can affect one or both travel circuits.
20. Centre Swivel / Rotary Joint
The centre joint transfers hydraulic oil between upper structure and undercarriage while allowing rotation.
Internal seal failure can cause:
- Weak travel
- Machine pulling to one side
- Cross-leakage between circuits
21. Travel Motor Case Drain
Case-drain flow can be used to assess internal motor leakage.
Excessive case drain may indicate:
- Motor wear
- Internal seal failure
- High internal leakage
Use the exact test procedure and limits.
22. Hydraulic Filters
Excavators may use:
- Return filter
- Pilot filter
- Suction strainer
- Case-drain filter
Blocked filters can cause slow operation, heat or cavitation.
23. Hydraulic Oil Condition
Check for:
- Milky oil
- Foam
- Burnt smell
- Metal particles
- Wrong viscosity
24. Cavitation
Cavitation may result from:
- Low oil level
- Blocked suction strainer
- Suction hose collapse
- Cold thick oil
- Air leak on suction side
It can rapidly damage pumps.
25. Aeration
Air entering the hydraulic oil may cause:
- Foaming
- Jerky functions
- Noise
- Overheating
- Poor control
26. Hydraulic Overheating
Possible causes include:
- Relief valve held open
- Internal leakage
- Restricted oil cooler
- Pump-control fault
- Wrong oil
- Continuous auxiliary flow
27. Auxiliary Hydraulic Circuits
Auxiliary circuits may power:
- Breaker
- Auger
- Grab
- Tilt bucket
- Rotator
Flow and pressure must match the attachment.
28. Breaker / Hammer Circuits
A hydraulic breaker may require:
- Specified flow
- Specified pressure
- Low-restriction return
- Correct attachment mode in controller
Incorrect settings can damage both attachment and machine.
29. Best Hydraulic Diagnostic Order
- Check oil level and condition.
- Check filters and suction restrictions.
- Confirm pilot pressure where fitted.
- Record which functions are weak.
- Check main pressure at correct test point.
- Compare pump flow where necessary.
- Check cylinder or motor internal leakage.
- Check control valve and relief valves.
- Only then condemn main pumps.
30. Excavator Hydraulic Checklist
| Area | Check |
|---|---|
| Hydraulic oil | Correct level, type and condition. |
| Filters | Return, pilot and strainers serviced. |
| Pilot pressure | Checked where system uses pilot control. |
| Main pressure | Measured at correct test point. |
| Pump flow | Checked if pressure alone is inconclusive. |
| Cylinders | No excessive drift or internal leakage. |
| Swing/travel | Motors and brake circuits checked. |
| Centre swivel | Checked if travel faults are unequal. |
| Manual data | Exact pressures and settings used. |
Related Agrimanual Manuals
Hydraulic circuits, test ports, pressure settings and pump-control procedures are excavator-specific.
- Search Agrimanual excavator hydraulic workshop manuals
- Search Agrimanual excavator service manuals
- Request a manual if your machine is not listed
Frequently Asked Questions
Why are all my excavator functions weak?
Check oil level, filters, pilot pressure, engine speed and main pressure before condemning the main pumps.
Why is only one hydraulic function weak?
A single weak function may point toward its cylinder, spool, port relief valve or local circuit rather than the main pump.
Why does the boom drift down?
Internal cylinder leakage, control-valve leakage or a load-holding/check valve fault are common possibilities.
Can low pilot pressure stop the excavator controls?
Yes. On pilot-operated machines, low pilot pressure can prevent the main control-valve spools from moving correctly.
Should I turn up the main relief valve to make the machine stronger?
No. Diagnose the cause of low performance and adjust only to the exact manufacturer specification if adjustment is required.
Test the system in order: oil, pilot pressure, main pressure, flow and internal leakage. Main pumps are expensive and should be condemned only after the simpler causes are ruled out.