By Dovydas Kersys · Chapter 37 of 40 in Hydraulic Troubleshooting.
Before you begin: Hydraulic equipment can contain lethal stored energy. Only trained and authorized personnel should perform hydraulic work. Isolate energy, support loads, verify the pressure state, use pressure-rated instruments and follow site-specific lockout/tagout procedures. The diagrams are simplified training illustrations; use the actual machine schematic and manufacturer instructions.
Good hydraulic preventive maintenance focuses on contamination, temperature, leakage, hose condition, fastener/security checks and trend data. It does not mean randomly turning valves or replacing parts on an arbitrary calendar.
In practice, the fastest diagnosis comes from deciding what the component is supposed to control and then measuring the variable on both sides of that component. Pressure tells you about load and restriction; flow tells you about speed; temperature tells you where power is being lost; leakage tells you whether clearances or seals are allowing oil to bypass the intended path.
Figure 37. Simplified diagnostic view for 37. Preventive Maintenance That Actually Prevents Failures.
What the technician may observe
- same failures recur each year
- minor leaks become major shutdowns
- hose failures occur without warning
- oil samples are taken inconsistently
- settings drift because adjustments are unsealed
Diagnostic sequence
- Inspect fluid level, color and abnormal aeration. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Inspect hoses, clamps, rods, breathers and visible leaks. Record the result before changing the next variable so the test actually narrows the fault.
- Trend oil temperature and filter differential pressure. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Sample oil using a repeatable clean method. Record the result before changing the next variable so the test actually narrows the fault.
- Check critical pressures and cycle times at planned intervals. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Inspect accumulator precharge and cooler performance per manufacturer guidance. Record the result before changing the next variable so the test actually narrows the fault.
How to interpret the measurements
Begin by asking whether the symptom affects the whole machine or only this function. If other functions sharing the same supply remain normal, a common pump or reservoir fault becomes less likely and the local valve, actuator, plumbing and mechanical load deserve more attention.
Next, compare pressure upstream and downstream while oil is actually flowing. A large pressure difference across a component means that component or passage is consuming hydraulic energy. If flow is low everywhere, investigate pump delivery, inlet conditions, speed and major internal leakage before blaming a local valve.
Finally, repeat the test at the condition that creates the complaint: hot oil, full load, a particular direction, low speed or rapid cycling. Hydraulic clearances, viscosity, pilot pressures and valve forces change with operating condition, so a static test may look normal while the production fault remains.
Corrective actions after the cause is proven
- Create condition-based triggers where practical.
- Fix small external leaks before dirt adheres and enters the system.
- Keep fill oil clean and filtered.
- Train technicians not to adjust pressure without measurement.
Common mistakes
- Adjusting the relief or compensator before recording the original setting and proving that the setting is wrong.
- Replacing a component because it is hot, even though the heat may be arriving from an upstream restriction or continuous bypass flow.
- Calling a problem “low pressure” when the actual complaint is low flow or excessive mechanical load.
- Testing only with cold oil when the production fault appears after the system reaches operating temperature.
Field rule: For 37. preventive maintenance that actually prevents failures, do not replace a component until one measurement or isolation test shows how it fails to perform its intended function.
Symptom-to-next-test table
| Observed condition | First useful test |
|---|---|
| Same failures recur each year | Measure the controlled variable before and after the suspected component. |
| Minor leaks become major shutdowns | Measure the controlled variable before and after the suspected component. |
| Hose failures occur without warning | Measure the controlled variable before and after the suspected component. |
| Oil samples are taken inconsistently | Measure the controlled variable before and after the suspected component. |
Field practice
Before leaving the machine, write down three numbers that would let another technician judge whether the repair truly restored normal operation. Good choices are usually operating temperature, a pressure at a defined machine state, and either flow or cycle time. This creates the baseline that makes the next fault faster to diagnose.
Continue the series
- Previous chapter: Hydraulic Commissioning After Pump, Valve or Cylinder Replacement
- Next chapter: Hydraulic Troubleshooting: 10 Worked Case Studies
- Browse all Hydraulic Troubleshooting chapters
Adapted from Hydraulic Troubleshooting for Maintenance Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.