By Dovydas Kersys · Chapter 6 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.
Parts swapping is expensive because many hydraulic symptoms have several possible causes. A disciplined process moves from the simplest system-level checks toward component isolation, using each measurement to narrow the field.
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 6. Simplified diagnostic view for 6. The Hydraulic Troubleshooting Funnel.
What the technician may observe
- maintenance team repeatedly replaces the same valve
- machine restarts after cooling and fault returns
- no baseline data exists
- several adjustments have been changed
- fault description is vague such as “hydraulics bad”
Diagnostic sequence
- Define exactly what the machine fails to do and under what load. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Check fluid level, temperature, obvious leaks and recent work. Record the result before changing the next variable so the test actually narrows the fault.
- Compare the faulty function with another function that shares the same pump. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure pump pressure, work-port pressure and return pressure during the fault. Record the result before changing the next variable so the test actually narrows the fault.
- Isolate the suspected component with controlled tests rather than assumptions. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- After the repair, restore settings and verify the original symptom is gone. 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
- Write down readings before turning adjustments.
- Change one variable at a time.
- Use known-good baselines rather than memory.
- Keep replaced parts for failure analysis when practical.
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 6. the hydraulic troubleshooting funnel, 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 |
|---|---|
| Maintenance team repeatedly replaces the same valve | Measure the controlled variable before and after the suspected component. |
| Machine restarts after cooling and fault returns | Measure the controlled variable before and after the suspected component. |
| No baseline data exists | Measure the controlled variable before and after the suspected component. |
| Several adjustments have been changed | 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 Gauges, Flow Meters and Test Points
- Next chapter: Hydraulic Building a Normal-Condition Baseline
- 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.