By Dovydas Kersys · Chapter 13 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.
A filter can protect the system only when oil actually passes through usable media. A clogged element, wrong rating, stuck bypass, high cold viscosity or an undersized housing can all change system behavior.
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 13. Simplified diagnostic view for 13. Filters, Bypass Valves and Differential Pressure.
What the technician may observe
- filter indicator stays in bypass
- machine slow only when oil is cold
- element collapses or distorts
- frequent filter changes with heavy debris
- return line pressure increases over time
Diagnostic sequence
- Measure differential pressure across the filter at known flow and temperature. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Check whether the indicator is mechanical, electrical, or temperature-compensated. Record the result before changing the next variable so the test actually narrows the fault.
- Verify element part number, micron rating and collapse rating. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Inspect bypass valve condition and orientation. Record the result before changing the next variable so the test actually narrows the fault.
- Check for a surge-flow condition that exceeds housing capacity. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Cut open failed elements safely and inspect debris patterns. 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
- Replace elements based on condition and manufacturer guidance.
- Correct the contamination source rather than repeatedly changing filters.
- Use proper element rating for servo/proportional circuits.
- Fix bypass valves that do not reseat.
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 13. filters, bypass valves and differential pressure, 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 |
|---|---|
| Filter indicator stays in bypass | Measure the controlled variable before and after the suspected component. |
| Machine slow only when oil is cold | Measure delivered flow and work-port pressure during motion. |
| Element collapses or distorts | Measure the controlled variable before and after the suspected component. |
| Frequent filter changes with heavy debris | 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.
PART IV – PUMP TROUBLESHOOTING
Practical diagnosis from symptom to measurement to root cause.
Continue the series
- Previous chapter: Hydraulic Contamination and Repeated Component Failures
- Next chapter: Hydraulic Pump Fundamentals: Flow, Leakage and Efficiency
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Adapted from Hydraulic Troubleshooting for Maintenance Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.