By Dovydas Kersys · Chapter 14 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.
Pumps wear internally. As clearances increase, more outlet oil leaks internally toward the inlet or case. A worn pump may look acceptable unloaded but lose flow badly when pressure and temperature rise.
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 14. Simplified diagnostic view for 14. Pump Fundamentals: Flow, Leakage and Efficiency.
What the technician may observe
- good speed unloaded but slow under load
- flow falls as oil warms
- case drain increases
- pump needs higher rpm to maintain cycle time
- pressure can be reached only with very low flow demand
Diagnostic sequence
- Measure flow at low and high pressure with oil at operating temperature. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Compare delivered flow with expected displacement and shaft speed. Record the result before changing the next variable so the test actually narrows the fault.
- Check case-drain flow on designs where this is a valid diagnostic. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Verify inlet conditions before blaming wear. Record the result before changing the next variable so the test actually narrows the fault.
- Inspect drive coupling and shaft speed. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Check pressure-compensator or load-sense controls on variable pumps. 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
- Repair or replace pump only after suction and control problems are excluded.
- Correct excessive case pressure.
- Use clean oil and proper priming after replacement.
- Document post-repair flow at a defined pressure.
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 14. pump fundamentals: flow, leakage and efficiency, 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 |
|---|---|
| Good speed unloaded but slow under load | Measure delivered flow and work-port pressure during motion. |
| Flow falls as oil warms | Measure the controlled variable before and after the suspected component. |
| Case drain increases | Measure the controlled variable before and after the suspected component. |
| Pump needs higher rpm to maintain cycle time | 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 Filters, Bypass Valves and Differential Pressure
- Next chapter: Hydraulic External Gear Pump Faults
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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.