By Dovydas Kersys · Chapter 25 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.
Actuator speed comes from flow, but flow controls can behave differently with changes in load, temperature and direction. Meter-in and meter-out arrangements are not interchangeable for every load condition.
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 25. Simplified diagnostic view for 25. Flow Controls and Unexpected Speed Changes.
What the technician may observe
- cylinder speed changes with load
- actuator runs away downhill
- one direction much faster
- needle valve adjustment has little effect
- valve becomes very hot
Diagnostic sequence
- Identify meter-in, meter-out or bleed-off arrangement. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure pressure before and after the flow-control element. Record the result before changing the next variable so the test actually narrows the fault.
- Check pressure-compensator operation if fitted. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Verify check-valve bypass direction in one-way flow controls. Record the result before changing the next variable so the test actually narrows the fault.
- Measure actual pump flow and actuator leakage. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Check for contamination or erosion in small orifices. 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
- Use meter-out control for overrunning loads when the design calls for it.
- Restore compensator and bypass components.
- Avoid using excessive throttling as a substitute for correct pump control.
- Record final cycle speed and setting.
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 25. flow controls and unexpected speed changes, 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 |
|---|---|
| Cylinder speed changes with load | Measure delivered flow and work-port pressure during motion. |
| Actuator runs away downhill | Measure the controlled variable before and after the suspected component. |
| One direction much faster | Measure the controlled variable before and after the suspected component. |
| Needle valve adjustment has little effect | 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
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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.