By Dovydas Kersys · Chapter 2 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.
Pressure and flow are related but they are not interchangeable. A pump primarily creates flow; pressure develops when that flow meets resistance. A technician who separates pressure problems from flow problems can eliminate half the possible causes very quickly.
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 2. Simplified diagnostic view for 2. Pressure, Flow, Force and Speed.
What the technician may observe
- actuator moves slowly but still develops force
- actuator stalls under load
- pressure rises while motion stops
- machine is fast unloaded but weak loaded
- one direction is slower than the other
Diagnostic sequence
- Compare unloaded and loaded pressure. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure or estimate flow when speed is the complaint. Record the result before changing the next variable so the test actually narrows the fault.
- Calculate cylinder force from pressure and effective piston area. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Remember that rod-side area is smaller than cap-side area. Record the result before changing the next variable so the test actually narrows the fault.
- Look for throttling losses when pressure is high but speed is low. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Use temperature as evidence of energy being wasted across a restriction. 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
- Correct pressure settings only after proving the load requirement.
- Repair sources of internal leakage that reduce delivered flow.
- Correct undersized or restricted lines where pressure drop is excessive.
- Record normal cycle pressure and cycle time as a baseline.
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 2. pressure, flow, force and speed, 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 |
|---|---|
| Actuator moves slowly but still develops force | Measure delivered flow and work-port pressure during motion. |
| Actuator stalls under load | Measure pressure at pump and actuator under the actual load. |
| Pressure rises while motion stops | Measure pressure at pump and actuator under the actual load. |
| Machine is fast unloaded but weak loaded | Measure pressure at pump and actuator under the actual load. |
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.
Core Equations and Rules of Thumb
Use equations to test whether a diagnosis is physically possible. The examples below are intentionally general; use consistent units and the exact efficiency assumptions required by your machine documentation.
| Quantity | Relationship | Troubleshooting use |
|---|---|---|
| Cylinder force | Force = pressure × effective area | Shows whether available pressure should overcome the load. |
| Cylinder speed | Speed = flow ÷ effective area | Links slow motion to missing flow or changed cylinder area. |
| Hydraulic power | Power ≈ pressure × flow | Explains why throttling and relief flow create heat. |
| Motor torque | Torque is proportional to pressure difference × displacement | Separates torque faults from speed faults. |
| Pump theoretical flow | Flow = displacement × shaft speed | Baseline for volumetric-efficiency checks. |
| Pressure drop | ΔP = upstream pressure – downstream pressure | Locates restrictions and wasted energy. |
PART I – FOUNDATIONS
Practical diagnosis from symptom to measurement to root cause.
Continue the series
- Previous chapter: Hydraulic Safety and Stored Energy
- Next chapter: Hydraulic Cylinder Area, Force and Load Calculations
- 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.