By Dovydas Kersys · Chapter 5 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 hydraulic diagnosis becomes much more reliable when measurements are taken at deliberate points. One gauge at the pump can show that pressure exists; it cannot prove that pressure and flow reach the actuator.
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 5. Simplified diagnostic view for 5. Gauges, Flow Meters and Test Points.
What the technician may observe
- normal pump pressure but weak cylinder
- large pressure drop across manifold
- fault appears only at high flow
- return pressure higher than expected
- pressure spikes too fast for a slow gauge to show
Diagnostic sequence
- Use gauges with suitable pressure range and transient rating. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure before and after suspected restrictions. Record the result before changing the next variable so the test actually narrows the fault.
- Use a flow meter with correct direction, range and pressure rating. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Check return-line backpressure where valve or motor performance depends on low tank pressure. Record the result before changing the next variable so the test actually narrows the fault.
- Use a data logger or fast transducer for transient faults. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Minimize hose length and trapped air in test-gauge lines. 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
- Install permanent test points at critical locations.
- Replace damaged gauge hoses and couplers.
- Calibrate or cross-check suspect instruments.
- Record normal hot and cold readings for comparison.
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 5. gauges, flow meters and test points, 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 |
|---|---|
| Normal pump pressure but weak cylinder | Measure pressure at pump and actuator under the actual load. |
| Large pressure drop across manifold | Measure pressure at pump and actuator under the actual load. |
| Fault appears only at high flow | Measure the controlled variable before and after the suspected component. |
| Return pressure higher than expected | 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.
PART II – SYSTEMATIC TROUBLESHOOTING
Practical diagnosis from symptom to measurement to root cause.
Continue the series
- Previous chapter: Reading Hydraulic Schematics Without Guessing
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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.