By Dovydas Kersys · Chapter 36 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.
Many replacement components are damaged during the first minutes after installation because of dry starts, contamination, incorrect adjustment or trapped air. Commissioning should be treated as a controlled procedure.
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 36. Simplified diagnostic view for 36. Commissioning After Pump, Valve or Cylinder Replacement.
What the technician may observe
- new pump noisy immediately
- new cylinder moves violently
- replacement valve overheats
- system cannot build pressure after service
- fresh oil becomes contaminated quickly
Diagnostic sequence
- Confirm correct part, rotation, port orientation and adjustment positions. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Clean and cap open lines during assembly. Record the result before changing the next variable so the test actually narrows the fault.
- Prefill/prime components where the manufacturer requires it. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Start at low pressure or unloaded condition when the design allows. Record the result before changing the next variable so the test actually narrows the fault.
- Bleed air through approved points while controlling motion. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Set pressure and flow controls using gauges, then perform a gradual load test. 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
- Stop immediately if the new pump does not prime.
- Recheck fluid level as cylinders fill.
- Inspect filters after major contamination repairs.
- Record post-repair baseline readings.
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 36. commissioning after pump, valve or cylinder replacement, 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 |
|---|---|
| New pump noisy immediately | Measure the controlled variable before and after the suspected component. |
| New cylinder moves violently | Measure the controlled variable before and after the suspected component. |
| Replacement valve overheats | Measure temperature and pressure drop at the suspected heat source. |
| System cannot build pressure after service | 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.
Continue the series
- Previous chapter: Intermittent Hydraulic Faults
- Next chapter: Hydraulic Preventive Maintenance That Actually Prevents Failures
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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.