By Dovydas Kersys · Chapter 39 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 quick-reference chart is useful only if it points to measurements, not guesses. Use this chapter at the machine to choose the next gauge, flow or temperature check, then return to the detailed chapter for interpretation.
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 39. Simplified diagnostic view for 39. Quick Reference: Symptom-to-Measurement Guide.
What the technician may observe
- no movement
- slow movement
- weak under load
- excessive heat
- noise / foam / vibration
Diagnostic sequence
- No movement: verify command, pump flow, pump pressure and valve work-port pressure. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Slow movement: verify flow, pressure drop across restrictions and actuator leakage. Record the result before changing the next variable so the test actually narrows the fault.
- Weak under load: verify required load pressure, relief path and pressure at actuator. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Heat: find where high flow crosses a large pressure drop. Record the result before changing the next variable so the test actually narrows the fault.
- Noise/foam: inspect inlet vacuum, air ingress and reservoir return behavior. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Drift: secure load and isolate valve leakage from actuator leakage. 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
- Carry a rated gauge kit and clean test couplers.
- Keep normal machine values accessible.
- Use temperature and sound as clues, not final proof.
- Escalate safety-critical load-holding faults immediately.
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 39. quick reference: symptom-to-measurement guide, 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 |
|---|---|
| No movement | Measure the controlled variable before and after the suspected component. |
| Slow movement | Measure delivered flow and work-port pressure during motion. |
| Weak under load | Measure pressure at pump and actuator under the actual load. |
| Excessive heat | 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.
Quick Reference Tables
| Symptom | Likely categories | Measure first |
|---|---|---|
| No motion | No command; no pump flow; valve not shifting; blocked load-holding valve; mechanical jam | PLC/coil state, pump flow/pressure, P-A-B-T pressure |
| Slow motion | Low flow; restriction; internal leakage; wrong flow-control setting | Flow, pressure drop, return pressure |
| Weak under load | Insufficient pressure; relief/unloading path; pump leakage; cylinder leakage; overload | Pump and actuator pressure under load |
| Excess heat | Relief flow; throttling; internal leakage; cooler fault; high backpressure | Standby pressure, pressure drop, temperature differential |
| Pump noise | Cavitation; aeration; coupling; wear; wrong viscosity | Inlet vacuum, oil condition, shaft/coupling |
| Cylinder drift | Valve leakage; check/counterbalance leak; piston bypass; thermal effect | Port pressures and safe isolation test |
| Erratic motion | Air; contamination; sticking valve; unstable control; variable load | Oil aeration, command vs pressure/motion trace |
| Repeated seal failure | Rod damage; side load; heat; contamination; pressure spike | Rod/alignment inspection, pressure trace, oil condition |
Pressure-drop interpretation
| Finding | Meaning |
|---|---|
| High pressure before component, much lower after, with flow present | Restriction or intentional throttling across that component. |
| Low pressure everywhere despite known heavy load | Missing flow or an unintended low-resistance path to tank. |
| Pressure normal, flow low | Pump flow loss, restriction, low shaft speed, or internal leakage. |
| Pump pressure high, actuator port pressure low | Loss is between pump and actuator: valve, line, coupler, manifold passage. |
| Return pressure high | Blocked return/filter/cooler, undersized line, or tank-path restriction. |
Hydraulic troubleshooting worksheet
Machine / asset: ________________________________________________________
Date / shift: ________________________________________________________
Exact symptom: ________________________________________________________
Operating condition / load: ________________________________________________________
Oil level / fluid type: ________________________________________________________
Oil temperature: ________________________________________________________
Pump outlet pressure: ________________________________________________________
Work port A pressure: ________________________________________________________
Work port B pressure: ________________________________________________________
Return pressure: ________________________________________________________
Flow or cycle time: ________________________________________________________
Electrical command / coil voltage: ________________________________________________________
Recent work or changes: ________________________________________________________
Test performed: ________________________________________________________
Result: ________________________________________________________
Root cause: ________________________________________________________
Corrective action: ________________________________________________________
Post-repair baseline: ________________________________________________________
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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.