By Dovydas Kersys · Chapter 31 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 hose can look normal outside while its inner liner has collapsed. A fitting can be assembled incorrectly, a tube can be crushed, and a quick disconnect can open only partially. These faults often create heat and direction-specific slowness.
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 31. Simplified diagnostic view for 31. Hoses, Tubes, Fittings and Hidden Restrictions.
What the technician may observe
- hose gets hot in one small area
- actuator slow after hose replacement
- pressure high upstream but low downstream
- quick coupler difficult to connect
- hose outer cover blistered or abraded
Diagnostic sequence
- Measure pressure before and after suspect hose/fitting under flow. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Check hose size, bend radius, routing and length against the original. Record the result before changing the next variable so the test actually narrows the fault.
- Inspect quick couplers for full engagement and damaged poppets. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Look for crushed tubes or fittings with excessive thread sealant intrusion. Record the result before changing the next variable so the test actually narrows the fault.
- Check hose age, abrasion and movement. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Use thermal comparison cautiously to locate concentrated pressure-loss areas. 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
- Replace internally damaged or wrongly sized hoses.
- Correct clamps and bend radius.
- Use compatible fittings and assembly procedures.
- Never repair pressure hose with improvised clamps.
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 31. hoses, tubes, fittings and hidden restrictions, 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 |
|---|---|
| Hose gets hot in one small area | Measure temperature and pressure drop at the suspected heat source. |
| Actuator slow after hose replacement | Measure delivered flow and work-port pressure during motion. |
| Pressure high upstream but low downstream | Measure pressure at pump and actuator under the actual load. |
| Quick coupler difficult to connect | 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.
PART VII – AUXILIARY COMPONENTS AND CONTROLS
Practical diagnosis from symptom to measurement to root cause.
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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.