By Dovydas Kersys · Chapter 40 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 written worksheet reduces repeated work across shifts. It also protects the next technician from inheriting undocumented pressure changes, temporary test lines or assumptions that were never proven.
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 40. Simplified diagnostic view for 40. Technician Worksheets and Troubleshooting Checklists.
What the technician may observe
- fault changes between shifts
- several technicians test same points repeatedly
- pressure settings are undocumented
- replaced parts cannot be tied to evidence
- repair cannot be verified against original symptom
Diagnostic sequence
- Write exact symptom, time, load and oil temperature. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- List recent maintenance or machine changes. Record the result before changing the next variable so the test actually narrows the fault.
- Record pump, work-port and return pressures. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Record flow or cycle time and important electrical commands. Record the result before changing the next variable so the test actually narrows the fault.
- List one change at a time and its observed result. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Write final root cause, repair and post-repair baseline. 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
- Attach readings to the work order.
- Tag any nonstandard temporary configuration.
- Return adjustment locks and guards to service condition.
- Use the worksheet during shift handover.
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 40. technician worksheets and troubleshooting checklists, 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 |
|---|---|
| Fault changes between shifts | Measure the controlled variable before and after the suspected component. |
| Several technicians test same points repeatedly | Measure the controlled variable before and after the suspected component. |
| Pressure settings are undocumented | Measure pressure at pump and actuator under the actual load. |
| Replaced parts cannot be tied to evidence | 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.
Glossary
| Term | Meaning |
|---|---|
| Aeration | Air entering and circulating with hydraulic oil. |
| Cavitation | Formation and collapse of vapor cavities when local pressure becomes too low. |
| Case drain | A line that carries internal leakage from a pump or motor housing to tank. |
| Compensator | A control that changes variable-pump displacement in response to pressure or another signal. |
| Counterbalance valve | A pressure valve used to control or hold an overrunning load. |
| Cracking pressure | Pressure at which a check or pressure valve begins to open. |
| Differential pressure | Pressure difference between two measurement points. |
| Flow | Volume of fluid delivered per unit time; directly related to actuator speed. |
| Load sense | A control signal that communicates load pressure to a variable pump or control system. |
| Pilot pressure | A smaller control pressure used to shift or regulate another valve. |
| Pressure | Force per unit area created when flow meets resistance. |
| Relief valve | A valve that limits maximum pressure by diverting flow when its setting is reached. |
| Return backpressure | Pressure in the tank/return path that can affect valve and actuator behavior. |
| Viscosity | Resistance of a fluid to flow; strongly dependent on temperature. |
| Volumetric efficiency | Actual flow divided by theoretical flow; reduced by internal leakage. |
Troubleshoot with measurements, not guesses.
Make safe → define symptom → measure → isolate → prove → repair → verify → document
Continue the series
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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.