By Dovydas Kersys · Chapter 15 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.
Gear pumps are simple and robust, but they are not immune to wear, suction problems, shaft seal damage and pressure overload. Their fixed displacement makes flow diagnosis straightforward when shaft speed is known.
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 15. Simplified diagnostic view for 15. External Gear Pump Faults.
What the technician may observe
- gear pump whine or growl
- low flow at operating temperature
- shaft seal pushes outward or leaks
- pressure ripple and vibration
- metallic wear debris
Diagnostic sequence
- Check suction line size, strainer condition and air leaks. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure pump outlet flow at several pressure points. Record the result before changing the next variable so the test actually narrows the fault.
- Inspect coupling alignment and shaft condition. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Check whether shaft-seal leakage is caused by excessive inlet vacuum or case pressure. Record the result before changing the next variable so the test actually narrows the fault.
- Inspect system relief setting against pump rating. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Look for scoring in housing, gear side plates and bearings after confirmed failure. 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
- Correct inlet restriction before fitting a new pump.
- Do not raise relief pressure above pump or component ratings.
- Flush wear debris from the circuit.
- Prime the replacement pump according to manufacturer procedure.
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 15. external gear pump faults, 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 |
|---|---|
| Gear pump whine or growl | Check inlet condition, air ingress, fluid condition and pump flow. |
| Low flow at operating temperature | Measure temperature and pressure drop at the suspected heat source. |
| Shaft seal pushes outward or leaks | Measure the controlled variable before and after the suspected component. |
| Pressure ripple and vibration | 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: Hydraulic Pump Fundamentals: Flow, Leakage and Efficiency
- Next chapter: Hydraulic Vane Pump Faults
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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.