By Dovydas Kersys · Chapter 33 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 cooler can be perfectly clean and still provide poor cooling if oil is bypassing it, if airflow is reversed, if coolant flow is inadequate, or if the thermostat never directs oil through the core.
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 33. Simplified diagnostic view for 33. Coolers, Heat Exchangers and Thermal Bypass Faults.
What the technician may observe
- reservoir temperature high
- cooler fan running but little temperature drop
- cooler pressure drop excessive
- system overheats only at high ambient temperature
- oil bypass line remains hot
Diagnostic sequence
- Measure oil temperature at cooler inlet and outlet. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Verify fan direction, fan speed and clean airflow path. Record the result before changing the next variable so the test actually narrows the fault.
- Check water flow and fouling on water-cooled units. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Verify thermostat or bypass-valve operation. Record the result before changing the next variable so the test actually narrows the fault.
- Measure pressure drop across the cooler at known flow. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Confirm heat load has not increased because another fault is wasting power. 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
- Clean or repair the cooler.
- Restore bypass/thermostat operation.
- Correct fan or coolant-flow problems.
- Eliminate upstream heat generation before upsizing the cooler.
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 33. coolers, heat exchangers and thermal bypass 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 |
|---|---|
| Reservoir temperature high | Measure temperature and pressure drop at the suspected heat source. |
| Cooler fan running but little temperature drop | Measure temperature and pressure drop at the suspected heat source. |
| Cooler pressure drop excessive | Measure pressure at pump and actuator under the actual load. |
| System overheats only at high ambient temperature | Measure temperature and pressure drop at the suspected heat source. |
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 Accumulators and Precharge Problems
- Next chapter: Solenoids, PLC Outputs and Electro-Hydraulic 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.