By Dovydas Kersys · Chapter 34 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.
Modern hydraulic machines fail at the boundary between electrical commands and hydraulic response. A lit PLC output or connector LED proves very little by itself; the complete chain must be checked.
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 34. Simplified diagnostic view for 34. Solenoids, PLC Outputs and Electro-Hydraulic Faults.
What the technician may observe
- PLC output turns on but no actuator motion
- coil burns repeatedly
- valve works with manual override only
- intermittent motion when cable moves
- command correct but spool position feedback wrong
Diagnostic sequence
- Verify PLC logic state and interlocks. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure voltage directly at the coil while it is energized. Record the result before changing the next variable so the test actually narrows the fault.
- Check current draw and coil resistance against expected values. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Confirm the coil develops magnetic force and the armature moves. Record the result before changing the next variable so the test actually narrows the fault.
- Measure hydraulic P/A/B/T pressures to prove spool function. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Inspect connector pins, DIN plugs, suppressors and grounding. 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
- Repair wiring voltage drop and loose connectors.
- Use the correct coil voltage and duty rating.
- Correct spool contamination if electrical command is proven.
- Document I/O and hydraulic test results together.
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 34. solenoids, plc outputs and electro-hydraulic 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 |
|---|---|
| Plc output turns on but no actuator motion | Measure the controlled variable before and after the suspected component. |
| Coil burns repeatedly | Measure the controlled variable before and after the suspected component. |
| Valve works with manual override only | Measure the controlled variable before and after the suspected component. |
| Intermittent motion when cable moves | 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.
Continue the series
- Previous chapter: Hydraulic Coolers, Heat Exchangers and Thermal Bypass Faults
- Next chapter: Intermittent Hydraulic Faults
- Browse all Hydraulic Troubleshooting chapters
Adapted from Hydraulic Troubleshooting for Maintenance Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.