Solenoid coil burnout occurs when abnormal current, poor heat dissipation or mechanical sticking drives the internal conductor temperature past the limit of its insulation class (Class F or Class H, for example), degrading or melting the insulation, causing turn-to-turn shorts, and ultimately leaving the solenoid unable to generate enough magnetic force to function. As coil temperature climbs, the enamel insulation on the magnet wire can age, carbonise, crack or melt — leading to turn-to-turn shorts, an open circuit or an abnormal resistance reading. The coil can then no longer produce the force needed to shift the spool, and the valve fails.
In a hydraulic system, energising the coil creates a magnetic field that pulls in the armature and shifts the spool to redirect flow. Under normal conditions the heat the coil generates dissipates continuously through its housing, keeping temperature within the design range.
On automated equipment, CNC machine tools and hydraulic systems generally, coil burnout is one of the most common failures encountered in the field. Most plants have experienced a machine stopping mid-cycle, only to find a burnt-out solenoid coil. In reality, the burnt coil is rarely the root cause — it is the consequence of something else going wrong in the hydraulic system, the power supply or the way the equipment is used. Replace the coil without eliminating the real source, and the same failure is likely to recur.
This article covers the five most common causes seen in industrial settings and how to prevent each, to help plant and maintenance staff find the root cause quickly and reduce downtime.
Why do solenoid coils burn out? The causes behind the overheating
Contaminated hydraulic oil causing the spool to stick
After extended operation, hydraulic oil accumulates metal wear particles, seal debris, sludge and oxidation products. These contaminants circulate with the oil into the valve and settle in the precision clearance between spool and body.
Once contamination reaches a certain level, the spool can begin to stick, move sluggishly or fail to shift fully. At that point, even with the coil correctly energised, the magnetic force cannot pull the armature fully home.
On an AC solenoid valve, an armature that does not seat fully leaves an air gap in the magnetic circuit. Coil inductance falls and operating current rises. Held in that high-current, high-temperature state for any length of time, the insulation ages rapidly, and the coil eventually overheats, shorts or burns out. Many failures that look like coil defects therefore trace back to oil contamination and a sticking spool.
How to prevent it.
Manage the hydraulic oil properly: schedule regular cleanliness testing and change oil and filter elements according to the equipment specification. If a valve starts behaving abnormally — hesitant shifting, unusual noise — check the spool and the circuit immediately, before the problem escalates. A sound oil management regime reduces failure rates far more effectively than repeatedly replacing coils.
Excessive oil and ambient temperature
Many hydraulic power units are installed in poorly ventilated spaces, or run at high load for long periods, driving oil temperature steadily upward. Sustained oil temperatures above 60°C — and certainly above 70°C or 80°C — accelerate oil degradation and raise the valve’s operating temperature.
Because the coil is mounted directly on the valve, heat from hot oil passing through the body conducts to the coil through the armature tube. When that external heat source is added to the coil’s own temperature rise from being energised, internal temperature can keep accumulating, accelerating insulation ageing. Operating in high ambient temperature for extended periods increases the risk of shorting, failure and burnout.
Insulation class matters when specifying a coil. TAI-HUEI coils are wound throughout with high-grade PEWH polyesterimide enamelled copper wire, giving generous thermal margin even under continuous duty or high oil temperature.
Common industrial insulation classes: Class B allows a maximum operating temperature of 130°C, suited to lightly loaded, intermittent duty in well-ventilated installations. Class F allows 155°C and is the common industrial grade, suitable for most automated lines and general CNC machine tools. Class H allows 180°C and suits high oil temperature, continuous energisation or heavy-duty systems in enclosed installations.
A note on margin: IEC 60085 defines the permissible operating temperature for Class H as 180°C. Building on that Class H baseline, TAI-HUEI winds its coils with PEWH magnet wire rated to 200°C — roughly 20°C of headroom above the standard, giving the coil greater thermal buffer and reliability under demanding conditions such as high oil temperature.
How to prevent it.
Monitor oil and reservoir temperature regularly, and make sure the power unit has adequate ventilation and heat dissipation. Check that coolers, heat exchangers and related equipment are working properly, so that reduced cooling efficiency does not drive system temperature up. Where operating conditions allow, avoid running equipment above capacity for extended periods, and specify valves and coils rated for the actual thermal environment.
Abnormal supply voltage
Running a solenoid valve on a supply that is too high, too low or unstable affects how the coil operates and raises the risk of overheating and failure.
Overvoltage: when supply voltage stays above the coil’s rated specification, coil current and heat generation rise with it and internal temperature climbs. Prolonged overvoltage accelerates insulation ageing, leading to shorting, failure or burnout.
Undervoltage: when supply voltage falls well below the rated value, the coil may not generate enough force, leaving the armature unable to seat fully or the valve shifting erratically. On an AC solenoid, an unseated armature keeps the coil in a high-current state, generating still more heat. Left unresolved, this accelerates insulation ageing and raises the risk of burnout. On a DC solenoid, an unseated armature does not usually cause a marked current rise, but the coil can still overheat and fail through prolonged energisation, poor heat dissipation or overvoltage.
How to prevent it.
Measure supply voltage regularly to confirm it matches the equipment specification, and check that power supplies, transformers and wiring are functioning correctly. Where the line includes large motors, variable-frequency drives or other high-power equipment, avoid sharing an unstable supply with solenoid valves. If a valve runs abnormally hot, shifts erratically or fails repeatedly, check supply quality alongside the valve itself.
High switching frequency — inrush current and accumulated heat
On automated presses, injection moulding machines, punching equipment and robotic handling, solenoid valves often switch at high frequency. When switching frequency is too high, the heat the coil generates cannot dissipate quickly enough, internal temperature keeps rising, and the coil can overheat or burn out.
On AC solenoids, every energisation produces a brief inrush current well above normal operating current. If actual duty exceeds the coil’s recommended duty cycle (ED%), heat accumulates faster than it dissipates and the failure risk rises.
How to prevent it.
For high-cycle equipment, assess the coil’s duty cycle (ED%), energised time and cooling conditions in addition to confirming the valve specification, and avoid running beyond design load for long periods. Where equipment requires sustained high-frequency operation, specify valves rated for continuous energisation or high cycle counts, and pair them with good installation and cooling conditions to extend coil life and improve system stability.
Oil leakage and moisture ingress causing a short circuit
Prolonged exposure to high pressure and vibration can age valve body seals such as O-rings, allowing hydraulic oil to leak. Humidity, water vapour or coolant can also find its way in through gaps in the coil housing or terminal box. Once liquid or moisture reaches the coil interior, insulation absorbs moisture and degrades, reducing the dielectric strength of the magnet wire and its insulation.
Failures of this kind are usually progressive. Early symptoms may be no more than erratic operation or intermittent faults, developing later into a short circuit or full burnout.
How to prevent it.
Inspect seals and valve body sealing regularly so that ageing or wear does not admit oil or moisture. Where the working environment allows, avoid leaving equipment in high humidity, oil mist or coolant spray for extended periods. In harsh environments, specify connectors rated IP65 or better. Clean oil and coolant residue from valve surfaces regularly to limit corrosion and ingress paths. If a coil shows signs of moisture, terminal corrosion or abnormal heating, stop the machine and investigate immediately before the fault worsens.
How to reduce the risk of coil burnout
With the common causes understood, the risk can be reduced on three fronts: selection, oil management and electrical protection.
Specify the coil and ingress protection for the actual duty
Choose the coil specification during design or maintenance based on real operating conditions. For continuous duty or high oil temperature, choose a coil designed for 100% ED (continuous energisation), preferably with Class F insulation or better. In wet, oil-mist or dusty environments, specify IP65 or better ingress protection to reduce the risk of moisture and oil reaching the coil.
Maintain oil quality and stable temperature
Oil quality and temperature are among the biggest influences on valve life. Fit appropriate filtration, change filter elements on schedule, and keep the oil clean so that fewer contaminants reach the valve. Where oil temperature runs high, control it with an oil cooler or by improving heat dissipation, and keep oil temperature within the range specified for the equipment to limit heat conducted into the coil.
Strengthen electrical protection
Adding suitable protection to the control system materially reduces the risk of abnormal coil damage. Protective devices in the control circuit provide overload protection when abnormal current occurs. For DC solenoids, fit a flyback diode or snubber to suppress the inductive voltage spike generated at de-energisation and protect the coil insulation.
A four-step diagnostic procedure
When a valve fails to operate, operates erratically or the coil runs abnormally hot, work through these four steps to locate the source quickly.
Step 1 — Measure coil resistance.
Start by confirming the coil itself. Measure resistance across the coil terminals with a multimeter to identify burnout, a short circuit or an open circuit. An abnormally low reading suggests an internal short; an infinite reading suggests an open circuit. Confirm the power is isolated before measuring, to protect both equipment and personnel.
Step 2 — Confirm supply voltage.
If coil resistance is normal, check the control signal and supply. Measure the actual voltage at the valve connector and confirm it matches the coil rating (AC 110V, AC 220V, DC 12V, DC 24V, etc.). Insufficient voltage leaves the coil unable to develop enough force; excessive voltage can drive the coil to overheat and burn out.
Step 3 — Check spool movement manually.
With the electrical side confirmed, move on to the mechanical side. Isolate power and system pressure, then use the manual override to check that the spool moves freely. Where the spool sticks, the usual causes are oil contamination, foreign matter, internal wear or seizure after a long shutdown.
Step 4 — Check hydraulic oil cleanliness.
If both coil and electrical control are sound but the valve still operates abnormally, examine oil quality and system cleanliness. Metal particles, contaminants or sludge in the oil can block the spool clearance, causing sticking, internal leakage or erratic operation. Maintaining the filtration system reliably reduces valve failure rates and extends service life.
Following the sequence electrical check → signal check → mechanical check → oil analysis avoids replacing valves blindly, locates the root cause quickly, and reduces both downtime and repair cost.
Case study: what was really behind the repeated coil failures
At an injection moulding plant in northern Taiwan, an auxiliary hydraulic power unit burnt out the same solenoid coil four times in six months. The initial response was straightforward: replace the failed coil, restart, carry on. But each replacement lasted only three to five weeks before failing again. The problem kept recurring and was never resolved.
After the fourth failure, the site supervisor began to suspect the coil was not the problem and asked the TAI-HUEI engineering team to assess the machine on site. Our engineer first measured coil resistance and confirmed it was within the normal range, ruling out a manufacturing defect. Stripping the valve revealed heavy dark brown sludge on the mating surfaces between spool and body, with the spool clearly binding as it moved. Tracing back to the power unit reservoir uncovered the root cause: oil quality had gone unmanaged for a long time. The hydraulic oil had not been changed in over two years, the reservoir had accumulated substantial sludge and metal wear debris, and the filter element was blocked and no longer filtering effectively. These contaminants were carried into the valve with the oil, obstructing spool movement and holding the coil in an abnormal operating state until temperature rose far enough to destroy it.
The recommended remedy was a complete oil change, new filter elements, and a proper oil management regime — moving from replace-on-failure to scheduled maintenance with oil cleanliness testing. Nearly six months of follow-up showed no further coil failures on that valve.
The case illustrates the point again: a burnt-out coil is usually the result of a system fault, not simply a component quality issue. Replacing the coil without eliminating oil contamination or spool sticking leaves the failure free to recur.
Coil burnout — frequently asked questions
Q: What surface temperature is normal for a hydraulic solenoid coil?
In normal operation, coil surface temperature typically sits between 60°C and 85°C, varying with oil temperature, ambient temperature and energised time. A surface temperature approaching or exceeding 95°C for extended periods indicates the system may already be running abnormally hot. Most solenoid coils use Class F (155°C) insulation or better, often Class H (180°C), so a short temperature excursion does not necessarily cause immediate damage — but sustained high temperature accelerates insulation ageing. If the coil housing shows deformation, discolouration or a distinct burnt smell, stop the machine and investigate immediately.
Q: Why are AC solenoids more prone to burnout than DC solenoids?
The difference lies in how the two types work. An AC solenoid relies on the inductance created once the magnetic circuit closes to limit operating current. When the armature seats normally, coil current falls. If contamination, wear or mechanical sticking prevents it from seating fully, the magnetic circuit cannot close, the coil stays at a higher current, and it generates more heat. A DC solenoid is governed mainly by coil resistance, so current stays relatively stable — though it can still overheat and burn out through prolonged energisation, overvoltage or poor heat dissipation.
In an AC solenoid, operating current is determined primarily by inductance once the magnetic circuit closes; with the armature unseated, the air gap widens, reactance falls and current rises markedly. There is also a brief inrush current at switch-on. In a DC solenoid, current is determined primarily by coil resistance, stays largely stable when the spool sticks, and rises smoothly at switch-on with no pronounced inrush. AC solenoids are therefore considerably more sensitive to oil contamination and mechanical resistance, with correspondingly higher failure and burnout rates in harsh plant environments.
Q: How can I tell whether the coil has burnt out or the spool is stuck?
The quickest field check is to measure resistance across the coil terminals with a multimeter. A reading near 0Ω or infinity generally indicates the coil has shorted or gone open circuit — a burnout. A normal reading indicates the coil is broadly sound, though you should still confirm it develops proper force; check manually whether the spool moves, to establish whether it is sticking. Where the coil is sound but the valve does not operate, the usual causes are oil contamination, spool sticking or insufficient pressure — not the coil.
If your equipment is suffering repeated coil failures or intermittent operation with no obvious cause, contact the TAI-HUEI engineering team. We can help identify the root cause at system level and reassess selection and operating conditions to reduce the risk of unplanned downtime.
Q: What should I check when replacing a solenoid coil?
Before replacing, confirm rated voltage and supply type (AC/DC), rated power, connector type, coil dimensions and mounting specification, and check that the valve series and coil model correspond — so the valve operates correctly and retains its original performance and reliability.
Coil voltage: confirm the rated voltage marked on the original coil (DC 24V, AC 110V, etc.) and that the supply type matches. Coil power: confirm rated power matches the original specification, since different power ratings affect magnetic force, heat generation and stability. Connector type: confirm the wiring arrangement — DL DIN Plug Type, LW Terminal Box Type, M4 Pin Type Fast Connector (M12-4PIN), 2W Two Wires Type or DT Deutsch Connector Type — as this affects wiring method, ingress protection and installation space. Coil dimensions: confirm bore size, coil height, retention method and mounting orientation. Valve series: confirm the valve body series, for example G02/D03 (NG6) or G03/D05 (NG10), and that the coil suits it.
Q: How long should a solenoid coil last?
There is no single service life for a solenoid coil. Life depends mainly on coil design, insulation class, operating temperature, energised time and operating environment. Broadly, running at rated voltage, within the specified temperature range and correctly installed, coil insulation life typically extends to several years or more.
Whether a coil needs replacing should not be judged on age alone, but on symptoms: abnormally high coil temperature, abnormal resistance, no magnetic force when energised, falling insulation resistance or repeated burnout. Where a coil fails repeatedly over a short period — several failures within weeks or months — that is not normal wear, and the supply, connector protection, spool movement and oil contamination should all be investigated.
Written by the Hydraulic Systems Application Engineering Team, TAI-HUEI Hydraulic. Last updated: July 2026.
References and applicable standards
IEC 60085 — Electrical insulation — Thermal evaluation and designation, edition 4.0 (2007). Coil insulation thermal classes follow this standard, with Class F at 155°C and Class H at 180°C.
IEC 60529 — Degrees of protection provided by enclosures (IP Code), edition 2.2 (2013). The IP65, IP67 and IP68 definitions in this article follow this standard.
ISO 4401:2005 — four-port directional valve mounting surfaces; valve body and interface correspondence when changing coils follows this standard.
TAI-HUEI Hydraulic coil certification reports and product catalog — coil voltage, power, connector type and dimensional specifications. Information verified: 2026-07-28.