How do I tell remanence from a mechanical bind without special equipment?

With the coil de-energised, apply a gradually increasing axial pull to the plunger and record the force at which it breaks free. Near zero means the plunger is not held magnetically — look at grease, guide surfaces, and pressure lock. A clearly measurable force that scales with how long the coil was energised points at remanence. Watching whether it 'feels stuck' tells you nothing, because both cases look identical.

Can I fix remanence by just increasing the return spring?

It will mask it, and it costs you force on the pull-in stroke, because spring force peaks where solenoid force is weakest. It is a legitimate temporary field fix but a poor design answer. A non-magnetic shim to establish a defined residual gap is usually cheaper and more effective than changing the core grade.

The plunger sticks only in winter. Why?

Grease. Viscosity rises steeply as temperature falls, and a grease chosen at 25 °C can be near-solid at 0 °C. This is a lubricant selection problem, not a magnetic one, and it is why the failure looks seasonal and batch-independent.

It releases on the bench but not in the machine. What is different?

Pressure. A closed chamber on the plunger side can hold it in place through vacuum or trapped pressure, and the release force available may be a fraction of a newton. Check whether the mechanism vents. A small breather hole or a vented seal often removes the symptom entirely.

Solenoid sticks after power off: remanence or mechanical bind?

14 September 2026

Measure the release force with power off — do not judge by watching the plunger. If release force is near zero it is remanence and you need a material or air-gap change. If release force is normal and the plunger still stays put, it is a mechanical bind, a pressure lock, or dried grease. The remedies have nothing in common.

Why this happens

“Sticks after power off” gets treated as one failure. It is two, and they are opposites.

Remanence: the magnetic circuit retains enough flux after the coil is de-energised to hold the plunger against the return spring. The cause is in the material and the geometry — coercivity of the core and plunger, and whether there is any defined residual gap for the flux to work against.

Mechanical bind: the plunger is not held magnetically at all. It is held by friction, by a pressure differential, by vacuum, or by hardened lubricant. Removing the current changes nothing because the current was never what was holding it.

The reason this distinction matters so much is that the two remedies share nothing. Buying a lower-coercivity core for a grease problem wastes a qualification cycle and fixes nothing. Cleaning the guide on a genuine remanence problem gets you the complaint back in two weeks.

How to tell them apart

De-energise the coil and apply a steadily increasing axial pull to the plunger. Record the force at which it breaks free.

Near zero release force, plunger stays put → not magnetic. Go to the mechanical list.
A measurable release force that grows with how long the coil was energised → remanence. Go to the magnetic list.
A measurable release force that is independent of energising time → probably a spring or a pressure condition rather than remanence, even though it feels like a hold.

Repeat this after the unit has run at working temperature. Remanence and lubricant behaviour both change with temperature, and a cold-only test will mislead you.

If it is mechanical

Hardened or dried lubricant. The most common finding, and the one that gets misdiagnosed as magnetism because the force needed to break free is small and sudden, exactly like a magnetic release. Section the unit or simply clean the guide and retest.
Pressure or vacuum lock. A closed volume on the plunger side holds it by pressure, and the available release force may be a small fraction of a newton. Check whether the chamber vents. A breather hole or a vented seal frequently removes the symptom completely.
Corrosion product or swarf on the guide. Especially after storage in humid air, and especially on steel guides without a coated surface.
Deformed guide bore or bent plunger. Usually a handling or assembly problem, and it shows up as a rub that changes with rotation.
Return spring that is weak, or that has taken a set. Measure its force, do not read its drawing.

If it is magnetic

Material coercivity. Softer magnetic material (lower coercivity) reduces residual flux. This is the change people reach for first, and it is often not the cheapest effective one.
No defined residual gap. A non-magnetic shim that guarantees a small, controlled gap at the closed position gives the residual flux somewhere to work against. In my experience this is cheaper and more reliable than a material change, and it is the first thing I try.
Residual field from the housing or a plated part. Galvanised or plated components can carry their own field. Check whether the symptom follows the plating batch.
Coil driven with a slow decay. A freewheeling diode across the coil extends current decay and can hold the plunger briefly. If the “stick” lasts a fraction of a second rather than indefinitely, this is your answer and it is a drive-circuit change, not a solenoid change.

What actually to change

Release force	Likely cause	First thing to change
Near zero	Grease, pressure lock, corrosion	Clean and re-lubricate with a temperature-appropriate grease; vent the chamber
Small, grows with energising time	Remanence	Add a non-magnetic shim to define a residual gap
Small, independent of energising time	Spring or pressure	Measure the spring; check the chamber
Present for a fraction of a second only	Slow current decay	Remove or resize the freewheeling diode

When it IS the harder problem

Seasonal and batch-independent. If it only happens in winter and it affects units from every batch, you are looking at lubricant viscosity, and no amount of magnetics work will help. This is the single most misdirected investigation I see.

Position-dependent. If it sticks in one orientation but not another, gravity is participating — which means the holding force is already marginal. Fixing the force margin is the answer; fixing the material is not.

Only in the assembled machine. The solenoid is probably being asked to release against a pressure differential it was never specified against. This is a system-level problem presented as a component defect, and it is worth measuring the pressure on the plunger side before touching the solenoid at all.

A note on what this page is

This is a personal notebook, not a product page. I write down the checks that actually decide the outcome and the order to run them in, including the ones I got wrong first.

solenoid sticks after power offsolenoid residual magnetismsolenoid remanencesolenoid will not releasesolenoid plunger stuck

Frequently asked

How do I tell remanence from a mechanical bind without special equipment?: With the coil de-energised, apply a gradually increasing axial pull to the plunger and record the force at which it breaks free. Near zero means the plunger is not held magnetically — look at grease, guide surfaces, and pressure lock. A clearly measurable force that scales with how long the coil was energised points at remanence. Watching whether it 'feels stuck' tells you nothing, because both cases look identical.
Can I fix remanence by just increasing the return spring?: It will mask it, and it costs you force on the pull-in stroke, because spring force peaks where solenoid force is weakest. It is a legitimate temporary field fix but a poor design answer. A non-magnetic shim to establish a defined residual gap is usually cheaper and more effective than changing the core grade.
The plunger sticks only in winter. Why?: Grease. Viscosity rises steeply as temperature falls, and a grease chosen at 25 °C can be near-solid at 0 °C. This is a lubricant selection problem, not a magnetic one, and it is why the failure looks seasonal and batch-independent.
It releases on the bench but not in the machine. What is different?: Pressure. A closed chamber on the plunger side can hold it in place through vacuum or trapped pressure, and the release force available may be a fraction of a newton. Check whether the mechanism vents. A small breather hole or a vented seal often removes the symptom entirely.