Testing a solenoid coil takes a basic multimeter and about five minutes. The most reliable method is measuring the coil’s resistance (in ohms) and comparing it to the manufacturer’s spec. If you get an “OL” (overload) reading, the coil is open and dead. If you get zero or near-zero ohms, the coil is shorted. Either way, it needs replacing. Beyond resistance, you can verify voltage supply, check for ground faults, and listen for telltale sounds that point to specific problems.
What You Need Before You Start
A digital multimeter with resistance (Ω) and voltage settings covers every test described here. You’ll also want insulated gloves and safety glasses, especially if the solenoid will be energized at any point during diagnosis. Before touching anything, disconnect the solenoid from its electrical connections. If you’re working on a solenoid that’s part of a larger system (an irrigation valve, a hydraulic circuit, an automotive starter), de-energize the whole circuit first. Solenoid coils can produce voltage spikes when power is suddenly removed, so treat even a “dead” coil with respect.
Have the manufacturer’s documentation handy if possible. Resistance values vary widely between solenoid types. A small 12V DC solenoid valve might read 20 to 60 ohms, while a large 240V AC coil could read in the thousands. Without a spec to compare against, you can still catch open and shorted coils, but you won’t be able to confirm a coil is within its healthy range.
The Resistance Test (Most Important)
This is the core test. Set your multimeter dial to the ohms (Ω) setting. Most solenoids have two electrical terminals for the coil winding, plus sometimes a separate ground terminal that looks different from the other two. If you’re unsure which terminals are which, check the manual. Touch one multimeter probe to each of the two coil terminals.
Here’s how to read the result:
- A reading within the manufacturer’s spec range: The coil winding is intact and healthy.
- An “OL” or infinite reading: The wire inside the coil is broken somewhere. No current can flow, so the coil can’t create a magnetic field. It’s dead and needs replacing.
- Zero or near-zero ohms: The coil’s windings are shorted together, bypassing most of the wire. This means the coil draws too much current and won’t generate the correct magnetic field. Replace it.
If you don’t have a spec sheet, a reading somewhere between roughly 10 and 5,000 ohms (depending on the coil’s size and voltage rating) at least tells you the winding is continuous. An OL or zero reading is always bad regardless of the specific coil type.
Testing for a Ground Fault
A coil can pass the resistance test but still fail because its internal insulation has broken down, allowing current to leak to the metal housing. This is called a short to ground, and it can blow fuses, trip breakers, or create shock hazards.
To check, keep your multimeter on the ohms setting. Touch one probe to either coil terminal and the other probe to the solenoid’s metal body or mounting bracket. The multimeter should display “OL,” meaning no electrical connection exists between the coil winding and the housing. Any resistance reading here means the insulation has failed and current is leaking to ground. The coil needs to be replaced.
Checking the Voltage Supply
If the coil tests fine on resistance but the solenoid still isn’t working, the problem might not be the coil at all. The next step is verifying that the coil is actually receiving the voltage it needs.
Reconnect the solenoid to its circuit. Set your multimeter to the appropriate voltage setting (DC or AC, matching your system). With the circuit energized and the solenoid commanded to activate, measure the voltage across the two coil terminals. The reading should match the coil’s rated voltage. A 24V coil should see close to 24 volts. If you’re reading significantly less, something upstream is stealing voltage: a bad relay, corroded wiring, a weak power supply, or a failing control switch.
Low voltage doesn’t just mean weak performance. In AC solenoids, undervoltage can prevent the plunger from fully pulling in, causing it to oscillate between open and closed. This produces an audible hum or buzz and generates excess heat that will eventually burn out the coil.
The Quick Magnetic Field Check
If you want a fast confirmation that a coil is energizing without pulling out a multimeter, there’s an even simpler approach. With power applied to the coil, bring a small steel screwdriver or a thin piece of steel wire close to the coil body. If the coil is working, you’ll feel the magnetic pull drawing the metal toward it. No pull means no magnetic field, which points to either a dead coil or no power reaching it.
This test won’t tell you if the coil is weak or partially shorted. It’s a pass/fail check, useful for quick triage in the field before you break out more precise tools.
What the Sounds Tell You
Solenoids are surprisingly talkative when something goes wrong, and learning what each sound means can speed up diagnosis considerably.
A single firm click when the solenoid activates (and another when it deactivates) is normal. That’s the plunger snapping into position. If you hear rapid clicking, every second or faster, that typically points to a power supply issue. The coil is getting just enough voltage to pull the plunger in, but not enough to hold it. It releases, gets pulled in again, releases again, creating a rapid stutter. Check your voltage supply and wiring connections.
A continuous hum or buzz from an AC solenoid usually means undervoltage, as described above. The plunger can’t fully seat, so it vibrates in place. A chattering or rattling noise can indicate loose or missing internal parts, which is a mechanical problem rather than an electrical one. And complete silence when the solenoid should be activating means either the coil is dead or it’s not receiving power at all.
Separating a Coil Problem From a Mechanical Problem
Sometimes the coil is perfectly fine but the solenoid still won’t operate because the valve or plunger is stuck mechanically. Many solenoid valves have a manual override, a small button or knob that lets you physically push the plunger into its activated position without any electrical input.
If pressing the manual override moves the valve and the system responds correctly, you know the mechanical side is working. The problem is electrical: the coil, the wiring, or the control signal. If the manual override also fails to move the plunger, the issue is mechanical. Debris, corrosion, or a damaged spring could be jamming things up. In that case, no amount of coil testing will find the problem because there isn’t one on the electrical side.
Not every solenoid has a manual override, but when one is available, using it early in your troubleshooting saves time by immediately narrowing down which half of the system to focus on.
Common Reasons Solenoid Coils Fail
Coils don’t usually fail randomly. The most frequent killer is overheating, which happens when a coil is energized for longer than it’s rated for, operated at excessive voltage, or installed in an environment that’s too hot for adequate cooling. The insulation on the internal wire degrades, adjacent windings touch, and the coil shorts out.
Moisture is another common cause, especially in outdoor or washdown environments. Water infiltrates the coil housing, corrodes the wire, and eventually causes an open circuit or a ground fault. Voltage spikes from nearby equipment, repeated mechanical shock, and simply running a coil past its expected service life can also do the job. If you’re replacing coils frequently, it’s worth investigating the root cause rather than just swapping in another one.