What a multimeter tells you about a coil

A coil is a wire wound in loops, and it either conducts electricity or it doesn't. A multimeter measures resistance — the opposition to electrical flow — and that's what you need to know. If a coil reads infinite resistance (the meter shows "OL" or "∞"), the wire is broken inside and the coil is dead. If it reads zero or very close to zero, the coil is shorted. A healthy coil reads a specific resistance value that depends on what kind of coil it is.

The test itself takes about two minutes. You don't need to remove the coil from whatever it's in — you can test it in place on an ignition coil, a relay, a transformer, or an inductor. The multimeter does the work; you just touch the probes to the right terminals and read the number.

Key Takeaways

  • Set your multimeter to the ohms setting (Ω symbol) before you touch the coil, and make sure the device is powered off.
  • Infinite resistance (OL or ∞ on the display) means the coil wire is broken and the coil has failed.
  • Zero or near-zero resistance means the coil is shorted and also needs replacement.
  • A healthy coil reads a specific resistance value — check your equipment manual or the coil label to know what number to expect.
  • If the coil is part of a larger assembly, you may need to disconnect it first so you're only measuring the coil itself, not other components in parallel.

Setting up the multimeter correctly

Turn off the device the coil is in. This is not optional — testing a live coil can damage the multimeter or give you a false reading. If the coil is removable, take it out. If it's soldered to a board or bolted in place, you can test it where it sits, but disconnecting it gives you a cleaner result because you're not measuring other components at the same time.

Set the multimeter dial to ohms (Ω). Most multimeters have a dial with several ranges: 200Ω, 2kΩ, 20kΩ, 200kΩ, and 2MΩ. Start with 200Ω if you expect a low resistance (like an ignition coil, which usually reads 1 to 10 ohms). Start with 2kΩ or higher if you expect medium to high resistance (like a relay coil, which might read 100 to 1000 ohms). If you're not sure, start high and work down — a high range won't hurt, it just shows "OL" if the resistance is too low for that scale.

Make sure the probes are plugged into the right jacks. The black probe goes into the jack marked COM (common). The red probe goes into the jack marked Ω or V (the same jack works for both). If your multimeter has a separate jack for high resistance, use that instead.

Finding the coil terminals and taking the reading

Look at the coil and identify the two terminals where current enters and exits. On an ignition coil, these are usually marked as primary and secondary, or labeled with + and −. On a relay or transformer, they're the two wire leads or the two pins. If the coil is part of a larger component, check the manual or the label on the part itself — it will show you which terminals belong to the coil you're testing.

Touch the red probe to one terminal and the black probe to the other. The order doesn't matter for resistance — you'll get the same reading either way. Wait a second or two for the meter to stabilize. The number on the display is the resistance in ohms. Write it down or take a photo of the meter so you have it for comparison.

If the meter shows "OL" or "∞", the coil is open (broken). If it shows 0 or 0.1 or any number very close to zero, the coil is shorted. Either way, the coil has failed and needs to be replaced. If the meter shows a steady number in the range you expected, the coil is probably good — but compare it to the specification in the manual to be sure.

Understanding what the numbers mean

Every coil has a resistance specification. An ignition coil primary winding typically reads 0.5 to 2 ohms. A relay coil might read 100 to 500 ohms. A transformer secondary might read 1000 to 5000 ohms. The exact number depends on the wire gauge, the number of loops, and what the coil is designed to do. Your equipment manual, the coil label, or the manufacturer's datasheet will tell you what to expect.

If your reading is within about 10 percent of the specification, the coil is good. If it's way off — say, the manual says 50 ohms but you read 200 — the coil is degraded or failing. If it reads infinite or zero, it's dead and must be replaced. If you're testing a coil that's part of a larger circuit and you get a reading that's much lower than expected, you may be measuring other components in parallel with the coil; disconnect the coil completely and test again.

When the coil is still in the circuit

You can test a coil without removing it, but you need to understand what you're measuring. If other components are wired in parallel with the coil — meaning they share the same two terminals — the multimeter will measure the combined resistance of everything in parallel, not just the coil. This gives you a false low reading.

To avoid this, disconnect at least one terminal of the coil from the circuit before you test. If the coil is soldered to a board and you can't disconnect it without desoldering, you have two choices: desolder it carefully, or accept that your reading might be lower than the true coil resistance. If the reading is infinite or very close to zero, the coil is definitely bad regardless of what else is in parallel. If the reading is in the normal range, the coil is probably fine.

Troubleshooting a reading that doesn't match the manual

If your reading is much higher than the specification, the coil wire is partially broken or corroded. The coil has failed and should be replaced. If your reading is much lower, either the coil is shorted, or you're measuring other components in parallel. Disconnect the coil completely and test again.

If the meter shows "OL" but you expected a low number, double-check that you're touching the right terminals. Some coils have more than two terminals — a primary winding and a secondary winding, for example. You need to test each winding separately. Check the manual to see which terminals belong to which winding, then test each pair.

If you're testing an ignition coil and the reading is way off, the coil may be temperature-sensitive. Ignition coils sometimes read differently when hot than when cold. If the coil is warm from running, let it cool for a few minutes and test again. If the reading changes significantly, the coil is failing and should be replaced.

Frequently Asked Questions

Can I test a coil while it's powered on?

No. Always turn off the device before testing. A live coil can damage the multimeter, give you a false reading, or cause injury. If you're not sure how to turn off the device safely, consult the manual or ask someone familiar with it.

What does "OL" mean on the multimeter display?

OL stands for "overload" or "open line." It means the resistance is too high for the current meter range to measure — usually infinite, which indicates a broken wire inside the coil. The coil has failed and needs replacement.

Do I need to disconnect the coil to get an accurate reading?

Disconnecting gives you the cleanest reading because you're measuring only the coil, not other components in parallel. If the coil is hard to remove, you can test it in place, but understand that the reading might be lower than the true coil resistance if other parts share the same terminals.

What if the coil has more than two terminals?

The coil may have a primary winding and a secondary winding, each with its own pair of terminals. Check the manual to identify which terminals belong to which winding, then test each pair separately. Each winding should read its own specification.

Can a coil read the right resistance but still be bad?

Yes, though it's rare. A coil can have the correct resistance but fail under load or when hot. If you've tested the resistance and it's in spec, but the device still doesn't work, the problem may be elsewhere — or the coil may be failing in a way the multimeter can't detect. In that case, you'll need to test the coil under operating conditions or replace it as a troubleshooting step.