How to Test a 12V Relay with a Multimeter

As an Amazon Associate we earn from qualifying purchases.

To test a 12V relay with a multimeter, remove power from the circuit the relay switches, check the coil, then check the switch contacts with the relay off and again with the coil energized at its rated voltage. The contacts should be open with the coil off and should close when the coil is powered. The two sources we opened agree on this sequence, but they disagree on how to check the coil, so the article states both approaches and does not pick one silently.

Quick answer

• Remove power from the load the relay switches, and make sure the relay is not energized before you connect the meter.
• Check the coil. One source measures coil resistance, and the other energizes the coil at its rated voltage and listens for the click.
• With the coil off, the switch contacts should read open. With the coil energized, they should close and read near zero.
• If the contacts do not change state between those two conditions, replace the relay with one of the same model.

The person typing this wants to know

The person typing this phrase has a relay that is not switching a 12V load, such as a fan, a pump, a light, or an accessory circuit, and wants a way to tell whether the relay itself has failed. They need a bench-level check they can do with a multimeter and a 12V supply, and they need to know what result means a bad relay. This guide covers that low-voltage test. It does not cover mains-voltage relays.

How we chose

We opened two sources and read them. The first is a Docsie tutorial, Test a relay with a multimeter. It is a short procedure for a four-terminal relay, and it applies the rated voltage to the coil and checks the contacts before and after. The second is a Hackaday page titled How to test a relay. It describes measuring coil resistance, then checking the contacts without power and with power applied. We did not use search snippets, and we did not use any vendor shop page. Where one source gives a number and the other does not, we say so.

Before you start: remove power and identify the pins

The Docsie tutorial begins by saying to keep the relay on a flat, non-conductive surface, and to make sure the relay is not energized before you connect the multimeter. The Hackaday page says to remove power from any load on the relay before you test it. Both advise you to check the markings on the relay body, because pin layouts differ between relays.

A four-terminal relay has two coil terminals and two switch terminals. The Docsie tutorial says to use the markings on the body and the circuit diagram to tell them apart. The Hackaday page labels the common terminal as 30 and the normally open terminal as 87, and it uses those labels in its contact steps. If your relay uses different numbers, use the diagram on its datasheet rather than these labels.

Step 1: check the coil

There are two methods in the sources, and they do not give the same result.

The Hackaday page measures coil resistance. It sets the meter to resistance, uses a range around 2 kΩ, and probes the two coil terminals. It says a standard 12V relay has a coil resistance of around 50 to 200 ohms. A reading far outside that range, or an open reading, suggests the coil is damaged. The Hackaday page gives this single range, and we could not confirm it against a second source, so treat it as a rough check and not a pass limit.

The Docsie tutorial does not measure coil resistance. It checks the coil by applying the rated DC voltage, which it gives as 12V in its example, from a supply that matches the coil’s rating, and then confirming the relay responds. It says to listen for a click, which shows the switch has moved. The Docsie tutorial does not give a resistance figure for the coil.

Use the datasheet for the exact coil resistance and rated voltage of your relay. If the two methods disagree on your relay, the datasheet decides.

Step 2: check the contacts with the coil off

With no power on the coil, set the meter to continuity or resistance mode and probe the switch terminals. The Docsie tutorial says the display should read 1 or OL, which means an open circuit. The Hackaday page says to measure the common terminal, 30, against the normally open terminal, 87, and expects an open reading, shown as OL. Both sources agree that the open reading is the expected state with the coil off.

If the contacts read closed with the coil off, the relay may be stuck closed, and the sources we read do not describe a repair. Remove the relay from service in that case.

Step 3: check the contacts with the coil energized

Apply the rated voltage to the coil terminals from a separate supply. The Docsie tutorial says to use a supply within the relay’s rated voltage, and to check the polarity of the coil connection before you apply power. Make secure connections before you apply power, and keep the meter probes on the switch terminals.

With the coil energized, the switch should close. The Docsie tutorial gives a reading near 0 ohms as confirmation, and its example reads 0.04 Ω. The Hackaday page says to probe the contacts again with the relay still mounted and powered. If the contacts stay open while the coil is energized, the relay has failed to switch.

When you are finished, remove the coil power and check the contacts again. The Docsie tutorial says the switch should return to open. A contact that stays closed after the coil is off has a fault, and the source says to replace the relay with one of the same model.

Reading the results

• Coil resistance outside the stated range, or an open coil: suspect the coil. Check the datasheet first.
• Contacts open with the coil off, closed near zero with the coil on, and open again after the coil is off: the relay is working.
• Contacts that never close with the coil on: the relay has failed to switch.
• Contacts that stay closed with the coil off: the relay may be stuck.

Safety notes

• Test only a relay that is removed from the circuit or with the load disconnected.
• Match the supply voltage to the coil’s rating. Applying a higher voltage can destroy the coil. The sources do not give a figure for this, so use the datasheet.
• Keep the relay on a non-conductive surface, and do not hold it in your hand while the coil is energized.
• Disconnect the supply and the wiring when you are finished.
• The Docsie tutorial gives few explicit safety warnings. It does not cover mains-voltage relays, and this article does not cover them either.

When a relay test does not settle the question

A relay can pass the bench test and still fail in the vehicle or the equipment, for example because of a bad connection in the socket or a fault in the control signal. The sources we read do not describe those cases. Check the wiring and the socket, and confirm the control signal is reaching the coil at its rated voltage, before you blame the relay.

If you want a broader view of what a meter can and cannot tell you about a circuit, our article on continuity testers covers the basic continuity method. For an outlet-related check, see GFCI open ground.

What we could not verify

• The 50 to 200 ohm coil range comes from the Hackaday page only. We could not confirm it against a second source or a datasheet, and the Docsie tutorial gives no coil resistance.
• The two sources check the coil differently. We could not reconcile the resistance method with the rated-voltage method, so both are described.
• The Docsie tutorial’s example reading of 0.04 Ω is for its own example relay. It is not a pass limit for every relay.
• Neither source gives a value for the maximum coil voltage, the contact rating, or the switching current. This article does not state them.
• Neither source covers automotive relays with a diode or resistor across the coil. Check your relay’s diagram for that.

FAQ

What coil resistance should a 12V relay have?

The Hackaday page gives around 50 to 200 ohms for a standard 12V relay. The Docsie tutorial does not give a number. Check the datasheet for your relay, because the figure depends on the model.

Can I test a relay while it is wired into the circuit?

The sources say to remove power from the load and to make sure the relay is not energized before you connect the meter. Test the relay on the bench, out of the circuit, if you can.

Why does the relay click but the contacts stay open?

The sources we read say a click shows the switch has moved, but they do not explain why the contacts might stay open. Check the contact reading with the meter, and check the wiring to the switch terminals.

What if the contacts stay closed with the coil off?

The Docsie tutorial says the switch should return to open when the coil is off. If it does not, the source says to replace the relay with one of the same model.

Is this test safe on a mains relay?

The sources do not cover mains relays, and this article does not give steps for them. Mains wiring must be de-energized and locked out first, and it should be handled by a qualified person using a rated tester.

Bottom line

Remove power, check the coil, then check that the contacts are open with the coil off and closed with the coil on at its rated voltage. A relay that does not switch between those two states should be replaced with one of the same model.

Sources

• Docsie, Test a relay with a multimeter: https://www.docsie.io/tutorials/field-service/test-a-relay-with-a-multimeter/ (opened 2026-10-30)
• Hackaday, How to test a relay: https://hackaday.io/page/12738-how-to-test-a-relay (opened 2026-10-30)