What a relay test tells you

A relay is an electromagnetic switch that uses a small electrical signal to control a larger one — like a car starter motor or an air conditioning compressor. When a relay fails, the device it controls stops working. Testing one with a multimeter tells you whether the relay's coil (the electromagnet) is working and whether its contacts are opening and closing as they should.

A multimeter can perform two useful checks: measuring the coil's resistance to see if it's burned out, and testing the contacts to see if they're stuck open or closed. You won't know the exact reason a relay failed, but you'll know whether it's the problem or whether you should look elsewhere.

Key Takeaways

  • A relay has two parts to test: the coil (which should show a specific resistance value) and the contacts (which should switch between open and closed).
  • Set your multimeter to the resistance or ohms setting (Ω) to measure the coil; a reading of zero or infinity usually means the relay is bad.
  • Testing contacts requires either a continuity setting or resistance mode, depending on your multimeter, and the relay must be unpowered during the test.
  • Always remove power from the circuit before testing, and consult the relay's datasheet to know what resistance value the coil should show.

Locating the relay and identifying its pins

Relays come in different shapes, but most automotive and household relays are rectangular blocks with four to eight pins sticking out the bottom. The pins are numbered, usually printed on the relay itself or on the socket it plugs into. Before you test, you need to know which pins belong to the coil and which are the contacts.

The relay's datasheet or the device manual will show a diagram of the pin layout. If you don't have the datasheet, search online for the relay's part number (printed on top of the relay) plus "pinout" or "datasheet". Common automotive relays follow standard layouts — pins 1 and 3 are often the coil, while pins 2, 4, 5, and 8 are contacts — but this varies, so verify before you start.

Once you know the pin layout, remove the relay from its socket by pulling straight up. Keep it nearby so you can reference the socket's pin numbers if the relay itself isn't clearly marked.

Testing the coil with resistance mode

Set your multimeter to the resistance setting, marked with the Greek letter omega (Ω). The resistance of a relay coil typically ranges from 50 to 200 ohms, but the exact value depends on the relay type — this is why the datasheet matters.

Touch one multimeter probe to one coil pin and the other probe to the second coil pin. A healthy coil will show a steady resistance reading that matches the datasheet value. If the multimeter shows zero ohms, the coil is shorted and the relay is bad. If it shows infinity (often displayed as "OL" for open line), the coil is broken internally and the relay is bad.

If the reading is close to the expected value, the coil is probably working. Move on to testing the contacts. If the reading is way off — much higher or lower than expected — the relay is likely faulty, but contact testing can confirm.

Testing the contacts with continuity or resistance

Relay contacts can be either normally open (NO) or normally closed (NC). Normally open contacts are open when the coil is unpowered and should close when power is applied. Normally closed contacts are closed when unpowered and should open when power is applied. Your datasheet will tell you which type you have.

For an unpowered relay (which is what you're testing now), set your multimeter to continuity mode if it has one — this mode beeps when two points are electrically connected. Touch the probes to the two contact pins. For a normally open contact, you should see no continuity (no beep, infinite resistance). For a normally closed contact, you should see continuity (beep, near-zero resistance).

If you don't have a continuity setting, use the resistance mode instead. The same rules apply: normally open contacts show infinity, normally closed contacts show near-zero ohms. If a normally open contact shows continuity when it shouldn't, or a normally closed contact shows no continuity when it should, the contacts are stuck and the relay is bad.

What to do if the relay fails the test

If the coil shows zero or infinite resistance, or if the contacts are stuck in the wrong state, the relay needs to be replaced. Buy a replacement relay with the same part number or one that matches the pinout and coil voltage of the original. Relays are inexpensive — usually between five and twenty dollars — and swapping one takes seconds.

Before you install the new relay, double-check that you've tested the right component. A relay that fails testing is almost certainly the problem, but if the device still doesn't work after you replace it, the issue may be elsewhere in the circuit — a blown fuse, a bad wire, or a failed component the relay controls.

Common mistakes to avoid

The most common mistake is testing a relay while it's still powered. Always disconnect power from the circuit first. Testing a live relay can damage your multimeter or give you a false reading because the coil is energized.

Another mistake is not knowing what resistance value to expect. If you test the coil and get a reading of 120 ohms but the datasheet says it should be 75 ohms, you might think the relay is bad when it's actually fine. Always check the datasheet before you decide a reading is wrong.

A third mistake is confusing the coil pins with the contact pins. If you measure resistance between a coil pin and a contact pin, you'll get a meaningless reading. Use the pinout diagram to make sure you're testing the right pins.

Frequently Asked Questions

Can I test a relay without removing it from the circuit?

No. Testing a relay in-circuit can give false readings because other components in the circuit affect the multimeter's measurement. Always disconnect power and remove the relay from its socket first. This also protects your multimeter from voltage spikes.

What if my multimeter doesn't have a continuity setting?

Use the resistance mode instead. Continuity is just a convenience feature that beeps when resistance is near zero. In resistance mode, zero ohms means the contacts are closed, and infinite ohms means they're open — the same information, without the beep.

How do I know if a relay is normally open or normally closed?

The relay's datasheet will specify this, usually in a diagram or table. If you can't find the datasheet, search the part number online. The relay itself may also have markings like "NO" or "NC" printed near the contact pins, though this isn't always clear.

Can a relay pass the coil test but still be bad?

Yes. A relay can have a healthy coil but stuck contacts. This is why testing both the coil and the contacts matters. If the coil resistance is correct but the contacts won't switch, the relay is still faulty and needs replacement.

What resistance should I expect from a relay coil?

This depends entirely on the relay type. Automotive relays typically range from 50 to 200 ohms, but industrial relays can be higher or lower. Always check the datasheet for the specific relay you're testing. A general rule: any reading between 10 and 500 ohms is probably normal, but zero or infinity is definitely bad.