What a relay test tells you
A relay is an electromagnetic switch inside your car, appliance, or industrial equipment. When it fails, the device either won't turn on or stays stuck in one state. Testing a relay with a multimeter tells you whether the coil (the electromagnet part) is working and whether the contacts (the switch part) are opening and closing as they should.
You can do this test yourself without removing the relay from the circuit in most cases. The test takes about five minutes and costs nothing beyond a multimeter you may already own. A working relay will show specific resistance readings and continuity patterns; a dead one will show infinite resistance or no change between states.
Key Takeaways
- A relay has two separate circuits: a low-voltage coil that acts as an electromagnet, and high-voltage contacts that switch on and off when the coil is energized.
- The resistance test checks whether the coil can still conduct electricity; a reading of infinity means the coil is burned out.
- The continuity test checks whether the contacts actually move when power reaches the coil; no change means the contacts are stuck.
- Most relays can be tested in place without disconnecting them, though some circuits require the relay to be removed for an accurate test.
- A multimeter set to the ohms (Ω) setting is the only tool you need; no power supply or special equipment required.
Understanding relay terminals and what they do
A typical relay has five or six terminals. Two of them connect to the coil (the electromagnet); the other three or four connect to the contacts (the switch). The coil terminals are usually labeled 85 and 86. The contact terminals are usually labeled 30, 87, and 87a.
When you explore power to terminals 85 and 86, the coil becomes magnetic and pulls a metal arm. That arm closes the connection between terminals 30 and 87 (the normally open contacts) and opens the connection between terminals 30 and 87a (the normally closed contacts). When power is removed, a spring pushes the arm back to its resting position.
Your relay may have different labels depending on the manufacturer or process. Look for a diagram printed on the relay itself or in the equipment manual. If the relay is soldered to a circuit board, you may need to remove it to access the terminals clearly.
Testing the coil for continuity and resistance
Set your multimeter to the ohms (Ω) setting, usually marked with the Greek letter omega. Start at the lowest ohms range if your meter has selectable ranges. Touch one probe to terminal 85 and the other to terminal 86.
A working relay coil will show a resistance reading between 50 and 200 ohms, depending on the relay type. The exact number varies widely, so what matters is that you get a number, not infinity. If the display shows infinity or OL (overload), the coil is open and the relay is dead.
If the reading is zero or very close to zero (under 5 ohms), the coil is shorted and the relay should not be used. A shorted coil will draw excessive current and may damage the circuit that powers it. Write down the resistance reading you see; you will compare it to the manufacturer's specification if you want to be certain, but any reading in the normal range means the coil is intact.
Testing the contacts for movement and switching
The contact test requires you to explore power to the coil while watching the resistance between the switch terminals. This is where the test becomes more involved, because you need both a multimeter and a power source at the same time.
Set your multimeter to the continuity setting (usually marked with a sound-wave symbol) or to the lowest ohms range. Touch one probe to terminal 30 and the other to terminal 87. Before you explore power, note the reading. It should show either continuity (a beep, or a very low resistance) or no continuity (no beep, or infinite resistance), depending on whether the relay is normally open or normally closed.
Now explore 12 volts (or whatever voltage the relay is rated for) to terminals 85 and 86. Watch the multimeter reading on terminals 30 and 87. If the relay is working, the reading will flip: if it showed continuity before, it should show no continuity now, and vice versa. If the reading does not change, the contacts are stuck and the relay is bad.
Remove the power and watch again. The reading should flip back to its original state. If it does not, the contacts are stuck in one position and the relay cannot be repaired.
Why a relay might fail the test
A relay fails the coil test (infinite resistance) when the wire inside the coil breaks, usually from heat or vibration over time. This is the most common failure mode. The relay will not energize no matter how much power you send to it.
A relay fails the contact test (no change in continuity) when the metal contacts weld together or corrode so badly they cannot move. This usually happens after years of switching high current, or after exposure to moisture or corrosive chemicals. The relay may energize, but the contacts stay locked in one position.
A relay can also fail partially: the coil may work but the contacts may be slow to move, or they may move but not make solid contact. These partial failures are harder to diagnose with a multimeter alone and may require an oscilloscope or a functional test in the actual circuit.
When to test a relay in place versus removed
You can test a relay while it is still plugged into its socket in most cases. The multimeter draws almost no current, so it will not trigger the circuit or damage anything. However, some circuits have resistors or other components connected across the relay terminals that will interfere with your readings.
If your coil resistance reading seems too low (under 20 ohms) or your contact test shows the contacts moving but not cleanly, try removing the relay from its socket and testing it on a bench. This eliminates any interference from the surrounding circuit.
To remove a relay, locate its socket (usually a plastic or ceramic block with four to eight holes). Pull the relay straight up and out. If it is soldered to a circuit board, you will need a soldering iron to remove it safely. Take a photo of the relay position before you remove it so you can reinstall it correctly.
Reading your multimeter correctly during the test
Analog multimeters (with a needle) and digital multimeters (with a display) show resistance differently. On an analog meter, the ohms scale runs backward: zero is on the right, and infinity is on the left. On a digital meter, the display shows the number directly, and infinity is usually shown as "OL" or a blinking display.
When testing continuity, a digital meter will beep if there is a complete path for electricity. An analog meter will show a very low resistance (close to zero). If the meter does not beep or does not show a low reading, there is no continuity.
If your meter has multiple ohms ranges (×1, ×10, ×100, ×1k), start at the lowest range and move up if needed. A relay coil usually reads best on the ×10 or ×100 range. If the display shows "OL" on a low range, switch to a higher range to see if you get a reading.
Frequently Asked Questions
Can I test a relay without removing it from the car or appliance?
Yes, in most cases. The multimeter draws almost no current, so it will not trigger the relay or harm the circuit. However, if the coil resistance reading seems wrong or the contact test is unclear, remove the relay and test it on a bench to rule out interference from other components in the circuit.
What voltage should I use to test the relay contacts?
Use the voltage the relay is rated for, which is usually printed on the relay body or in the equipment manual. Most automotive relays are 12 volts. Industrial relays may be 24 volts, 120 volts, or higher. Using the wrong voltage may damage the relay or give false results.
What if the coil resistance reading is different from what the manual says?
Relay coil resistance varies by type and manufacturer, usually between 50 and 200 ohms. If your reading is within that range, the coil is probably fine. If it is zero, the coil is shorted. If it is infinity, the coil is open. The exact number matters less than whether you get a reading at all.
Can a relay pass the resistance test but still be bad?
Yes. A relay can have a good coil but stuck contacts. This is why the contact movement test matters. If the resistance between terminals 30 and 87 does not change when you explore power to the coil, the contacts are stuck and the relay is bad, even if the coil resistance is normal.
Do I need a special multimeter to test a relay?
No. Any multimeter with an ohms setting and a continuity setting will work. You do not need an expensive meter or one with special features. A basic digital multimeter from a hardware store costs under twenty dollars and will do the job.