How to Test a Relay With a Multimeter: A Detailed Guide
A multimeter tests a relay in two ways. First, set it to ohms (Ω) mode and measure the coil resistance, which should read 50-120 Ω on most 12 V relays. Then switch to continuity mode and confirm the contacts open and close correctly when the coil is energized versus de-energized.
A relay that won’t switch can shut down everything from a car’s fuel pump to a bench-top automation project. The good news is you don’t need specialized gear to diagnose one. A multimeter with resistance, continuity, and diode modes can tell you whether a relay is healthy, dead, or hiding an intermittent fault, usually in under five minutes.
The same basic test works whether you’re checking a relay on the bench or chasing a no-start problem in a car. Solid-state relays work differently, but you can test those with a multimeter too, using one additional diode-mode check.
What is a relay and how does it work?
A relay is an electrically operated switch. Current flowing through a coil creates a magnetic field that pulls a movable armature. That armature closes (or opens) a separate set of contacts, letting a low-power signal switch a much larger load.
Inside a standard electromechanical relay, you’ll find four key parts. There’s a coil, an armature, a spring, and a contact set.
When current flows through the coil, it becomes an electromagnet and pulls the armature against the spring’s tension, switching the contacts. Cut the power, and the spring returns the armature to its resting position.
The magic here is isolation. A tiny current, often just a few milliamps from a microcontroller or a switch on your dashboard, can control circuits carrying many amps.
Relays show up everywhere, from car electrical systems to industrial control panels. If you want to see this in action, try a hands-on relay project that uses a relay to switch between two LEDs.
Normally open (NO) vs normally closed (NC) contacts
At rest, with no power applied to the coil, the armature sits against the normally closed (NC) contact. Energizing the coil moves it to the normally open (NO) contact. This is the switching behavior you’ll check during the continuity test.
Automotive relays use standardized pin numbers, and 85 and 86 are the coil terminals:
- 30: The common contact
- 87: The normally open (NO) contact
- 87a: The normally closed (NC) contact, found on 5-pin relays
So at rest, pin 30 connects to 87a. Once the coil energizes, pin 30 jumps over to connect with 87.
Common relay types you’ll encounter
Relays come in a few standard shapes, and the pinout is what tells them apart.
- 4-pin SPST: One switched output and one common contact. Common in automotive fans and horns.
- 5-pin SPDT: One common plus both an NO and NC contact. Common in fuel-pump and starter circuits.
- 6-pin / 8-pin DPDT: Two independent switch pairs in one package. Common in industrial gear and DIY audio.
- 3-pin: Less common and mostly found in hobbyist builds. The pinout varies by design, so check the datasheet before testing.
- Reed relay: A sealed glass tube with magnetically actuated contacts, often with just two leads. Common in RF and instrumentation.
- Solid-state relay (SSR): Uses an LED, an optocoupler, and a TRIAC or MOSFET, with no moving parts. Common in AC-load control.
Tools and safety before you start
You’ll need a digital multimeter with Ω, continuity, and diode modes, a set of jumper wires, an appropriate DC power source (9-12 V for most coils), and insulated probes.
If your bench only has room for one accessory beyond the meter, make it the jumper wires. Half of relay testing is getting the coil powered without holding two loose leads in place by hand.
Before you pull a relay from any live system, de-energize the circuit. Turn off the ignition, unplug the device, or disconnect the battery.
Relay contacts can carry serious current, and yanking a relay under load risks arcing, blown fuses, or a nasty shock. Once everything is powered down, you can safely remove the relay and move it to your bench for accurate testing.
How to test a relay with a multimeter (step-by-step)
A multimeter can tell you whether the problem is in the relay coil or the contacts. The steps below cover both.
Step 1: Remove the relay from the circuit (or leave it in?)
For an accurate coil-resistance reading, bench testing is the way to go. Any component wired in parallel, such as a bulb, motor, or another coil, creates an alternate path that can corrupt the measurement, and you might pass a relay that’s actually bad.
In-circuit voltage checks have their place, though. You can confirm the relay is receiving switching voltage at pins 85 and 86 while it’s still socketed, which tells you whether the control side is doing its job.
But for coil resistance and contact continuity, pull the relay first. It only takes a second and saves you from chasing false readings.
Step 2: Measure coil resistance
Set your multimeter to the ohms (Ω) range and touch the probes to the coil terminals, pins 85 and 86. For a typical 12 V automotive relay, expect a reading between 50 and 120 Ω. Some datasheets narrow this to 60-95 Ω for specific parts, so check the spec sheet if you have it.
Two readings spell trouble. If the meter shows OL (over limit), the coil is open, since there’s a break somewhere in the winding, and the relay is dead.
A reading below 5 Ω usually means the coil is shorted. Either way, it needs replacing.
Step 3: Check contact continuity (de-energized)
With the coil unpowered, switch your DMM to continuity mode. Probe the common contact (pin 30) to the NO contact (pin 87). No beep is what you want here, since these contacts sit open at rest.
Next, probe pin 30 to the NC contact (pin 87a). This time you should hear a beep, confirming that the contacts are closed at rest.
Step 4: Energize the coil and re-check the contacts
Now apply the rated DC voltage, typically 9-12 V, to the coil terminals at pins 85 and 86. If the relay includes a coil-side flyback diode, watch your polarity. Pin 86 is usually positive and pin 85 negative. You should hear an audible click as the armature moves.
With the coil powered, the contacts switch positions. Probe pin 30 to pin 87 again. The meter should now beep, confirming that the normally open (NO) contact has closed.
Probe 30 to 87a, and the beep should disappear, confirming the NC contact has opened. A relay that clicks but won’t swap contacts is still faulty.
Step 5: Interpret the results
The truth table below sums up every outcome you’re likely to see.
| Coil test | Contact test (energized) | Verdict |
|---|---|---|
| 50-120 Ω | 30→87 closes / 30→87a opens | Relay OK |
| Open (OL) | N/A | Coil open, replace relay |
| Short (< 5 Ω) | N/A | Coil shorted, replace relay |
| 50-120 Ω | 30→87 does NOT close | Contact stuck open, replace |
| 50-120 Ω | 30→87a does NOT open | Contact welded, replace |
Testing a 4-pin vs 5-pin vs 6-pin relay
Nothing about the core method changes here. Measure coil resistance, then check contacts before and after energizing. What changes is how many continuity checks you run.
4-pin relay (SPST, one switched output)
Testing a 4-pin relay with a multimeter follows the same basic method, just with fewer contacts to check. This one has pins 85 and 86 for the coil, plus 30 and 87 for a single switched output.
Measure coil resistance across 85 and 86, then run one continuity check between 30 and 87. With the coil off, expect no continuity. Energize it, and 30 should connect to 87 with an audible click.
5-pin relay (SPDT, one common + NO + NC)
A 5-pin relay is what you’ll usually find behind a fuel pump or starter circuit, and it adds pin 87a for the normally closed contact. The full pinout is 85, 86, 30, 87, and 87a.
Test the coil the same way, then run two continuity checks. When de-energized, 30 connects to 87a but not 87. When energized, 30 connects to 87 but not 87a.
6-pin / 8-pin relay (DPDT and beyond)
A 6-pin or 8-pin relay packs two independent switch pairs into one housing. Treat each pair like a separate SPDT relay.
Measure the shared coil resistance, then run continuity checks on each pole, de-energized and energized, to confirm both sets of contacts switch together. If one pair works and the other doesn’t, the relay is bad.
Testing a solid-state relay (SSR) with a multimeter
SSRs have no moving coil, so the usual resistance check doesn’t apply. Test them differently. Use diode mode on the input pins to verify the optocoupler LED, measure the output in the OFF state with ohms (it should read OL), then apply control voltage and confirm the output conducts.
Why the coil test doesn’t apply
A solid-state relay has no coil, armature, or spring. Instead, it uses an input LED, an optocoupler, and a TRIAC or MOSFET on the output.
When current drives the LED, its light triggers the photo-sensitive switch, letting current flow through the load side with no mechanical movement at all. That’s why there’s no coil resistance to measure and no click to listen for.
Diode-mode check on the input
Set your DMM to diode mode and probe the input pins. A healthy input LED shows a forward voltage drop of roughly 1.0-1.4 V in one direction and OL in the other.
If both directions read OL, or both read near zero, the input stage is dead and the SSR needs replacing.
For a reference design that shows the LED-plus-optocoupler architecture in action, see Digilent’s Pmod SSR.
Testing specific automotive relays
Whether it’s a starter, a fuel pump, or another car relay under the hood, the core method stays the same.
How to test a starter relay
Testing a starter relay with a multimeter means measuring coil resistance across 85 and 86, then checking that the contacts switch when energized, same as any other relay.
Watch the pinout, though. Some starter relays are non-standard 4-pin bar-type units. A classic symptom is a single click without the engine cranking, usually a dead coil or contacts that can’t handle the starter’s heavy current.
How to test a fuel pump relay
Fuel pump relays fail for the same kind of no-start reasons, and testing one with a multimeter uses the same coil and contact checks.
Run the standard coil and contact tests on the bench. But if you’re stuck roadside without a meter, the swap trick beats probing every time.
Pull an identical relay from elsewhere in the fuse box, put it into the pump’s socket, and see if the pump starts working. This won’t tell you exactly what failed inside the relay, but it will tell you whether you’ve found the problem.
How to test a horn/flasher/AC relay
A horn relay, a flasher relay, and an AC relay all follow the same procedure. Check coil resistance, then verify contact continuity when de-energized and energized.
Flasher relays can be trickier because some are electronic rather than purely mechanical, so a simple continuity test may not tell the whole story. When in doubt, substitute a known-good relay and see whether the horn honks, the blinker blinks, or the compressor clutch engages.
For a relay with an AC coil, the resistance check itself doesn’t change. With the circuit de-energized, measure across the coil in ohms mode just as you would with a DC relay.
A software-defined alternative: testing relays with the WaveForms DMM
The Analog Discovery 3 running WaveForms gives you a full Digital Multimeter tool on your PC for coil resistance and contact continuity, and WaveForms’ Data Logger can run alongside it to record readings over time, which is ideal for catching intermittent relay faults.
A handheld multimeter gives you a snapshot. But many relay failures are intermittent. A contact might chatter only under vibration, or a coil might fail when it gets hot. Those faults can hide from a single probe.
Run WaveForms from a laptop instead, and it logs coil resistance and contact continuity over time. Leave it recording while you tap the relay, warm it up, or cycle it repeatedly, and you can catch the exact moment a healthy-looking relay drops out.
That’s the advantage of a software-defined instrument: it can keep recording over time instead of giving you a single reading.
For setup details, see Digilent’s guide to using a Digital Multimeter tool.
Common mistakes when testing relays
Three mistakes are especially common. These are mixing up the NO and NC contacts, reversing the polarity on a DC coil with a flyback diode, and measuring resistance while the relay is still in the circuit. Any of these can make a good relay seem faulty or a bad relay seem fine.
Confusing NO and NC contacts
The classic slip-up is reading “good continuity” when the coil is off and assuming the relay is healthy, when in fact you were probing the normally closed contact, which is supposed to be closed at rest.
Always test both 87 and 87a on a 5-pin relay, so you know which contact is which. Most relay cases print a small schematic on the side, worth taking a look at before you probe.
Reversing polarity on a DC coil with a flyback diode
Many modern automotive relays include a coil-side diode to suppress the inductive kick when the coil switches off. Wire the coil backward, and that diode conducts, so the coil never sees full voltage and the relay just sits there silent.
Identify pin 86 (+) and pin 85 (-) from the case marking before applying power, and the armature will click as expected.
Trusting a resistance reading with the relay still in-circuit
Any parallel path in the circuit, bulbs, motors, capacitors, or other coils, corrupts an ohm reading. You’ll see a plausible-looking number that’s actually wrong, and pass a relay that’s genuinely bad.
Pull the relay from its socket, or lift one coil pin off the PCB pad, before measuring coil resistance. That small step can save you from swapping the wrong part.
Test with confidence, catch the faults others miss
A handheld meter covers the usual suspects for everyday checks. But if you’re chasing gremlins, the kind of fault that only shows up under heat or vibration, the Analog Discovery 3 with WaveForms lets you log measurements over time and catch what a single reading would have missed.
Frequently asked questions
How many ohms should a relay coil read?
For a 12 V coil, expect roughly 50-120 Ω. A 24 V coil typically reads 200-400 Ω.
Anything below about 5 Ω suggests a shorted coil, and an OL reading means the coil is open. Always check the datasheet for the exact value on your specific part.
Can I test a relay without removing it from the circuit?
Only partway. You can check the control side while the relay is still in the circuit by probing pins 85 and 86 to see whether it’s receiving switching voltage.
Coil resistance and contact continuity are a different story, since parallel paths elsewhere in the circuit will skew the reading and mask a genuine fault. For those two, pull the relay first.
What are the signs of a bad relay?
Watch for intermittent operation, no audible click when the coil is energized, or a component that’s dead with no other explainable cause. A relay that chatters or rapidly cycles its contacts is also failing and should be replaced.
How do I test a relay without a 12 V battery?
Nine volts is usually enough to make the armature click on most 12 V relays, and it’s worth a shot if that’s all you have on hand. Confirm by ear.
For accurate contact testing, energize the coil at its rated voltage, since a lower voltage may not pull the contacts fully into position.
Why did my relay pass the coil test but the circuit still doesn’t work?
The contacts can be pitted or oxidized even when the coil is perfectly healthy. Measure contact resistance while the relay is energized. It should read under 0.5 Ω.
A high reading means the contacts aren’t passing current cleanly, so the relay still needs replacing.
Do solid-state relays wear out like mechanical ones?
Yes, but through a different mechanism. Instead of mechanical wear, heat degrades the TRIAC or MOSFET over time.
A common failure mode is a shorted output. You’ll measure near-zero ohms across the load side even when the SSR is switched off.
Is a clicking sound enough to prove a relay works?
No. The click only proves the coil energizes and moves the armature. Contacts themselves can be welded, pitted, or too high in resistance to pass current, yet still click. Confirm the contacts with a continuity check, not just your ears.
