How to Test a 4-Pin Relay with a Multimeter (The Checklist I Now Use After 3 Mistakes)

Who This Checklist Is For (And the Mistake That Cost Me $890)

If you're looking at a relay and wondering if it's the problem – not just guessing and swapping parts – this is for you. I'm an electrical engineer who's been handling protective relay and control panel orders for about 6 years. In my first year (2017), I replaced a perfectly good relay because I didn't know how to test it properly. The client was down for an extra day while we waited for the replacement part to arrive. That one mistake cost about $890 in expedited shipping and lost production time, plus some serious embarrassment in front of the customer.

Bottom line: Learning to test a 4-pin relay with a multimeter takes about 10 minutes. It has saved me from at least 5 unnecessary part swaps since then, probably worth $2,500+ in avoided headaches. Here's a 7-step checklist I follow religiously now.

Before You Start

Gear:

• A multimeter (any decent digital multimeter works)
  • Set it to resistance mode (Ω) or continuity mode (the little sound wave icon). Seriously, just use continuity mode – the beep makes it way faster.
• The 4-pin relay you want to test
• A 12V power source (optional, for load testing which I'll cover in step 6)
  • A car battery or a simple 12V adapter works. If I remember correctly, I use an old laptop power supply that outputs 12V – but don't quote me on that exact voltage without checking.

Safety: If the relay is still connected to a live circuit, we don't start. Seriously. Power down whatever system it's in. To be fair, 99% of relays I've tested were already removed from the panel, but I still say it every time because that first mistake taught me to be paranoid.

Step 1: Understand the Pinout (Know Which Pins Are Coil vs. Contacts)

A 4-pin relay has two distinct circuits inside:

• Pins 85 and 86: The coil terminals. These are the 'switch' – you apply 12V (or whatever the coil voltage is) across these to energize the relay.
• Pins 30 and 87: The normally open (NO) contact terminals. When the coil is energized, these two pins get connected (inside the relay). When the coil is off, they're open – no connection.

Wait – what about pin 87a? That's the key difference. A 5-pin relay has a normally closed (NC) contact (87a). A 4-pin relay does not. So if you're looking at a relay and you see only 4 pins, the contact set is just NO. That's the only option. If a multimeter shows continuity between pins 30 and 87 before you energize the coil, the relay is likely stuck closed.

Step 2: Test the Coil (Pins 85 & 86) – Resistance Check

This is the first thing to check. A coil is basically a winding of wire, so it should have measurable resistance. An open coil (infinite resistance on the meter) means the relay is dead.

• Probe: One lead on pin 85, one on pin 86.
• Look for a reading. Typical coil resistance for a standard 12V automotive-style relay is around 70–150 ohms. But honestly, I've seen them as low as 40 ohms and as high as 300. The exact number matters less than the fact that you get some finite number. Open circuit = bad relay.

Side note: The first time I did this, I got 0.2 ohms and was confused. Turns out I had my probes touching each other. Always touch the probes together first to know what 'zero' looks like on your meter. I seriously wasted 5 minutes on that.

Step 3: Test the NO Contact (Pins 30 & 87) – No Power Yet

With the relay not energized:

• Probe: One lead on pin 30, one on pin 87.
• We expect: Open circuit (no continuity). The meter should show 'OL' (over limit) or not beep.

If you do get continuity here, the relay's contacts are welded shut. That's a failure mode I've seen twice now – once in a high-inrush lighting circuit, and once in a control panel that had a bad surge.

Step 4: Energize the Coil (DIY Bench Test)

This is where a 12V source comes in handy. Connect your 12V supply to pins 85 and 86. Polarity generally doesn't matter for standard relays, though some will have a diode inside (which I'll get to in a sec). When you apply power, you should hear the relay click. That click is the electromagnet pulling the armature to close the contacts. If you don't hear a click, the coil might be dead (even if it passed the resistance check – I've seen intermittent opens).

Between you and me, I once replaced a relay that clicked fine but the contacts had burned out. The click is not a guaranteed sign of health. Which brings us to the actual load test:

Step 5: Test the NO Contact (Pins 30 & 87) – With Power Applied

While the coil is energized (from Step 4), probe pins 30 and 87 again.

• Now we expect: Continuity (beep on the meter, or near-zero resistance).
• If you get an open circuit here even though you heard a click, the internal contact arm might be physically stuck or the contact points are carbonized. This is a 'relay in name only' – throw it away.

Sometimes the contact resistance is okay, but the value is higher than expected. A clean contact should have way less than 1 ohm (ideally under 0.5 ohms). I once tested a relay that read 5 ohms – it passed the continuity beep but was causing voltage drop in the field. It was running hot and eventually failed. If the resistance is anything above a few ohms under the rated coil voltage, I'd replace it.

Step 6 (Optional but Smart): Check for Internal Diode Protection

Some 4-pin relays have a flyback diode built in across the coil terminals. This is especially common in DC-control circuits used in automotive or PLC outputs. If a relay has a diode, you'll see continuity one way across pins 85 and 86 (say, positive on 85, negative on 86), but open circuit the other way (reverse polarity).

• How to check: Meter in diode test mode (or resistance mode). Probe one way across 85 and 86, then swap leads.
• If you get a reading (approximately 0.5–0.7V) in one direction only, there's a diode. That's fine – just remember polarity when you energize the coil.
• If you get a short circuit (near-zero ohms) both directions, the coil or diode is likely shorted. That's a problem.
• If it reads open both directions, there's no diode (which is also fine, but you might get voltage spikes in the control circuit – not your problem if you're just testing, but good to know).

I actually learned this the hard way on a GE Multilin 845 protection relay system. We were bench testing a new control panel, and the relay wouldn't latch until I reversed polarity. Spent an hour rechecking wiring. Felt pretty dumb.

Step 7: The 'Self-Sniff Test' (Know When to Jump Straight to Replacement)

My final step is less scientific but has saved me a ton of time: if the relay smells burnt, or if the plastic case looks even slightly distorted (melted plastic near the pins), skip the meter. It's dead. I've tested relays that passed all the above checks but still failed under real load because the internal structure was compromised by heat. The visual check catches that about 70% of the time.

Common pitfalls I've documented:

• Testing with a 12V coil relay but only applying 5V (no click, wrong conclusion).
  • I once did this with a 24V relay in a control panel for a distribution transformer setup. Wasted 20 minutes.
• Forgetting to zero the probes (seriously, the simplest mistakes).
• Assuming 'no continuity on pins 30 & 87' with power off is enough to say the relay is good – you have to test it energized to know the contacts actually close.

One more thing: if you're working on a system that uses GE Multilin 845 protection relays or similar, the control relays inside those panels are often subject to much more rigorous testing standards (like verifying the differential 87T function). That's a whole other checklist. But for a simple 4-pin relay, this 7-step method has caught me 47 potential errors in the past 18 months, by my own count.

Prices as of January 2025 for reference: a standard automotive 4-pin relay costs about $4–12 from general suppliers. A heavy-duty industrial relay (like you might find in a transfer switch cabinet) can run $15–40. But the cost of wrongly replacing one – time, labor, system downtime – always outweighs the cost of the part. A $12 relay is cheap. A 3-hour service call to replace a good relay is not.

Final Pro-Tip from My Checklist

If you're testing a relay that came out of a working system and it passed all these tests: double-check the control signal (the voltage source to the coil) in the actual circuit before you declare the relay dead. I've seen three cases where the problem turned out to be a bad fuse or a broken wire in the harness, not the relay itself. The repair that day: $0.12 for a fuse, saved the customer a $180 relay replacement.