How a relay works — and how to test one

Two circuits, not one

Every relay is two separate circuits sharing one plastic box. The control circuit is a small electromagnet — terminals 85 and 86. The load circuit is a heavy switch those magnets throw — terminals 30, 87 and sometimes 87a.

This is the whole point of a relay: a few hundred milliamps through a dash switch controls 30 amps to a headlight, winch or pump. The switch never sees the big current, so it can be small, cheap and far away.

What each terminal does

30 is the common — battery power in, almost always fused and hot at all times. 87 is normally open: it connects to 30 only when the coil is energized. 87a is normally closed: connected to 30 at rest, and it disconnects when the relay pulls in. Many relays have no 87a at all.

85 and 86 are the coil. On a plain relay they are interchangeable. On a relay with a built-in diode they are not — the diode has a polarity, and backwards it either does nothing or blows the fuse. Look for the diode symbol printed on the case.

Why the swap test lies to you

Swapping in a known-good relay is fast, and it is how most people 'test' one. The trouble is that it only proves something when the fault is currently present. A relay that drops out when hot, or whose contacts arc under load but pass a resistance check, will swap-test perfectly in a cold bay.

Worse, swapping tells you nothing about why. A relay that has cooked its contacts usually did so because the load is drawing more than it should — a dragging motor, a shorted winding. Replace the relay and you will replace it again.

How to test it

1. Check the coilOhms across 85 and 86. Expect roughly 60–120 Ω. Open circuit means a burned coil. A few ohms means shorted turns, which will pop the control fuse.
2. Check the rest stateOhms from 30 to 87 should be open. 30 to 87a, if present, should be near zero. Anything else means welded or corroded contacts.
3. Energize itApply 12 V and ground to 85/86 (mind the diode). You should hear the click, and 30 to 87 should now read under 0.2 Ω.
4. Test it under loadThe one that matters. With the circuit running, measure voltage drop across 30 and 87 with the relay closed. Over 0.2 V means the contacts are burned even if the bench test passed.

Numbers to work to

Common mistakes

• Reversing 85 and 86 on a diode-equipped relay and assuming the relay is dead.
• Passing a relay on a bench ohms test when its contacts fail under load — resistance at 1 mA tells you nothing about 30 A.
• Replacing a burned relay without asking what burned it. Check the load's current draw before fitting the new one.
• Forgetting that 30 is usually hot at all times — probing it and reading 12 V proves nothing about the relay.