Relay pinout 30 · 85 · 86 · 87 · 87a

Automotive relay wiring diagram: 4-pin and 5-pin, explained

A relay lets a small switch control a big load: a thin wire from the switch drives the coil, and a short thick wire from the fuse feeds the headlights, fan or horn. Here is what every terminal does, how to wire it, and a live diagram you can switch on.

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Relay terminals (DIN 72552)

Terminal	What it is	Connect to
30	Common contact, power in	Battery + through a fuse sized for the load
87	Normally open output	The load (headlights, fan, horn)
87a	Normally closed output (5-pin only)	A load that must be on while the relay is off
86	Coil +	The switched control wire from the switch or ECU
85	Coil −	Ground (or the switch, if it switches to ground)

The coil of a 12 V relay draws only about 0.15 A, so the switch and its wire carry almost nothing. Contacts are usually rated 30 or 40 A. Some relays have a diode or resistor across the coil; with a diode, 86 must be + and 85 must be −.

4-pin relay wiring

Battery + → fuse → terminal 30.
Terminal 87 → load + ; load − → ground.
Switched + (switch output, ignition) → terminal 86.
Terminal 85 → ground.
Close the switch: the coil pulls the contact, 30 connects to 87 and the load runs.

5-pin (changeover) relay wiring

A 5-pin relay adds 87a, which is connected to 30 while the coil is off. When the coil is energised, 30 moves from 87a to 87. Use it to switch between two loads — for example low and high beam, or to turn something off when something else turns on. A dual-87 relay has two outputs, 87 and 87b, that both switch on together.

Common relay wiring mistakes

No fuse on terminal 30 — a short in the load wire has nothing to stop it.
30 and 87 swapped — it still clicks, but power waits on the wrong side of the contact.
The coil fed through the relay’s own contact — the relay chatters on and off.
Too little voltage on the coil from a long thin control wire — it never pulls in.
The load wire as thin as the switch wire — the whole point of the relay is lost.

WireForge recognises each of these and says so in plain words when you run the simulation.

Build a relay circuit in WireForge

Open the “Headlights through a relay” demo, or start the editor and drag in a Relay.
Connect battery + → fuse → 30, 87 → headlights, 86 → switch, 85 → ground.
Press Simulate and click the switch: the relay pulls in and the lamps light.
Click the thin control wire and the thick lamp wire to compare their currents.
Swap 30 and 87 or remove the ground on 85 and watch the warning appear.

Questions

Which relay pin goes to ground?

Normally 85, the coil’s negative side. 86 gets the switched positive. If the switch switches to ground instead, 86 is fed permanently and 85 goes to the switch.

Does it matter which way the coil is wired?

For a plain coil, no. If the relay has a suppression diode across the coil, yes: 86 must be positive and 85 negative, or the diode shorts the control circuit.

Where does the fuse go?

On the power feed to terminal 30, as close to the battery as possible, rated for the load and no higher than the wire can carry.

What is the difference between 87 and 87a?

87 is connected to 30 only while the coil is energised (normally open). 87a is connected to 30 while it is not (normally closed); only 5-pin relays have it.

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