Relay Wiring Diagrams

Free relay wiring diagrams for 12 volt vehicles. Pick the job, and the tool draws what goes on pins 30, 87, 87a, 85 and 86, with the fuse and wire size and the parts you need. Below it, how a relay works, the six diagrams we wire most, and how to test one.

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How it works

  1. Pick the jobLights, polarity flips, starter kill, amp turn-on delay or door locks.
  2. Answer a question or twoTrigger type and load. The diagram redraws as you go.
  3. Wire it, or send it to usPrint it, copy the link, or send it to us to check before you cut anything.
Free tool

Draw your relay wiring diagram

Pick the job and answer a question or two. We'll draw the wiring, size the fuse and wire, and list the parts.

1What do you want to do?

Relay pinout: what 30, 87, 87a, 85 and 86 mean

Every standard automotive relay has the same five pin numbers, printed on its base. Two pins run the coil. The other three are the switch.

Does the switching

Pins 85 and 86: the coil

The coil is a small electromagnet. Put +12V on one pin and ground on the other, and it pulls the switch over. It draws roughly 0.15 A, which is why a low-current output from an alarm or remote starter can run it.

On a plain relay the coil works either way round. Relays with a built-in diode have a positive side, usually 86, so wiring 86 to positive and 85 to negative works with all of them. That's how every diagram on this page is drawn.

Carries the load

Pins 30, 87 and 87a: the switch

30 is the common connection. 87a is normally closed: it's connected to 30 while the relay is at rest. 87 is normally open: 30 moves over to it when the coil switches on, and springs back to 87a when it switches off.

A 4-pin relay has no 87a. It's a plain on-off switch between 30 and 87.

Installer tip

Why our diagrams feed power into 87

Contacts carry current either way, so you can feed a relay on 30 or on 87. We feed it on 87 and take the output from 30, and here's why. At rest, 30 is joined to 87a. Put +12V on 30 and the spare 87a lead is live the whole time the relay sits idle, ready to short against anything it touches if it isn't insulated properly.

Feed 87 instead and 87a is never live: at rest 30 has no power, and when the relay switches on, 87a disconnects. The exception is a timer relay like the BU510TD, which takes its own power from 30, so tape off its 87a lead.

Common relay wiring diagrams

The six setups we wire most. Each one opens in the tool above, ready to adjust.

Polarity flip

Negative trigger to a +12V output

Most alarm and remote start outputs are negative: they switch to ground when active. To turn one into +12V, wire 86 to fused +12V, 85 to your negative output, 87 to fused +12V, and take the new output from 30. When your output grounds, 30 goes live.

Open this diagram
Polarity flip

+12V trigger to a ground output

The reverse: your output shows +12V, but the device wants a ground. Wire 86 to your +12V output, 85 to ground, 87 to chassis ground, and 30 to the wire that needs the ground. 30 is grounded only while your output is on.

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Most common

Lights, horn or an accessory

The classic fog light, horn or light bar relay. 87 goes to the battery through a fuse sized to the load, 30 to the accessory, and the accessory's other wire to ground. Your switch only runs the coil, through 86 and 85. Over 25 A, use a 120 amp relay like the BU5078.

Open this diagram
Check first

Starter kill

Cut the crank wire, the one that's live only with the key at START. The ignition switch side goes to 30 and the starter side to 87a. At rest they're joined and the car starts normally. Switch the relay on and the circuit opens. Wire it to 87 instead and the car only cranks while the relay is on.

On push-button start and many newer vehicles, starting runs through a computer. Ask us before cutting anything on those.

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Timer relay

Amplifier turn-on delay

A delay ON timer relay like the BU510TD stops amplifier turn-on pop. 86 takes the head unit's amp turn-on wire, 85 goes to ground, 30 to fused +12V that stays live, and 87 to the amplifier's REM terminal. The amp comes on a couple of seconds after the head unit.

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Two relays

Door lock relays

Two relays, one to lock and one to unlock. For positive pulse locks, 87 goes to fused +12V and 30 to the vehicle's lock wire. For negative pulse, 87 goes to ground instead. For reversing polarity (5-wire) locks, cut each lock wire: motor side to 30, switch side to 87a, and 87 to fused +12V.

Open this diagram

How to wire a relay, step by step

The same four steps whatever the job.

1. Work out the job

Decide what switches the relay on, positive or negative, and how much current the load draws. The tool above turns that into a diagram.

2. Fuse the power

Fuse the +12V feed close to where you take it, sized at about 125% of the load. The fuse protects the wire, so never fit a bigger one.

3. Wire the coil

86 to +12V or your positive trigger, 85 to ground or your negative trigger. Thin wire is fine: the coil draws about 0.15 A.

4. Wire the switch and test

Power in on 87, out on 30 to the load, in wire rated for the fuse. A starter kill uses 30 and 87a instead. Test it before you tape everything up and refit the panels.

Testing, choosing and fitting a relay

Two minutes with a multimeter

How to test a relay

Take it out of the circuit and set a multimeter to ohms. Across 85 and 86, a 12 volt coil usually reads somewhere between 50 and 120 ohms. An open reading means the coil is dead.

At rest, 30 to 87a should show continuity and 30 to 87 shouldn't. Now put 12 volts across 85 and 86 through a fused jumper. It should click, 30 to 87 should now show continuity, and 30 to 87a should open.

SPST or SPDT

4-pin or 5-pin?

A 4-pin (SPST) relay is a plain on-off switch between 30 and 87. A 5-pin (SPDT) relay adds 87a, so it switches between two circuits. You need a 5-pin for starter kills, reversing polarity door locks, and anything that should be live while the relay is off.

A 5-pin relay does every 4-pin job too: just leave 87a unused.

Our advice

Fitting tips

Mount the relay with its terminals pointing down, so water can't sit in the socket. Solder or crimp every join properly and cover it with heat-shrink. Ground to clean, bare metal, and keep the fuse as close to the power source as you can.

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Need the parts?

Every diagram lists the parts it needs. For everything else, from 1 amp mini relays to 120 amp heavy-duty units, timer relays, harnesses and Compustar door lock adapters, see our relays page.

Shop relays and harnesses

Common questions

What's the difference between 87 and 87a?

87 is normally open: it's connected to 30 only while the relay is switched on. 87a is normally closed: it's connected to 30 while the relay is at rest, and disconnects when it switches on. Only 5-pin (SPDT) relays have 87a.

What size fuse do I need for a relay?

About 125% of what the load draws, rounded up to the next standard fuse, and never more than the wire can carry. A 10 A load gets a 15 A fuse and 14 AWG wire, for example. The tool above works it out for you.

Which way round does the relay coil go?

On a plain relay, either way. Relays with a built-in diode have a positive side, usually 86. Wiring 86 to positive and 85 to negative works with all of them, which is how our diagrams show it.

Why does my relay click but nothing turns on?

The click means the coil is working, so look at the switch side. Check the fuse and that power reaches the feed pin (87 in our diagrams), that the load is on 30, and that the load itself has a good ground. If all of that checks out, the relay's contacts may be burnt: swap in a new one.

Can a relay drain my battery?

Only while its coil is switched on. A relay draws roughly 0.15 A when on and nothing at rest. Left on around the clock, that can flatten a battery within a week or two, so feed the coil from an ignition source when it only needs to work with the key on.

Can I use these diagrams on a 24 volt truck?

The wiring is the same, but the relay must be a 24 volt one, like the SEM-2417. A 12 volt relay on 24 volts will burn out its coil.

Do I need a relay harness?

It makes the job much easier. A bare relay has spade terminals and nothing to plug into. The RH-200 is the matching socket with leads attached, and the RH-202 holds two relays for door locks.

Can you check my wiring before I install it?

Yes. Use the Email or Text button under your diagram and add your year, make and model. The link opens the exact diagram you drew. Or have us install it at our Markham shop.

Want us to check it?

Send us your diagram with your year, make and model. We wire relays every day, and we'd rather check it now than have you chase a fault later.

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