How do I wire a 12V relay (pins 30, 85, 86, 87)?

Quick answer

A relay uses a small current through its coil (terminals 85 and 86) to switch a large current through its contacts (terminals 30 and 87). Wire battery positive through a fuse near the battery to 30, connect 87 to the accessory and earth the accessory, then feed 86 from a fused or ignition-switched supply through your switch and connect 85 to earth. When the switch is on, the coil pulls in and 30 connects to 87; on a 5-pin changeover relay, 30 connects to 87a whenever the relay is off.

Is this a DIY job?

• Difficulty: Medium
• Time: 30 minutes to 2 hours, depending on the accessory and cable runs
• Tools: multimeter, crimp tool, wire stripper, side cutters, spanners for the battery terminals
• Skills: crimping, reading a simple wiring diagram, using a multimeter

A relay circuit is a good DIY job if you can crimp terminals well and follow a diagram. Get professional help for anything tied into the vehicle’s factory looms or body control module, and for high-current loads such as winches or inverters.

What each terminal does

These numbers come from the German DIN 72552 terminal-marking standard and are used on almost all automotive relays.

4-pin vs 5-pin. A 4-pin relay has 30, 85, 86 and 87: it simply turns a circuit on or off. A 5-pin relay adds 87a and is a changeover relay: the 30 contact swaps between 87a (off) and 87 (on). You can use a 5-pin relay as a 4-pin one by leaving 87a unconnected. Some 5-pin relays have two 87 terminals instead of an 87a, for feeding two outputs; check the diagram on the case.

Suppressed relays and polarity. When a relay coil switches off, it produces a voltage spike that can upset sensitive electronics. Many relays include a resistor or a diode across the coil to absorb it. A resistor-suppressed relay doesn’t care which way round 85 and 86 are wired. A diode-suppressed relay does: if you reverse it, the diode conducts straight across the coil when you switch on, the relay won’t work, and the trigger fuse or the diode can be destroyed. The negative terminal isn’t the same on every diode relay, so always follow the diagram printed on the relay.

Step by step

Schematic only. Pin positions on the relay base vary, so match terminal numbers, not positions.

1. Check the load and the relay. Find the accessory’s current draw and make sure the relay’s contact rating is higher. Note that the 87a contact often has a lower rating than 87.
2. Size the load cable. Choose cable by copper area in mm² for the current and run length, from the battery to 30 and from 87 to the load. (see What size cable do I need for my 12V accessory?)
3. Disconnect the battery. Negative off first.
4. Mount the relay. Use a relay base with crimped terminals, mounted with the terminals facing down (or sideways) so water can’t sit in it, away from exhaust heat.
5. Wire 30 (power in). Run fused cable from battery positive to terminal 30. The fuse goes as close to the battery as practical and is rated at or below the cable’s rating. (see Where should the fuse go when wiring an accessory?)
6. Wire 87 (power out). Run cable from 87 to the accessory’s positive.
7. Earth the load. Connect the accessory’s negative to a clean chassis earth or straight back to the battery negative with cable of the same size.
8. Wire 85 (coil earth). Connect 85 to a good earth. If the relay has a diode, check which terminal the diagram shows as negative.
9. Wire 86 (coil trigger). Run a light cable from a fused 12V feed, through your dash switch, to 86. Use an ignition-switched feed if the accessory should only work with the key on. Fit a small fuse in this trigger circuit sized to protect its thin wire.
10. Check everything before power. Make sure no bare copper is showing, and that 30 and 87 haven’t been swapped with the coil terminals.
11. Fit the fuses and reconnect the battery. Negative on last.
12. Test. With the switch off, the accessory should be off. Switch on: you should hear a click and the accessory should run. If you hear a click but nothing works, check the 30 feed fuse and the load earth. If there’s no click, check the trigger fuse, the switch and the 85 earth.

Testing a relay on the bench. Measure across 85 and 86 with a multimeter on ohms: a typical coil reads tens of ohms, and an open circuit means the coil has failed (a diode relay may read differently one way round). Then apply 12V to the coil (correct polarity for a diode relay) and check with the meter that 30 and 87 now read near zero ohms.

Specs table

Example figures from one maker’s 12V automotive changeover relay

Relays vary widely: heavy-duty types switch much more current. Always check the rating printed on the relay you buy, and remember that motors, compressors and some lights draw more current when they start than when they’re running.

What you’ll need

• 12V automotive relay 4 pin or 12V automotive relay 5 pin changeover
• relay base with terminals
• inline blade fuse holder (two: one for the load, one for the trigger)
• blade fuses
• 12V switch
• auto cable (heavy for the load circuit, light for the trigger)
• crimp terminals and adhesive lined heat shrink
• relay wiring harness (a ready-made option for lights)

When to call an auto electrician

• The relay will switch a high-current load such as a winch, inverter, air compressor or large light bar.
• You want to trigger the relay from a factory circuit such as high beam, reverse lights or a body control module output. Some factory circuits are monitored and can throw faults.
• Fuses blow, the relay clicks rapidly (chatters), or the relay or its base gets hot.
• The wiring passes near airbags or SRS looms, or the vehicle is a hybrid or EV (high-voltage systems are professional-only).

Related questions

Q: What do the relay numbers and symbols mean, and how do relays work in positive-earth vehicles? A: Our in-depth guide covers relay types, terminal numbers, ratings, suppressed relays and positive-earth wiring.

Q: What is the difference between pin 87 and 87a? A: 87 is connected to 30 when the relay is switched on. 87a, found only on 5-pin relays, is connected to 30 when the relay is off.

Q: Does it matter which way round 85 and 86 go? A: Not on a plain or resistor-suppressed relay. On a diode-suppressed relay it does, so follow the diagram on the relay case.

Q: Why use a relay instead of just a switch? A: A relay keeps the heavy current in short, thick cables near the battery, while the dash switch only carries the small coil current. That protects the switch and reduces voltage drop to the accessory.

Q: How do I wire driving lights through a relay? A: The relay switches the lights’ power, and the trigger usually comes from the high beam circuit so the lights go off on low beam.

Q: Why does my relay click but the accessory doesn’t work? A: The coil side is working, so check the fuse feeding 30, the connection at 87, and the accessory’s earth.

Sources

1. DIN 72552, “Terminal markings for motor vehicles” (Deutsches Institut für Normung) – terminal numbers 30, 85, 86, 87 and 87a. Referenced as the standard behind the numbering; not quoted.
2. Omron, G8JN automotive relay datasheet (via Farnell) – terminals 30, 85, 86, 87, 87a; coil resistance 72 Ω ±15%; contact ratings 35 A (NO) and 20 A (NC); pull-in voltage 8.0 V max at 23°C; versions with a suppression resistor or suppression diode. https://www.farnell.com/datasheets/103809.pdf (checked October 2026)
3. HELLA, “New relays” bulletin – relays are resistor- or diode-suppressed across the coil, identified from the printed schematic; on the diode type shown, 85 is negative and 86 positive. https://www.hella.com/forvia-us/assets/documents/BI_HELLA_New_Relays_EN.pdf (checked October 2026)