How to Wire a Car Horn: Relay Terminals, Fuse & Ground

A car horn is wired through a relay: the horn button only switches the small current to the relay (terminals 85 and 86), while the relay passes battery power from terminal 30 out of terminal 87 to the horns, which are grounded to the vehicle body. This guide gives the terminal-by-terminal wiring from Hella horn instruction sheets, a table of which layout fits which car, a check list for a silent factory horn, and the point where steering wheel and airbag work should go to a shop.

Understanding Your Car’s Horn System

A horn circuit has four parts that matter when you wire or rewire one: battery power through a fuse, a relay, the horn button, and the horns with their ground. The button carries only a switching signal. The relay carries the heavy current, which is why the Hella sheets send battery positive to relay terminal 30 through an in-line fuse holder, the horn-button or factory horn wire to terminal 86, and terminal 87 out to the horns.

Relay Terminals 30, 85, 86 and 87: What Each One Does

The roles below come from the Hella Twin Trumpet horn instructions supplied by Griot’s Garage.

Terminal	Role on the Hella sheet	What connects there
30	Power supply circuit	Wire to the battery positive post through an in-line fuse holder near the battery, ring connector on the post
87	Power output circuit	Wire to the horns; at the horn end a butt connector splits it into one lead per horn
86	Switching circuit	The existing horn wire from the car, or a wire to the horn button on a build with no factory horn
85	Switching circuit, second terminal	A wire to ground, or the second factory horn wire on a car whose horn has two wires

Wire gauge differs between the two Hella sheets. The Twin Trumpet sheet calls for 12 gauge wire on every relay lead. The other Hella Horns sheet calls for #12 gauge on terminals 30 and 87 and #16 gauge on terminals 85 and 86. Neither sheet gives a fuse rating; they only say an in-line fuse holder is supplied. Use the fuse rating printed in the instructions that came with your horns, and do not guess one.

Which Wiring Layout Fits Your Car?

The Hella sheets cover four situations. They disagree on whether a relay is needed for a simple swap, so the table shows both.

Your car	What the sheets say	Relay?
One existing horn with two wires	Horns sheet: move both factory wires onto the new horns, ground the horns. Twin Trumpet sheet: factory wires to relay terminals 86 and 85	Horns sheet: not necessary. Twin Trumpet sheet: yes
One existing horn with one wire	Horns sheet: factory wire to one horn, add a separate ground. Twin Trumpet sheet: factory wire to terminal 86 (use only one wire if you have two horns)	Horns sheet: not necessary. Twin Trumpet sheet: yes
Two existing horns, one positive wire each	Horns sheet: each factory wire to one new horn, plus a ground wire to each	Horns sheet: not necessary
No horn, or older than 1975	Horn button to terminal 86, fuse and wire from the battery to terminal 30, terminal 87 to the horns	Required on both sheets

For the no-horn build the two sheets also assign terminals 85 and 86 differently. The Horns sheet puts the horn button on 86 and a hot lead on 85. The Twin Trumpet sheet puts the horn button on 86 and a ground on 85. Because of that, follow the sheet packed with your horns rather than mixing the two. When you are reusing factory wires, keep them on the terminals the sheet names and you avoid deciding polarity yourself. A related walkthrough of the replacement itself is in our guide to installing a car horn yourself.

What You’ll Need: Tools and Materials

Most of this list comes from the materials list on the Hella Twin Trumpet sheet, and the horn kit supplies the relay and the in-line fuse holder.

• 12 gauge automotive wire
• Female spade connectors, eye-ring connectors and butt connectors
• Wire stripper and crimper
• A 12 volt test light
• Electrical tape
• The relay and in-line fuse holder from the kit
• Your owner’s manual, for the horn fuse and relay positions

Step-by-Step Guide: How to Wire a Car Horn

These steps follow the relay layout on the Hella Twin Trumpet sheet. If your car matches a layout where the sheet says no relay is needed, skip the relay steps and follow that layout instead.

Step 1: Safety First! Disconnect the Battery

Every Hella layout starts the same way: disconnect the ground lead from the battery before wiring. The order of terminals is explained in our guide on which terminal to handle first on a car battery, and the full routine is in how to safely disconnect a car battery. If your car has airbags, read your manual for the waiting time. Nissan, for example, requires at least 3 minutes with the ignition off before anyone works near the steering wheel airbag on the models in its horn bulletin.

Step 2: Locate Your Car’s Horn and Relay

Your owner’s manual shows the horn fuse and relay positions. The Hella sheet mounts new horns on a stable site inside the engine compartment, such as the firewall or the underside of the hood.

Step 3: Mount the Horns and the Relay

Bolt the horns to a stable mounting site. Mount the relay on a solid location near the horns, in an area protected from weather, as the sheet says.

Step 4: Wire the Relay

1. Terminal 86: crimp a female spade connector on the existing horn wire (or on a wire run to the horn button) and push it onto 86. If you have two factory horns and a single positive wire layout, use only one of the wires and cap the unused one.
2. Terminal 85: wire it to ground with an eye-ring connector. The sheet lets the relay’s own mounting screw serve as that ground.
3. Terminal 30: run the wire to the battery, away from sharp edges, put the in-line fuse holder near the battery, then finish at the positive post with an eye-ring connector.
4. Terminal 87: run the wire to the horns, end it at a butt connector, and split it into one lead for each horn.

It may be necessary to cut off a factory connector and crimp on a standard female spade connector, per the same sheet. If a connector near the grille is corroded, the fix in our guide to cleaning a corroded fuse box applies to connectors too.

Step 5: Ground Each Horn

Each horn needs one 12 volt wire and one ground wire. Put a female spade connector on a second 12 gauge wire for each horn, finish it with an eye-ring connector, and bolt it to a ground. The sheet allows the horn’s own mounting bolt to be that ground. Hella Twin Trumpet horns carry no polarity marking, so either terminal can take either wire as long as each horn gets one of each.

Step 6: Reconnect the Battery and Test

Reconnect the battery ground lead and check the installation by honking the horn at the steering wheel button, as the Hella sheets instruct. The sheets give no further test or troubleshooting steps.

If Your Factory Horn Is Silent

The Hella and Nissan documents used for this page do not give fuse, relay or horn test procedures, so none are listed here. Your owner’s manual shows where the horn fuse and relay sit, and the service manual for your exact vehicle gives the tests. For a wider look at causes, see our page on why a car horn is not working.

Steering Wheel Horn Pad and Airbag: Leave It to a Shop

A horn that is silent only from the button can be a steering wheel wiring problem. Nissan’s service bulletin NTB22-051B, hosted by NHTSA, covers an inoperative horn on the 2019 to 2023 Altima, 2018 to 2023 Kicks and LEAF, and 2020 to 2023 Sentra and Versa. Its action is to inspect the horn pad for a pinched ground wire and, if one is found, replace the high and low horns. You can read the NHTSA-hosted Nissan bulletin for the full procedure.

That procedure removes the driver airbag module with a dedicated release tool after a minimum 3-minute wait, and the bulletin says it is intended for qualified technicians, not do-it-yourselfers. The takeaway for a home repair is simple: do not open the steering wheel yourself; have a shop handle that side. More on the risks of this area is in our guide on damaging horn wiring in the steering wheel when repairing an airbag.

When to Call a Professional

• The fault points to the steering wheel, horn pad or anything near the airbag.
• The horn sounds on its own or will not stop, which is a different fault covered in why a car horn keeps going off and how to turn off a stuck car horn.
• You cannot find the horn relay or fuse in your manual.

Frequently Asked Questions (FAQ)

Do I need a relay to wire a car horn?

It depends on the car. One Hella instruction sheet says a relay is not necessary when you are replacing one existing horn, but states that a car with no horn, or one older than 1975, must use the relay or the horns will not get enough power. A second Hella sheet uses a relay in every layout. Follow the sheet packed with your horns.

Which relay terminal does the battery wire go to?

Terminal 30. The Hella sheets call it the power supply circuit: run the wire to the battery through an in-line fuse holder placed near the battery and finish it with a ring connector on the positive post. Terminal 87 is the output that goes on to the horns.

Does horn polarity matter?

Not for the Hella Twin Trumpet horns. Their sheet says the horns carry no positive or negative marking, and it does not matter which terminal gets the 12 volt wire as long as each horn has one 12 volt wire and one ground wire. Check the markings on your own horns before assuming this.

Where does the horn ground wire go?

Each horn needs its own ground wire taken to a suitable ground location on the vehicle. The Hella Twin Trumpet sheet allows the bolt that secures the horn in the engine bay to serve as the ground, and the relay’s mounting screw can serve as the ground for the relay’s ground terminal.

How long should I wait after disconnecting the battery before working near the airbag?

Follow your own owner’s or service manual. As one example, Nissan’s bulletin NTB22-051B for its Altima, Kicks, LEAF, Sentra and Versa models requires a minimum of 3 minutes with the ignition off after the negative terminal is disconnected, and it treats the work as a technician job.

Where can I find a wiring diagram for my car’s horn system?

Start with your owner’s manual for the horn fuse and relay positions, then use the service manual for your exact make, model and year for the full circuit diagram. The Hella sheets give no vehicle-specific wire colors.

Conclusion

Wiring a car horn comes down to four connections on a relay: battery power on 30 through a fuse, the horns on 87, the switching wire on 86 and a ground on 85, with every horn grounded to the body. Use the layout and wire gauge from the sheet packed with your horns, disconnect the battery first, and send anything involving the steering wheel or airbag to a shop. This guide does not cover air horns with compressors or alarm-integrated horn wiring.