Most vehicles only have a single horn. So, if you want to install an additional unit or replace the wiring for the existing one, you’ll have to deal with the car horn wire harness.
These wires are usually in the 10 to 14-gauge range. They are part of the steering column wiring harness because the factory horn button switch is on the steering wheel.
Let’s focus on the car horn wire harness, its components, and how to wire it.
Table of Contents
What Can Cause a Car Horn To Fail?
A car horn circuit consists of a horn switch, relay, and the horn. So, if the car horn fails, it could be either of these issues.
Blown Fuse
Like any other electric power-dependent component in a vehicle, the car horn’s circuit uses a fuse to safeguard the system.
If this fuse blows, power cannot flow from the battery to activate the horn when you press the switch. Therefore, you should check this circuit component first if issues arise.
Damaged Relay
Next in line is the relay. If the fuse is okay, the issue might be in this component, which is in the same fuse box as the fuse. Car horns have a relatively high power draw, and relays introduce a control circuit to separate the dangerous high current from the switch.
A motorcycle’s relay for its air horn
Although more expensive than fuses, relays are also affordable. So you won’t break the bank when replacing this component.
Broken Wire Harness or Connector
Since the car horn must be at the front of your vehicle, its connector gets exposed to dust, water, grime, oil, and other corrosive particles.
This exposure can damage the connector or the wire, resulting in poor conductivity.
The remedy might require a simple repair, such as cleaning the connector using WD-40. Disconnect the battery before performing this remedy.
Damaged Horn Switch
Most vehicles have their horn switch on the steering wheel. This circuit component can malfunction, and repairing it requires you to open the steering wheel.
So unless you know to open this part without deploying the airbag, get a certified mechanic to handle that task.
A driver pressing the horn switch on the steering wheel
Broken Horn
Car horns produce sound using a vibrating metal diaphragm driven by an electromagnet. If the solenoid fails, such as by shorting or having a wire cut, the horn will not work.
Expanded Airbag
An expanded airbag can affect the horn’s operations. So, if the horn does not work and the airbag dashboard light is on, visit a mechanic ASAP to get the issue sorted.
A technician removing the airbag unit to repair the steering wheel
Clock Spring Issues
The clock spring is a spiral-wound rotary electrical conductor that allows you to turn the steering wheel while maintaining electrical contact with the airbag and switches on the wheel. One of these switches is the car horn button.
A steering wheel’s clock spring
So, if the clock spring breaks, the horn will misbehave. Try diagnosing this issue by turning the steering wheel while pressing the horn.
Sporadic horn sounds mean a broken clock spring because the horn button has intermittent connections with it.
How To Wire a Car Horn Wire Harness
If you’ve narrowed the issue down to the car horn wire harness, here are its wiring steps to help you install a new bundle or repair the existing one. You can also use this procedure to install a new electric horn.
Step 1: Wire the Relay
The core component in the horn wiring system is the relay. So, you need to wire the 12V battery power source to the relay first. Four-pin relays have the following terminals.
- 85 (relay control coil negative terminal)
- 86 (relay control coil positive terminal)
- 30 (power or positive terminal input from the battery)
- 87 (relay to load)
Pin 85 should go to the car horn button in the steering wheel, pins 86 and 30 to the positive battery terminal, and pin 87 to the horn.
However, the connection to pin 30 to draw power from the battery should not be direct. Channel it through the fuse box to introduce a safety measure. You should also draw power for the control circuit via the fuse.
A boat’s stainless steel horn with its wires ready for connection to the relay switch
We recommend disconnecting the battery before proceeding with the wiring past the fuse box.
You can use a 5-pin relay, which splits terminal 87 into two. 87a is NC (normally closed), and 87 is NO (normally open). But there’s no point in using this relay because you won’t use pin 87a.
If you do, the horn will operate in reverse. It will produce sound throughout and turn off when you press the steering button.
Step 2: Wire the Horn Button
If using the factory horn switch, you’ll have to find the car horn wire harness in the steering column and connect one of the wires to pin 85 from the relay. Link the other cable to the ground.
These wires have no polarity because they handle the negative or ground connection.
When you press the horn button, you ground the relay’s control circuit, activating the coil to pull the armature and complete the load circuit.
But if installing an aftermarket horn switch, mount it at a convenient location near the driver, then wire it accordingly.
Step 3: Wire the Horn
The last step is to wire the horn, which comes in these different types and categories.
Terminal Categorization
- Single terminal horn (mounting bracket forms the negative terminal)
- Double terminal horn (terminals can or cannot have polarity markings)
Dual terminal diaphragm horns
Operating Mechanism Categorization
- Diaphragm horn (can be single or dual terminal)
- Air horn (consists of a compressor that pushes air into multiple trumpets to produce the sound, and some can have sequencers to play music)
An air horn
Regardless of type, the wiring is the same. Connect the relay’s pin 87 to the horn’s single terminal or positive pin if it has two. For the latter, connect the other pin to the car’s body for grounding.
Some car horn kits might have two horns (high and low note). Connect both of them to terminal 87, then ground their second terminals.
Reconnect the battery, then test the system.
Wrap Up
The car horn wire harness is a simple two-wire cable because it only powers and grounds the system. Therefore, setting up the system is probably the easiest in your vehicle.
The challenging part might be to wire the car horn button to the steering wheel. So let us know if you encounter issues installing this harness. We will help you sort it out and proceed to completion. Cheers!
