How Arduinos Helped Me Fix My Car Horn

Eli Fatsi, Former Development Director

Article Category: #Code

Posted on

There seems to be a common issue with 2002 Mitsubishi Montero Sports* where the horn will suddenly turn on if it’s cold out, and have no intentions of turning itself off. I discovered this first hand with my own Montero Sport earlier this winter on a particularly chilly night. The quick fix was to disconnect the battery, plug it back in when I wanted to drive, and then remember to unplug it again overnight (did not always go well). Unrelated, I have been experimenting with Arduinos here and there, and this is the story of how working with Arduinos let me (pretty much) fix my car horn.

* a few others online have posted their similar woes.

How/What I learned working with Arduinos

My introduction to Arduino was through the Sparkfun Inventor’s Kit, a fantastic resource for learning the basics of electronics and microcontrollers. Electronic circuits are for the most part straight forward and intuitive - electricity flows from + to - and if you make it go through something like an LED, you can make things turn on or move around. While it may not seem all that important to be able to turn a light on and off or move a motor back and forth, you can take this knowledge, get your hands on a few parts, and make some pretty cool things (maybe a blog post on that device later).

Aside from the raw knowledge I learned from working through the Inventor’s Kit, I’ve also gained a greater understanding of how electronics work in general. While this is common knowledge to many, it turns out that most electronic things are not in fact magic. So when my research of “my car horn turns on by itself in the middle of cold nights” led me to believe I would have to pay upwards of a thousand dollars to fix this, I instead thought “for Great Justice, I can probably hack something together myself!”

The fix

Looking around the car and the internet I found the relay supplying power to the horn. Big Win #1 - unplug this to disable my horn without disconnecting my battery. Relays (chapter 13 of the Inventor’s Kit guide) are switches which are “flipped” by relatively small amounts of electricity but can allow large amounts of electricity to pass through them. This relay is “flipped” when you press the horn button in your steering wheel and allows electricity from the battery to power the horn. My plan was to wire my own switch into this system so I could disable/enable the system from inside my car, and so the journey began.


I picked a random wire feeding the horn relay and cut it. When I plugged the relay back in, I was unable to activate the horn - SUCCESS! I then proceeded to solder two separate wires to the ends of the wire I’d just cut, and run those from the relay into the cabin of the car.


I then soldered the two wires to a simple rocker switch thus completing my side circuit. When I flip the switch closed, I complete the horn relay circuit and am able to blare my horn. Flip the switch back open (disconnecting the circuit) and my horn is rendered useless.


The last step was to make it look good. Another lesson from my past Arduino projects - no one likes messy projects. I found something that looked like I could carve a knife into without causing any new issues and hooked everything up!


The finished product is a switch that I flip when I need to use my horn, and an easy way to guarantee I won’t wake the neighborhood next time it gets cold in Boulder (video proof). Aside from being useful and 1/1000th the cost of having it repaired by a professional, coming up with a decent solution on my own was extremely satisying. So I’d recommend getting your hands on some intro-to-electronics resources if you haven’t already, and the next time your office microwave bites the bullet, see if you can be the one to bring it back to life*.

* electricity can be extremely dangerous - be sure to educate yourself and always use caution to avoid electrocuting yourself!

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