Why Do Motorcycle Fuses Blow?


Trying to do electrical work on your motorcycle can be extremely frustrating, especially when you feel like you’ve done everything you can and you can’t figure out why one of your fuses keeps blowing.

So why do motorcycle fuses blow? A motorcycle fuse blows when the current draw on a circuit exceeds the amperage rating of the fuse. In order to protect the wiring in that circuit from melting, the fuse will blow so no permanent damage is done to your motorcycle wires or accessories.

In other words, if a light or a starter or some piece of equipment gets a short circuit then there’s a rush of current down that particular wire (or wires).  If there is no fuse on that particular system then the current is going to make the wires get really hot and possibly cause major damage. Fuses are built in fail safes and they are a very cheap way to protect very expensive equipment.

Too often I have encountered fuse problems with my motorcycles which has given me a lot of experience with fuses. I can explain in further detail of particular ways and reasons a fuse blows.

Current Draw From An Accessory

It’s always helpful having the right tools when working on the electrical components to a motorcycle. Specifically, it’s handy to have a good wire stripper. Click here to see the wire stripper I suggest in my list of recommended tools.

The most common cause of a motorcycle fuse blowing is if there’s an excessive current draw from an accessory, usually something on the motorcycle that pulls a lot of power.  Some of the common accessories that cause blown fuses are things like the starter, the headlight, an improperly wired ignition coil, or one of the onboard computers that control the fuel injection or timing.

One example of a large current draw from an accessory is if the drive gear on your starter gets stuck or if your engine is seized and the starter is trying to turn it over but can’t.  The starter will pull more and more current from the battery to try and power through, but eventually there’s going to be so much current trying to go through that wire that the fuse will blow in order to protect the starter and the wire.

It’s a lot cheaper to replace a $0.25 fuse instead of a $40.00 starter.  Fuses have preset amperage ratings that are written on the fuse. The most common sizes on motorcycles are 5, 10, 15, 20, and 30 amp.  Somewhere on the fuse it will have the amp rating printed, like 5A or 20A. The amp rating means how much current that system can take before the fuse blows in order to protect it.

You should not mix and match fuses.  If the system originally had a 10 amp fuse, then put a 10 amp fuse back in.  A lot of people will blow fuses and then put in a bigger one so it will stop blowing.  That doesn’t mean you’re fixing the problem! You’re just covering it up for the time being.  This is dangerous and can lead to melted wires and fires.

I learned about melted wires the hard way when I was first starting out building motorcycles to pay for my college tuition.  I kept blowing fuses because of a poorly wired taillight, so I took out the fuse and just put a wire directly across the connection so that nothing could blow.  The next time I started up the motorcycle the whole wiring harness to the taillight caught on fire. That was a bad lesson to have to learn the hard way.

If you keep blowing fuses and are having a hard time figuring out why then invest in a small circuit breaker.  You can buy them for about $5.00 at any of your local auto parts stores or on Amazon. They come in different amp settings just like your fuses, so if a 10 amp fuse normally goes in that slot, then buy a 10 amp circuit breaker.  

A circuit breaker is similar to a fuse except after it trips off it will come back on after a few seconds so you don’t have to keep wasting money on blown fuses. If there’s an electrical fault somewhere in that system the circuit breaker will just keep clicking on and off until you have time to troubleshoot what the problem is.

Short Circuit To Ground

This is also a very common problem with older motorcycles.  As the motorcycle gets older, the connections start to get a little corroded and wires start to get more brittle.  After years of use the wires get heated up then cool down thousands of times during normal wear and tear. This makes the plastic around the wires get brittle and crack.

It’s fairly common that after a wire breaks there will be a section of exposed copper wire on the end of the wire.  If the wire gets shifted around and hits anything metal, like the frame, engine, or fuel tank, it will create an immediate path for that current from the stator or battery to rush to the ground without much resistance at all.  

This will immediately either cause the wire to burn or a fuse to blow. Hopefully there is a fuse somewhere in that system and just a fuse will blow.

Another common cause of a wire shorting to ground is when previous owners decide that they want to add aftermarket lights or gauges.  Most amateurs will just cut the lines, expose some copper, and use some electrical tape to hold the wires together. This is the worst way of joining wires together and ends in blown fuses most of the time.

Eventually that electrical tape comes off and exposes bare copper wires which get moved around and will probably bump into something metal, causing a short and blowing a fuse.  Human error is all too often the culprit when trying to find a short to ground.

If you’re trying to find a problem like this on your motorcycle and you keep blowing fuses, I would suggest getting a small circuit breaker and if you know the problem is on the light circuit then disconnect one light at a time until the problem stops.  

If you unplug the left blinker and you suddenly hear the circuit break stop clicking on and off then you found your problem! Somewhere on that light you have a wire that is shorting to something.

I recently was helping a friend with one of his motorcycles and he was having a hard time figuring out what kept bowing his 10 amp lighting circuit fuse.  I asked him which lights were original and which had been replaced. He told me the front turn signals were aftermarket.

I started at the left turn signal.  Turn signals only have two wires, so I looked for electrical tape on those wires and sure enough, i pulled it off and exposed a really bad connection that was twisted together by finger.

I disconnected both blinkers, and as I was disconnecting the right blinker I noticed that the positive wire electrical tape had become loose and the wire was rubbing up against the back of the headlight bucket, causing it to short circuit to ground.  After you’ve done a few of these fuse problems it becomes really easy to pinpoint what is causing the problem.

Wires Getting Crossed

There are two possible ways for a wire to short circuit.  It can either short circuit to ground (which I just explained in the last section) or it can short circuit to another circuit.  Not all circuits have the same voltage, so if two wires get crossed and those two wires have different voltages then it can create problems.

For example, I was once working on a vehicle that kept having problems with one of the sensors.  A fuse kept blowing but we couldn’t find anything wrong with the sensor itself. I looked at a wiring diagram and found that particular sensor should have a 5 volt reading at all times.  

So I used a simple multimeter and read the voltage on the sensor. It said 12.6 volts! So I knew there must be a crossed wire somewhere where that 5 volt line was getting 12.6 volts, and that increased voltage was also creating an increase in amps, which would occasionally blow the fuse.

After I followed the wire back far enough I found a small section of melted wire where these two wires had melted together.  They were sitting too close to the engine and had melted from the heat. So make sure when you have a wiring problem that you print off a wiring diagram and become familiar with all the wires in that system.

Again, electrical is daunting at first, I totally understand because I’ve been there.  It’s really hard to work on an electrical problem for hours and get nowhere, but I promise that if you keep working you’ll figure it out.

It helps me when I talk to myself, I talk about all the possible places that the short could be coming from and then check each one of those.  When you’re in your garage and talking to yourself there’s no one around to judge you, so go ahead.

Don’t Get Overwhelmed

My biggest piece of advice is to not give up and to not take it to a mechanic, you can figure this out.  It might take you 3 times longer than it would a professional, but there is huge value in figuring out a problem by yourself. Plus you’ll save a lot of money.

Once you learn how to diagnose your first electrical problem it’s like a light bulb goes on in your head and the next problems just get easier and easier to diagnose.

My first few electrical problems took me days to figure out.  I would go to bed upset that I couldn’t figure it out, but the next evening when I went out to work on the motorcycle again I would check a few other things and read more forums, then eventually it just made sense and I figured it out!

There are some basic electrical tools that I would recommend everyone get if you’re going to be doing motorcycle maintenance yourself, all of which you can get for cheap at harbor freight or an auto parts store (I’m all about saving money when I can).

Diagnostic tools to buy:

  • Test light
  • Multimeter
  • A small spool of electrical wire
  • Wire cutters
  • Circuit breakers
  • Assorted fuse kit
  • Long push pins

The test light will show you which wires have current running through them and which wires are ground.  The multimeter will tell you how much voltage is in each line. The spool of wire is for when you need to splice in new wire.  The long push pins come in handy when you’re trying to see if there is any current going through a certain connector and you can’t get the tip of the test light down into the hole; put the push pin down the hole and then touch the test light to the push pin.

Related Questions

How can you hide the wiring on a motorcycle? There are several ways to hide motorcycle wiring including routing the black wire loom along the frame, attaching them between frame rails underneath the seat, and creating a concealed electrical box as part of the seat. For more ways to hide your wiring, see my article here.

Where is the fuse box located on a motorcycle? On older motorcycles, the fuse box is usually found underneath the handlebar cover (the little plastic cover that goes over the middle of the handlebars). On newer motorcycles, the fuse box is either located under the seat or near the battery.


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