Why Do Motorcycle Fuses Burst?

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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 fuse blow? The fuse will blow if the circuit’s current draw exceeds its amperage rating. The fuse will blow to prevent any permanent damage to the wires and accessories of your motorcycle.

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).  The current can cause wires to heat up and may even cause serious damage if there’s no fuse. Fuses are designed to be fail-safes, and can protect expensive equipment at a fraction of the cost.

My motorcycles have had fuse problems too often, which has allowed me to gain a lot of knowledge about fuses. I can provide more detail on the reasons and ways that a fuse might blow.

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 for my recommendation of wire strippers

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. Blot fuses can also be caused by an incorrectly wired ignition coil, a starter, or one the onboard computers that controls the timing or fuel injection.

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 can have preset amperage ratings which are printed on the fuse. There are five, 10, 15, 20 and 30 amp sizes that motorcycles use.  You will find the amp rating printed on the fuse somewhere, such as 5A and 20A. An amp rating is how much current the system can take before it blows, in order to protect it.

It is not a good idea to mix and match fuse pieces. Replace the 10 amp fuse in an older system that had a 10amp fuse. Many people will blow fuse and then replace it with a larger one to stop the fuse from blowing.  That doesn’t mean you’re fixing the problem! You’re just covering it up for the time being.  This is dangerous as it can cause melted wires or fires.

When I first started building motorcycles to pay college tuition, I had to learn about melting wires.  A poorly wired taillight kept me from blowingfuses, so I pulled out the fuse and put a wire directly across it so nothing could go wrong.  When I tried to start the motorcycle, the entire wiring harness from the taillight caught fire. It was a terrible lesson that I had to learn.

You may be constantly blowing fuses, but you have difficulty understanding why. They are available at your local auto parts store or online for $5.00. They are available in different amp settings like your fuses. For example, if you have a 10amp fuse, then you can buy a 10amp 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 problem is very common on older motorcycles. As the motorcycle ages, the connections become corroded and the wires become more fragile. The wires become hotter over time and cool down many times throughout normal wear and tear. The wires’ plastic surrounds become 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 hits metal objects, such as frames, engines, fuel tanks, or other metals, it can create an immediate pathway for the current from either the stator/battery to reach the ground.

This will cause the wire to either burn or blow immediately. The fuse should be located in the system.

Another reason for a wire going to ground is when an owner decides to add aftermarket gauges or lights. Amateur electricians simply cut the wires, expose some copper and then use some electrical tape for holding the wires together. This is the most dangerous way to join wires, and it often results in blownfuses.

The electrical tape will eventually fall off, expose copper wires and cause a short circuit. When trying to find a short ground, human error is often the culprit.

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 the circuit break suddenly stops clicking, then unplug it. You have a shorted wire somewhere on that light.

Recently, I was helping a friend with his motorcycle. He was having trouble locating the problem that was causing his 10 amp lighting circuit to blow.  I asked him which lights were original, and which ones had been changed. He informed me that the front turn signals were aftermarket.

I began at the left turn signal.  I found that turn signals only had two wires. So, I looked for electrical tape. It was evident that there was a bad connection.

I disengaged both blinkers. As I was disconnecting right blinker, I noticed that the positive electrical tape on the positive wire had become loose. The wire was rubbing against the backside of the bucket, which caused it to short circuit to earth.  After you’ve done a few of these fuse problems it becomes really easy to pinpoint what is causing the problem.

Crossing of wires

A wire can short circuit in two ways.  It can either short-circuit to ground (which I have just described in the last section), or it can short-circuit to another circuit. Different circuits may have different voltages. This can lead to problems if there are two wires crossed with different voltages.

I remember working on a vehicle with problems with one of its sensors.  A fuse kept blowing but we couldn’t find anything wrong with the sensor itself. A wiring diagram showed that the sensor should always read 5 volts.

I took a simple multimeter to read the voltage from the sensor. The voltage reading was 12.6 volts. So I knew that there had to be a cross wire where the 5 volt line was receiving 12.6 V. This increased voltage was creating an increase of amps which could sometimes blow the fuse.

I was able to find a small amount of melted wire between the two wires after I had followed it back.  They had melted from heat because they were too close to the engine. Make sure you get a wiring diagram printed out and familiarize yourself with the wiring system when you encounter a problem.

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.

When I talk to myself, it helps me. I think about where the short could be coming and then go through each one.  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

I would say that you should not lose heart and not let a mechanic fix it. You can do this. Although it might take you three times as long to solve a problem than a professional, there are huge benefits to doing so. 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.

It took me several days to solve my first electrical problem.  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).

Buy diagnostic tools:

  • Check light
  • Multimeter
  • A small spool electrical wire
  • Wire cutters
  • Circuit breakers
  • Mix and match fuse kit
  • Push pins for long distances

The test light will indicate which wires have current and which are grounded.  The multimeter will show you the voltage in each line. When you need to splice wire, the spool is handy.  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.

It is easy to get overwhelmed by the electrical components of a motorcycle. I have created a video series that covers how to restore a motorcycle, including overhauling the electrical. Tutorials are also included on how to fix other difficult components like the body and rebuilding carbs. Click here for more information if you’re interested in in-depth electrical tutorials or if you’re simply interested in building your dream motorcycle.

Similar Questions

How do you conceal the wiring of a motorcycle? There are many ways to hide your motorcycle wiring. My article contains more information about hiding your wiring.

Is the fuse box on a motorcycle located? The fuse box can be found under the handlebar cover on older motorcycles. This is the little plastic covering that covers the handlebars. Newer motorcycles have the fuse box located either under the seat or close to the battery.

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