Car Fuses and Fusible Links Explained

Automotive fuses are the gatekeepers and bodyguards of automotive electronics. Whenever a sudden short or surge threatens the electronics of a modern car or truck, a fuse stands ready to throw itself into the fire.

In doing so, the fuse takes a figurative bullet for a valuable, complex, or indispensable component or device, such as a car stereo or amplifier. This often results in a temporary loss of functionality. Fuses, however, are cheap and usually easy to replace. Repeated failures of a fuse on the same circuit often reveals an underlying problem.

All Car Fuses Aren't the Same

Automotive fuses differ in design type and current rating, but they're all based on the standard ATO and ATC blade type fuses that Littelfuse patented in the 1970s. They have a similar appearance to the original ATO fuses, and many applications still use standard ATO and ATC fuses. Types differ primarily in size and number of terminals. Physically large fuses are typically used in high-current applications.

In the past, vehicles commonly used glass tube and Bosch fuses. They're found today in old vehicles that are still on the road.

A glass tube fuse is capped by metal terminals with a metal strip passing through the center. Bosch fuses are also roughly cylindrical, but they are made of a solid ceramic material with a metal strip on the surface.

Although it's possible to replace any ATO fuse with any other ATO fuse, this can be extremely dangerous if the wrong amperage fuse is substituted.

Similarly, replacing a Bosch fuse with an American-style glass tube type is sometimes physically possible, but sticking to the same amperage rating is imperative. Plus, a flat-capped glass tube fuse typically doesn't fit well into a fuse holder designed for conical end caps.

Types of Blade Fuses

For all blade fuses, the housing is opaque or clear. When the housing is clear, it's usually easy to tell whether the fuse is bad because the metal strip that connects the two terminals is visible. If the strip is broken, the fuse has blown.

Most modern cars and trucks use one or more of the following types of blade fuses, listed in descending order of size.

Maxi (APX) Heavy-duty Fuses

• The largest type of blade fuse.
• Used in heavy-duty applications.
• Available with higher amperage ratings than other blade fuses.

Regular (ATO, ATC, APR, ATS) Fuses

• The first and standard type of blade fuse. These fuses are wider than they are tall.
• Come in two main types that fit in the same slots. ATO fuses are open at the bottom; ATC fuses feature a plastic body that is enclosed.
• Found in most modern cars and trucks.
• Many applications started to replace ATO and ATC fuses with mini fuses in the 1990s, but they are still widespread.

Mini

• Smaller than regular blade fuses but available across a similar amperage range.
• Also available in a low-profile mini version.
• Low-profile and regular mini fuses share the same body height and width, but the spade terminals of low-profile mini fuses barely extend past the bottom of the body.

Micro

• Micro2 fuses are the smallest type of blade fuse. They're taller than they are wide.
• Micro3 fuses are larger than Micro2, low-profile, and mini fuses. They have three spade terminals. Every other type of blade fuse uses two terminals. They also include two fuse elements, which allows a single fuse to handle two circuits effectively.
• Available across the smallest range of amperage ratings.

Automotive Fuse Color Coding

It's possible to replace any ATC fuse with any other ATC fuse, any mini fuse with any other mini fuse, and so on. However, this isn't safe if you don't match the current ratings. Although fuses can blow under normal operating conditions because of age and wear, a blown fuse often indicates a deeper problem.

So, if you replace a blown fuse with another fuse with a higher amperage rating, you'll prevent the fuse from blowing again immediately. However, you also risk damaging some other electrical component or even starting a fire.

There are three ways to tell the amperage of a blade-type fuse:

• Look at the top of the fuse for the amperage rating printed on or stamped into the plastic.
• Look at the color of the fuse body If the rating has worn off.
• Check the fuse diagram to see what type of fuse belongs in that slot.

Colors and physical dimensions for blade fuses are laid out in DIN 72581, and not all colors and amperage ratings are available in all sizes.

Color coding is standard almost across the board for different types of automotive blade fuses, with two notable exceptions: 25 A and 35 A maxi fuses. These are gray and brown, respectively—colors that are also used for lower-amperage fuses. However, maxi fuses are not available in 2 A or 7.5 A, which are the ratings used by those colors, so there's no possibility of confusion.

What About Fusible Links?

Fusible links perform the same basic function as fuses, but in a slightly different way. In automotive applications, a fusible link is a length of wire that is several gauges thinner than the wire that it is designed to protect. When all goes well, this results in the fusible link failing and breaking the circuit before the protected wiring can fail.

Fusible links are also encased in special materials that are designed not to catch on fire when exposed to high temperatures. So, although extremely high current in a regular wire could cause a fire, a blown fusible link is less likely to do so.

You'll find fusible links in various places in cars and trucks, but they are commonly used in high-amperage applications such as starter motors, which can draw hundreds of amps. When this type of fusible link blows, the vehicle no longer starts, but the risks of fire are less. In other applications, the fusible link is easier to get at and replace than the wiring it is designed to protect.

Replacing Fuses and Fusible Links

Replacing a fuse is relatively easy, but it's still important to make sure you replace it with the correct style and amperage rating. Blade fuses are sometimes physically difficult to pull out. Still, most vehicles come with a fuse puller tool inside one of the fuse boxes or attached to a fuse box lid.

Although it is fairly easy to identify car fuses on sight, a visual guide can help you determine which type of fuse you need.

If you replace a fuse and it blows again, there's typically some underlying problem. Replacing the fuse with a higher amperage fuse might fix the problem temporarily. However, identifying the components present on that circuit and tracking down and fixing the underlying problem is the safer way to go.

Replacing fusible links is often a more involved job than pulling a fuse; they're typically bolted in place and are sometimes difficult to reach. You can do it at home with the right tools if you can physically locate the blown fusible link, but it's important to use the correct replacement.

Likewise, replacing a blown fusible link with the wrong part is dangerous. In a best-case scenario, the fusible link won't be able to handle the amperage of the application, and it will fail immediately. In a worst-case scenario, you could end up with a fire.

Never replace a fusible link with an electrical cable. Not even if you have a ground strap or battery cable lying around that looks the right size and length. Call a parts store, give them the application, and they'll come up with a fusible link designed for your job.

Fusible links often carry tremendous amounts of current. So, doing the job poorly or with any replacement wire or cable can result in a fire or a more costly repair when other wiring fails later on.