Know-How Notes: Automotive Fuse Guide
When you have a bad temper, people say you “have a short fuse”, in the world of automotive electrical components, a short fuse could mean a mini-ATC (technically ATM), GMA, or even an AGA style fuse. If you are not a regular in the automotive fuse section at your local NAPA store, you are probably scratching your head at the alphabet soup in that last sentence. Not to worry, we are here to help you make the connections you need without bogging down in the mire of complex electronical jargon.
A fuse is a mechanical breaker device that protects your vehicle’s electrical components and wiring in the event of a fault. A fault can be anything from a dead short (where the positive voltage shorts to ground), to a damaged component. When faults happen, the power feeding the component must be disconnected immediately. The most common cause of automotive fires is electrical shorts. Pulling too much amperage through a wire always leads to that wire getting hot, and eventually it catches fire. If you have ever experienced a wire fire, it is quite scary, especially if you are actively driving.
Auto makers design circuit breakers into every component in your vehicle, and most of them are housed in one or two central locations. These are the fuse boxes. Some vehicles have individual fuse boxes in various locations, but most vehicles use a centralized master fuse box, much like the breaker box in your home. There are many types of circuit breakers, ranging from sacrificial (fuses) to resettable (circuit breakers), and then there is the classically dreaded fusible link, which is the bane of many a mechanic. This article will address and identify the most common types of automotive fuses that you may encounter.
Sacrificial Circuit Breakers (Fuses)
All of these fuse types are destroy themselves in the process of breaking the circuit. This is done through a piece of metal that melts at a certain temperature point.
Most American vehicles went away from glass fuses in 1982. These fuses are rated for 32v DC and cover from 4 amps up to 30. There are numerous designs for automotive glass fuses. SFE and AG* type fuses are fast-blow fuses, they pop as soon as there is more draw than they are rated for. Slow-burn fuses, like the MDL type, are timed fuses.
SFE – These are the original glass fuses for automotive use. SFE stands for Society of Fuse Engineers, and have different lengths so that the wrong fuse cannot be installed into the wrong placement. They are ¼” in diameter, from 5/8” up to 1 7\16” long.
Outside of SFE fuses are AG type, which stands for Automotive Glass with a suffix letter to note the type.AG Type Fuses
|AGA (1 to 30 amp)||1/4” diameter x 5/8” long|
|AGB||.177” diameter x .588” long|
|AGC (0.125 to 50 amp)||1/4 diameter x 1-1/4” long|
|AGS||9/32” diameter x 1-1/4” long|
|AGU (1 to 60 amp)||13/32 diameter x 1-1/2” long (sometimes referred to as Midget fuses)|
|AGW (1 to 30 amp)||1/4” diameter x 7/8” long|
|AGX (1 to 30 amp)||1/4” diameter x 1” long|
|AGY (50 amp only)||1/4” diameter x 1 7/16” long|
|UK (35 to 50 amp)||1/4” diameter x 1 1/4” long|
MDL – This style is similar to the AGC fuse in size and rating, however AGC fuses are “fast blow”, meaning they pop instantly, whereas MDL fuses are time-delay fuses that burn slowly. This allows the MDL fuse to resist higher amperes for very brief moments, such as a high-draw from a compressor or fan kicking on.
Bosch – Used in older European vehicles, these plastic barrel fuses have exposed metal fuse link. They
install similar to a AA-type battery between two flat springs.
Lucas – The much maligned Lucas wiring system is used on British vehicles, and of course they use different fuses from everyone else with a convoluted rating system. These are ceramic barrel fuses similar to the Bosch style. Where many people go wrong is confusing the “continuous fusing” rating (where the fuse pops) with the smaller rating of “continuous amps”, which is where fuse operates normally. When replacing a Lucas fuse with a non-Lucas glass-type fuse, you use the smallest rating for the new fuse.Lucas Fuses
|Color||Continuous amps||Instantaneous fusing amps||Continuous fusing amps|
|Red on Yellow||2.5||6||5|
|Red on Green||5||12||10|
|Green on Black||5||12||10|
|Red on Brown||6||14||12|
|Purple & Yellow||25||60||50|
|Yellow & Red||30||75||60|
Micro2 – Also known as APT or ATR, these fuses have ratings from 5 up to 30 amp. Measurements are 9.1l × 3.8w × 15.3h mm
Micro3 – Also known as ATL, these 14.4 × 4.2 × 18.1 mm fuses run from 5 to 15 amp
LP-Mini/APS/ATT – Fuses measure 10.9 × 3.81 × 8.73 mm, and are rated from 2 to 30 amp.
Mini/ATM/APM – Fuses measure 10.9 × 3.6 × 16.3 mm, 2 to 30 amp.
Standard/ATC/ATO/APR/ATS – Fuses measure 19.1 × 5.1 × 18.5 mm, .5 to 40 amp.
Maxi/APX – These much larger fuses (29.2 × 8.5 × 34.3 mm) can hold more amps than standard or mini fuses. 20 to 120 amp.
When you need to manage high current, you need a big fuse. These fuses are large and generally mount with bolts or in some case, large blades.
MEGA/AMG/BF2 – Current ratings from 60 to 275 amp, these are used for starters, alternators, and batteries. They are a bolt-down fuse and are slow blow. These fuses measure 19 x 68 mm.
AMI/MIDI – These bolt-down fuses look just like MIDI fuses but are about half the size. Ratings range from 23 up to 200 amp. These slow blow fuses measure 10mmx 41mm.
ANL – These fuses measure 20 x 82 mm and cover a range from 20 to 500 amp. Unlike the other bolt-down fuses, this design has slotted ends, so the bolts do not have to come out completely.
MRBF/ZCASE – This is a single MEGA type fuse that is used for very high current application, most commonly the starter. It is mounted with a single bolt. Fuse ratings range from 40 to 250 amp.
Class-T – commonly used for EV and hybrid vehicles, the class-T fuse is a high voltage fuse. These are available in amperage ratings from 10 up to 50-amps, but the voltage capacity is much higher than 12-volts, as these are specifically for the battery packs, which are in the 250 to 450-volt range. Class T fuses are fast blow, and have the fastest blow speed of all high current fuses.
All non-glass sacrificial fuses are color coded for their amperage. The codes change based on the fuse type. The most common fuses you will encounter are blade type, so we have the basic color codes for them here:Blade Fuses
|Dark Blue||0.5 amp|
|Clear||25 amp (Grey for MAXI)|
|Blue/Green||35 amp (Brown for MAXI)|
You will notice that the color do repeat. In most cases this is because the fuse types have limits, such as a MAXI fuse does not go below 20 amps.
Resettable Circuit Breakers
Circuit breakers come in two types: manual or automatic reset. Manual reset breakers are mostly used for aftermarket applications such as car audio or lighting. Some manufacturers use auto-reset breakers for certain devices that have high current draw during start-up, such as cooling fans or electric motors. The benefit of an auto-reset breaker is that they reset themselves in the event that the initial draw is too high due to heat or other issues. This means your cooling fans will still work if they experience a brief high-draw issue, without have to replace the fuse. An auto-reset breaker is not a good idea for electronics because it will constantly try to reset in the event of a failure or short, and that is not a good thing.
Manual breakers operate just like a sacrificial fuse, but instead of replacing the fuse, you simply click the switch or button on the breaker and you are back in business. These are beneficial for aftermarket circuits where blown fuses are common (car audio, added lights, etc), as high-current fuses are not cheap.
Is it bad?
Most sacrificial fuses are obvious when it is bad, but not always. These fuses have a metal bar that literally burns up, disconnecting the power source from the rest of the circuit. If the little bar is melted, the fuse is bad. This does not always happen where you can see it. This is a particular problem that effect glass fuses. You cannot see the entire fusible element; the metal caps on either side prevent it. Unlike blade fuses, the element is soldered to the cap. If the solder melts before the element, you won’t see the break, but the circuit is dead anyway. This is most common with high-amp glass fuses.
You don’t have to pull a fuse to see if it is bad. Using a test meter (not a test light), set to continuity (where the unit beeps when the probes are touched together), and touch each lead to the ends or to the test terminals of the fuse. Your meter should beep (or read “0” if it is a silent meter). Any other reading means there is problem with the fuse and it should be replaced. High-amp glass fuses are notorious for melting the solder at lower amp ratings, so you may not have an actual problem, it could just be a bad fuse. This is why glass fuses are no longer used in most automotive applications.
Replacing a bad fuse is fairly easy in most cases, but you do need to take a few precautions. First, make sure the vehicle is off. You don’t want to replace a fuse while the vehicle is running or the equipment affected is active. This can cause the fuse to pop even if you have rectified the actual problem due to the initial connection. If the problem has not been rectified, the fuse is going to pop anyway, so it is best not to have it in your hand when that happens. If your circuit is always live (has power even with the key off), then you should disconnect the ground terminal of the battery.
Bladed, cartridge, and non-glass bladed fuses can be really tricky to get out of their sockets. Most vehicles come with a small plastic tweezer tool to remove the fuses, but you can use needle-nosed pliers in the likely case that the tool is missing.
Bolt-down fuses like ANL and Class-T types require tools. A socket or nut driver is the best option, as a wrench can accidentally bridge the gap on the fuse terminals and again, that is rarely a good thing to do. The most difficult to remove fuse is a glass-type barrel fuse. These fuses are delicate. When the fuse goes bad, the glass can break, but moreover, it can break when you are trying to get it out. Because the spring terminals are tight, these fuses require some effort to remove. It is a good idea to use a non-conductive tool to pry one side of the fuse out of the holder. Plastic or wood is perfect. Don’t use pliers on the glass tube, it will break.
When is it safe to use a bigger fuse?
The short answer is almost never. The fuses are there to protect your vehicle’s equipment, if there is a fault, you need to find the fault rather than just up the fuse rating. There are a select few instances where you can go up to a larger fuse, but only when you absolutely know that the wiring is capable of handling the extra current.
Example 1: You have a car audio system with an amplifier; The amp is wired with a 4-ga wire that is 16 feet long. The previous amp had a 40-amp fuse, the new amp requires 60 amps. The wire capable of handling the larger load, go ahead and increase the fuse size.
Example 2: You replaced the head light bulbs with fancy new high-output bulbs. DO NOT increase the fuse size for the headlights. In fact, you need to install a relay and re-wire the headlights with larger gauge wires. The current draw of high-output bulbs is more than the factory wires can handle and will result in a fire.
Armed with the knowledge we have provided, you should be able to test and replace any fuse with ease. Make sure you follow the safety protocols to protect yourself and your vehicle. One more cautionary note- NEVER EVER bypass the fuse for a component. Coins, aluminum foil, etc are not suitable circuit breakers in any situation. DO NOT bypass the fuse, that is a sure-fire way to end up in a catastrophic situation.
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