What Causes a Serpentine Belt to Break? Answered!

The serpentine belt is underestimated and overlooked by many as just something that needs replacing if it breaks. But this little belt might not look like much, but it plays a critical role in keeping your car running. So, what causes a serpentine belt to break, and what happens to the vehicle if it breaks? First, you should understand what the serpentine belt is…

What Is a Serpentine Belt?

A serpentine belt which goes by many other names, auxiliary belt, accessory belt, fan belt, and alternator belt, is a long single belt that drives many of the critical components of a car’s engine. It’s called a “serpentine” belt because it winds around several pulleys in a snake-like pattern. It is grooved on the inside, which will grip a matching designed pulley.

The serpentine belt is usually rubber with embedded fibers for strength and durability. It’s responsible for transmitting power from the engine’s crankshaft to various auxiliary components such as the alternator, power steering pump, air conditioning compressor, mechanical fan, and occasionally the water pump (on most cars, the cambelt drives the water pump).

Unlike older vehicles with multiple belts for each separate component, a serpentine belt replaces all the individual belts apart from the cambelt. This simplifies the whole system while reducing the number of maintenance components that require service—car dependant the serpentine belt is tensioned automatically by a spring-loaded or manually adjusted tensioner.

What Causes the Serpentine Belt to Break?

The most common cause of a serpentine belt breaking is the belt has gotten weak with age. Because the belt is constructed mainly of rubber, rubber cracks and perishes over the years; if left and never replaced, eventually the belt gets so weak it starts to fray and will eventually snap.

Another cause of a snapped belt is damage from an impact. Rocks and debris in the road can easily get flicked up into the engine bay if the engine’s dirt shield is missing or broken. Your car may not even have one to start with.

Although there isn’t much you can do to stop the belt from breaking from an impact, it is essential to check that your belt is always in good condition. A quick check now and again will stop the belt from ever getting to the point it has broken because of its condition.

Signs the Serpentine Belt Needs Replacing

A serpentine belt is very cheap and straightforward to replace; it can be done on the driveway with a basic tool set and a little bit of know-how, so it is essential to replace the belt if you spot any of the signs below of a problem looming.

Strange noises

Listen for high-pitched screeching and squeaking noises, especially those that change with revs; it indicates a problem with the serpentine belt. Because the belt is under constant tension, it will stretch, so most modern cars use an automatic tensioner that readjusts itself with age to keep the belt taught. If the belt stretches past the tensioner’s maximum adjustment, the belt will be loose on the pulleys, and you will have almost certainly heard the horrible screeching noise as a car revs. This is the belt slipping on the pulley while being forced around.

The same thing happens with a manually adjusted tensioner, but a simple readjustment can stop the horrible noises. However, the belt will need replacing if it’s at its maximum adjustment.

The other noise to pay attention to is the catching noise of the frayed bits of the belt hitting the engine. This noise indicates the belt is holding on by a small amount, and you should switch the car off and have it recovered. The bits of the belt being rotated at several thousand RPM will cause damage to other components in the engine bay, so it is not worth the risk of driving.

Visual damage

The serpentine belt is mainly made of rubber, and the downfall of rubber is it dries out and cracks with age. Cracks don’t necessarily mean the belt will snap at any moment or needs replacing immediately because of its working environment (heat and constant use). Small cracks may appear along the ribs even after a few months of replacing them with a new belt.

The thing to be cautious of is the depth of the cracks and if they are starting to appear on the shiny outer surface of the belt. If they are, it needs replacing asap; the belt will probably break at any moment.

Dashboard warning lights

Dashboard warning lights such as the battery, charging, or even the check engine light, although not directly associated with a belt problem, could indicate the belt has broken. For example, the belt turns the alternator which charges the battery; if the belt has snapped, the alternator cannot charge the battery, in which case both the charging light will come on because the alternator is not working and the battery light because the battery is being depleted of any charge.

The check engine light may also come on when the alternator is not operating, although not very common. It is more likely to come on because on cars that still have an engine-driven mechanical fan (Land Rovers commonly) fitted that is not operating because the belt has broken and the vehicle is now overheating.


Overheating can occur when a belt breaks for several reasons. One is because the fan is not operating if it is engine-driven. Another is when the serpentine belt drives the water pump, and the engine overheats because the coolant is not being pushed around the engine. The motor will overheat if no cold coolant passes through the engine’s water jackets.

Struggle starting

It’s pretty well known that serpentine belts break on starting a vehicle; that sudden jolt and being asked to move if weak can force a weak belt to snap. With a snapped belt, you may not be able to start the car or at least keep it running for long.

If you did get the car going and were to eventually turn it off if it was to die while driving, the battery would have received no further charge from the alternator because the belt would not be turning it. This will fail to restart the vehicle. Even if you replace the belt, the battery will require a jump start or even replacement, depending on it’s condition to get it going again.

what causes a serpentine belt to break

What Happens if the Serpentine Belt Breaks While Driving?

If the belt breaks while driving, the car will shut off. Unfortunately, the battery’s condition will determine how long it can hold up with no further charge being put back into it. Remember, one of the crucial jobs of the serpentine belt is to turn the alternator which in turn tops up the battery with charge. If the alternator is not operating, no charge is being put back into the battery, which will cause the battery to drain very quickly and result in the car randomly switching off, even while driving.

The other essential item the serpentine belt turns is the power steering pump if the car has it! As the pump will not rotate and move the fluid, the steering will become very heavy; you will effectively be driving a car without any power steering. But a vehicle becomes undrivable when designed to have power-assisted steering.

Can You Drive With a Damaged Serpentine Belt?

If the belt is still a complete unit, you can theoretically drive; however, there is no knowing how long it will last until it breaks. Once a belt is damaged, it may last a few minutes, or it may last months. There is no set time frame. So, if you spot damage to the serpentine belt, you should replace it immediately.

How Long Does a Serpentine Belt Last?

Serpentine belts generally have a serviceable lifetime of approximately 50,000 miles, but it is not uncommon to hear them last longer than 100,000 miles. Because the serpentine belt must be removed to replace the timing belt, water pump, power steering pump, alternator, and AC pump, it is usually replaced once removed. It’s one of those items that you wouldn’t normally refit the old one because they stretch and have to be stretched further to remove from the pulleys. Serpentine belts are cheap to buy and simple to replace, so it would be silly to refit an old belt regardless.


The serpentine belt is a single circular belt of rubber that snakes around pulleys, which turn vital components such as the alternator. Without the belt, a car will be undrivable and shut off quite quickly.

To summarise the question, what causes a serpentine belt to break? The most common cause is age-related cracking, which means the belt is weak. It will eventually start to fall apart and break without warning. It is essential to check the belt’s condition regularly to prevent it from breaking. Look out for any signs of damage, severe cracking, squeaking noises, and dashboard warning lights, all indicating it needs to be replaced.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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