How Tight Should A Serpentine Belt Be [Detailed Answer]

Like the alternator and the battery, the serpentine belt is one of the many parts that make up the engine of a vehicle. These various parts play different functions in the engine, but the serpentine belt plays a slightly more important role than other parts of the engine.

The purpose of the serpentine belt is to send power to different components in the vehicle. But without the right amount of tightness or tension, the serpentine belt won’t work properly.

This begs the question: How tight should a serpentine belt be for it to send power to the necessary components?

For the serpentine belt to effectively send power to the necessary parts, its tightness or tension should be half an inch of play. What this means is that if you are able to deflect the belt by just half an inch, then the tension is just right. If you aren’t able to deflect it to half an inch then,  it’s too tight, and if it deflects beyond half an inch, then it’s loose.

Every time the serpentine belt is too tight, it can cause pulleys to degrade faster. When it’s loose, it may slip off of its pulleys or cause your engine to overheat. Aside from the tightness or looseness of the serpentine belt, its condition also matters.

In this article, we’ll take a cursory look at the serpentine belt, what it does, and how tight it should be. We’ll also discuss the signs of a failing serpentine belt and how to adjust the tension on it. Keep reading!

What Is A Serpentine Belt?

how tight should a serpentine belt be

The serpentine belt is a long rubber belt that winds through different pulleys to power different parts of the engine. It’s often ribbed horizontally on one side for better grip.

The serpentine belt is known by many names. It’s sometimes called the fan belt, alternator belt, or accessory drive belt.

This belt performs the functions that multiple belts did in older vehicle models. Now, it is the one belt that sends power to the alternator, the AC, and other parts of the engine.

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The risk with this design is that when the serpentine belt breaks, various engine parts stop working.

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What Does A Serpentine Belt Do?

The function of the serpentine belt is to deliver rotational power from the engine crankshaft to different accessories in the engine system. It sends power to the alternator, power steering, the air conditioning compressor, and in some cars, the water pump.

The serpentine belt works with a number of pulleys. It works with the pulleys on the engine accessories, a belt tensioner pulley, and one or two idler pulleys.

It uses the pulleys and the belt tensioner to transfer power from the engine crankshaft to all the necessary components.

How Much Should You Tighten Your Serpentine Belt?

The standard tension on the belt should be approximately ½ inch of play.

However, most new vehicles have serpentine belts with an auto-tensioner. The auto-tensioner automatically keeps the right tension on the belt. Vehicles without an auto-tensioner have an adjustment screw that helps apply tension to the belt.

The tension needs to be right for it to move the pulleys and provide power to the necessary components.

When the serpentine belt is too tight, it can cause the bearings in the pulleys of the alternator, power steering pump, and water pump to wear out prematurely. However, if the belt is loose, it can slip off the pulleys, causing several parts to stop working.

If this happens, the water pump may not circulate coolant effectively, thereby leading to overheating.

Signs of a Failing Serpentine Belt (When To Replace)

There are a couple of things you’ll notice when the serpentine belt is failing. The sign most common in cars is a squeaking noise coming from the engine.

Below is a list of the signs you may notice when the serpentine belt goes bad:

1. Squeaking noise

When the serpentine belt is stretched out, it becomes loose, and it may slip off the pulleys. The result of this is a squeaking noise.

What you need to do when this happens is replace the belt. If the noise continues after that, then it could be from the pulleys.

To confirm which is the culprit, you can spray some water on the belt while it’s running. If the sound continues, then you have a bad pulley, but if the noise ceases, it means the belt is bad and it needs to be changed.

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2. Tears and cracks on the belt

When you begin to notice the serpentine belt is fraying at the edges or that there are cracks on the ribbed side it means it’s time to get a new belt.

The tears, cracks, and fraying are often caused by natural wear and tear. But the more the belt comes in contact with oil, heat, or dirt, the faster it will wear out.

3. Loss of power in electronic parts

When a serpentine belt fails, other machine components, such as pulley systems, air conditioners, water pumps, or alternators, tend to suffer.

The result is that these parts won’t get power, causing them to shut down. So you may find your AC is not working, or the steering is hard because the power steering pump is not working. Your car may even overheat because the water pump is not able to circulate coolant.

4. The battery warning light illuminates

When the battery warning light comes on, it could be a sign that the serpentine belt is failing. The serpentine belt powers the alternator, which in turn charges the battery as the engine runs.

When the belt stops powering the alternator, the alternator stops charging the battery. This will cause the battery to be drained because other electronic devices will be drawing power from it.

The battery warning light will then come on to warn you that the battery does not have enough power.

How To Check Serpentine Belt Tension

The tension on the serpentine belt needs to be right for it to move the pulleys effectively. Too tight or too loose can result in damage to parts of your engine. The rule of thumb for the tension is ½ inch of play.

You can check the tension on the belt by twisting it to a 90-degree angle. If you are able to twist it beyond 90 degrees, then the belt is loose. But if you can’t twist it to 90 degrees, then it’s too tight.

You can also use a ruler or a serpentine belt tension gauge to check the belt’s tension.

Locate the belt’s longest stretch between the pulleys and find the midpoint using the ruler. Now, move the belt from side to side while holding it between your fingers and thumb.

Watch how much it deflects in the run’s middle. It is too slack if it moves more than 1/2 in. (13 mm); if it moves less, it is too tight.

Compare it with the specifications in the car’s manual. Ensure you take the utmost care when doing this because the measurement is quite small, and you can easily get it wrong.

How To Adjust Serpentine Belt Tension

To adjust the serpentine belt’s tension, you need to know what type of tension adjustment system you have.

Newer cars come with an automatic drive belt tensioner that maintains the right amount of tension always, so it doesn’t require adjustment. However, if the belt needs adjusting, you either have the wrong belt size or the tensioner is faulty.

On the other hand, older cars use a tension adjustment fastener located on the alternator. But in other cars, it could be on the power steering pump or the air conditioner compressor.

Before you adjust the tension, you need to measure the tension on the belt. Attach a tension gauge or ruler to the longest belt span and pull it to measure the tension on your drive belt. Compare your reading to the specifications in your owner’s manual.

If the tension needs adjusting, follow these steps:

  • Loosen the alternator mounting bolts with a ratchet and socket.
  • Using a pry bar as a lever, pull or push the alternator away from the center till you get the desired tension.
  • Hold the pry bar steady with one hand to maintain the tension while tightening the bolts.
  • Cross-check the tension with that in the owner’s handbook. You should also turn on your engine after adjusting the tension to ensure there is no squeaking noise.
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How To Align Pulleys On A Serpentine Belt

The pulleys on a serpentine belt can have an angular or parallel misalignment. Whatever the case, a laser belt alignment tool will prove effective in realigning the pulley.

Laser equipment enables measurements and adjustments to be made with very high precision.

To align the pulley with the laser belt alignment tool, follow these steps:

  • Align the faces of the pulleys, or their grooves
  • Straddle the laser beam on the groove of the crankshaft and aim it at the misaligned pulley.
  • Use the beam to realign the pulley. Then use the right-sized bracket or socket to screw the pulley in place.
  • Ensure the beam meets the pulley in the same groove as the one the beam is in on the crankshaft pulley.
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Is The Serpentine Belt And The Timing Belt The Same?

Drivers can often mistake the timing belt and the serpentine belt for each other. But they are two different things that perform different functions.

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One way to differentiate them from each other is by their location and the role they play in the engine.

The timing belt is sometimes called a timing chain in some other vehicles. It is a stiff rubber with gears located inside or behind the engine. It connects the crankshaft to the engine valves, which supply air and fuel.

It ensures that the engine’s intake and exhaust valves work in sync with the pistons. This is what keeps the engine running smoothly

On the other hand, the serpentine belt is a stretchy horizontally ribbed rubber located outside the engine. It connects the engine crankshaft to all of the engine accessories.

The belt spins the alternator, which in turn converts the engine’s power to electricity to power other accessories in the vehicle.

One similarity between the timing belt and the serpentine belt is how long they last before needing to be changed. They can both last 50,000-100,000 miles with proper maintenance.

How Long Does A Serpentine Belt Last?

How long a serpentine belt will last depends on the material it is made of. A serpentine belt may be made of neoprene or ethylene propylene diene monomer (EPDM).

Those made of neoprene may last 50,000- 60,000 miles, while those made of EPDM could last up to 100,000 miles. So the serpentine belt could last you between 4-7 years, depending on your driving habits.

However, the lifespan of a serpentine belt can also be affected by its exposure to heat and friction.

The best way to maintain the lifespan of the belt is to ensure that you take your car in for regular maintenance, and get the belt inspected often.


What happens to a car when the serpentine belt breaks?

When the serpentine belt breaks, most of the electronic parts will lose functionality. It will stop the alternator from charging the battery, leaving the vehicle to run on reserve power until the battery dies.

You may also notice the headlights dim and the radio may not work. The following are other things you may experience when the serpentine belt breaks:

1. Overheating

The water pump is one of the parts that the serpentine belt powers, and when it breaks, it will stop sending power to it. This will stop the water pump from circulating antifreeze, causing the engine to overheat.

2. Air conditioner loses power

The air conditioner will not get power when the serpentine belt is broken, so it won’t work. This means when your serpentine belt breaks, your driving experience can go from pleasurable to undesirable.

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3. Hard steering wheel

When the serpentine belt breaks, engine accessories will not be able to spin at the proper speed, so the electrical system gets insufficient power. This can lead to a sudden loss of power assist for the steering system.

This will cause the steering wheel to become stiff all-of-a-sudden, making it very hard to turn. This can put you in a dire situation if it happens while driving.

Is replacing a serpentine belt expensive?

Replacing the serpentine belt does not take a lot of time, nor is it pricey. It could cost between $120-$150. The belt itself will cost up to $80, depending on the type of vehicle.

The total sum it will cost to replace a serpentine belt will depend on the area that you live in, the car you drive, and the overall condition of the vehicle. Government regulations could also impact the price.

If you live in the Midwest, it could cost you between $90 and $150, but if you live on the West Coast, you might spend between $100-$170. For those residing on the East Coast, the cost will range from $110-$220.

Can a serpentine belt cause the check engine light to come on?

The serpentine belt drives the alternator, which in turn powers the vehicle’s many electronic components. When the serpentine belt snaps or slides off the pulleys, the alternator loses power.

The result of this is that the alternator will not send power to the electronic devices, which will in turn cause the check engine light to come on.

Final Consideration

The serpentine’s tension must not be too tight or too slack for it to perform properly. As a rule of thumb, serpentine belt tension should have half an inch of play. You can check your owner’s manual to confirm what the ideal tension should be for your vehicle.

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