It is usually recommended that the serpentine belt and the belt tensioner pulley be replaced at the same time, as these parts have a similar life cycle and are designed to wear at approximately the same rate. Belt tensioner pulleys and idler pulleys are also used to maintain the routing of the serpentine belt to ensure that it runs in an unobstructed path. The belt tensioner pulley also uses spring tension to automatically adjust the tension of the belt to appropriate levels as it wears and stretches over time.
What is a Serpentine Belt?
The serpentine belt, also known as the drive belt, is likely the only engine belt you will see when you open the hood of your vehicle. It winds through the engine's accessory drive system and is one of the most important parts of keeping your vehicle running and its systems operational. When a vehicle is running correctly, it is very easy to forget about regularly inspecting the serpentine belt. Often, the condition of the serpentine belt is overlooked until it breaks, which could leave your vehicle stranded. Adding belt inspection to your regular maintenance routine is a must to prevent larger problems when you’re on the road.
What Does a Serpentine Belt Do?
A serpentine belt is used on almost all modern vehicles as the primary engine belt. The serpentine belt on your vehicle is a long, reinforced rubber belt that uses the motion of the crankshaft pulley to power the components of the accessory drive system. Serpentine belts are normally thin and flat with multiple grooves or “ribs” on the inside of the belt that match the rib pattern of the pulleys on the accessory units. These accessory systems may include the air conditioning compressor, power steering pump, water pump, and the alternator which provides power to the vehicle’s electrical systems and charges the battery. Complete failure of the serpentine belt can prevent these systems from receiving power and lead to a variety of symptoms, but even a belt that is too loose or worn may prevent these systems from working optimally.
When to Replace a Belt Tensioner Pulley
How Long Do Serpentine Belts Last?
Like any other part, serpentine belts wear out over time due to heat, friction, and age. The serpentine belt generates friction any time the engine is running. Most original equipment serpentine belts are rated for approximately 60,000 miles; however, some aftermarket belts can last up to 90,000 miles. Serpentine belts may also fail due to issues such as misaligned pulleys or accessories that fail or lock up.
As the ribs of the belt wear, or if the tensioner is failing, the belt may slip and suffer additional, accelerated damage. Contact with other parts of the vehicle could also cause belt damage, but this is often not a problem unless the belt or other parts near the belt are installed improperly. It is also possible for oil or coolant leaks around the belt to cause the belt to make noise or slip and suffer increased wear and damage.
When a serpentine belt is beginning to stretch, wear out, or fail, it will likely present one or more common belt failure symptoms. Diagnosing a bad serpentine belt may be relatively easy if you can see the problem, but a few symptoms may not be as easily recognizable. Inspecting the serpentine belt during regular vehicle maintenance can help you determine when it is time to replace the belt and tensioner.
Belt Wear
Signs of wear are the best indicators that the serpentine belt is reaching the end of its service life. When you visually inspect the belt, you may notice the ribs on the inside of the belt are starting to crack, the belt edges are beginning to fray, or the smooth side of the belt is starting to glaze over or take on a shiny appearance. The ribs on the inside of the belt may also begin to wear to the point they no longer make complete contact with the accessory pulley grooves. Rib wear can be checked using a serpentine belt gauge to be sure the belt is making complete contact with pulley grooves.
These signs usually occur due to age or wear from heat and friction, and these are indicators that the belt is reaching the end of its life and may fail unexpectedly. If you notice any of these signs, you should consider replacing the serpentine belt and tensioner as soon as possible. While these failure indicators are visible, there are also other symptoms which may not be as obvious when inspecting the serpentine belt, and may only occur while the engine is running or at higher RPMs.
Belt Squeal and Belt Slap
Belt squeal and belt slap occur when there is too much or not enough slack in the serpentine belt. Most modern vehicles use an automatic belt tensioner to maintain the correct serpentine belt tension, but some vehicles use a manual tensioner that uses an adjustment screw to apply tension. Automatic tensioners are designed to maintain appropriate tension as the belt stretches. However, they may reach the end of their travel if the belt stretches too much, and this can allow the belt to loosen. Manual tensioners must be adjusted periodically to keep the serpentine belt from becoming too loose. If the belt tension is not correct, you may notice a loud chirp or squeal when the engine is at idle, under load, or when turning a sharp corner.
Serpentine belts need to have the right amount of tension to drive the accessories and should not be too loose or too tight. There are two ways to test the tension of a serpentine belt. The first way is to find the longest free section of the belt and press in on the smooth side of the belt. The belt is too loose if it travels more than 1/4 inch, and it is too tight if there is no give. You can also take the same section of the belt and try to twist it in the middle. The serpentine belt is too loose if it twists a half a turn, causing the ribs to face outward, and is too tight if there is little to no twist. If you are not sure about performing this test, you can use a belt tension gauge to accurately measure the tension of the serpentine belt.
The Alternator and Electrical System
The engine and electrical systems of your vehicle rely on power from the alternator, which converts mechanical power from the serpentine belt to electrical power. When the alternator does not turn fast enough, it will not generate enough charge to power the battery, ignition system, or the vehicle’s electrical systems. If the serpentine belt is slipping on the alternator pulley and not spinning the alternator at the appropriate speed, you may notice the amp meter on the dashboard reading low, or the Check Engine light or battery light may activate. Too little power being produced by the alternator can cause the battery to eventually die or fail to fully recharge, and the engine could even stall while the vehicle is being driven. These symptoms may also be related to a failing alternator, but checking the condition of the belt is a great first diagnostic step. You can learn more about the alternator on your vehicle with our article on signs of a bad alternator. O'Reilly Auto Parts can also test your alternator for free to help determine if it needs to be replaced.
The Water Pump and Cooling System
If the water pump is not turning fast enough due to a loose or slipping serpentine belt, engine coolant will not circulate properly and the engine can run hot or overheat. If some coolant is circulating, it may prevent the engine from overheating to a dangerous level, but you may notice the temperature gauge on the dashboard displaying operating temperatures that are higher than normal. If there is not enough coolant circulating, the engine will eventually overheat and you may notice the temperature gauge rising toward “hot”, the Check Engine light or temperature warning light may activate, or you may smell hot coolant or see steam from under the hood. If you notice any signs that your engine is overheating, you should pull off the road as soon as possible in a safe location, such as a parking lot or driveway, and turn off the engine. Overheating can cause serious damage to the engine block, head gaskets, and other internal engine components, and any overheating problems should be addressed before the vehicle is driven again.
Diminished Performance and Other Symptoms
Some serpentine belt issues may not cause your vehicle to stop running or overheat, but they can create other symptoms that point to belt failure. You may notice a loss of performance, power steering failure, sudden battery drain, or a loss of air conditioning. These symptoms may present immediately if the belt fails or breaks, or they may occur gradually as the belt ages and stretches.
The Check Engine light may also activate when belt failure symptoms begin. You may also notice a low battery voltage indicator light if the belt is not turning the alternator correctly. Some symptoms may specifically point to a belt issue, but complete diagnosis may be needed to ensure that these issues are related to a belt failure. If you notice any of these symptoms, it is recommended that you drive your vehicle as little as possible until it can be repaired.
We hope this has helped you understand what your serpentine belt does and how to diagnose serpentine belt wear and failure symptoms. Remember, it is a good idea to include the serpentine belt and belt tensioners in your regular vehicle maintenance schedule, and to visually inspect the belt every time the hood is opened for other services. Check out our video on how to change a serpentine belt, tensioner and idler pulley for helpful instructions on changing a serpentine belt. If you are unable to or unsure of how to inspect or replace the serpentine belt or tensioners on your vehicle, O’Reilly can recommend a trusted local repair shop. You can also stop by your local O’Reilly Auto Parts, and our parts professionals will be happy to help find the right parts or provide possible repair options for your vehicle.