What a serpentine belt does and when to replace it
A serpentine belt is a single rubber belt that runs multiple pulleys under your hood, powering your alternator, power steering pump, air conditioning compressor, and water pump all at once. When it wears out, one or more of those systems stops working — your battery won't charge, your steering gets heavy, or your engine overheats. You'll usually hear a squealing noise first, especially when you start the car or turn the wheel hard.
Most serpentine belts last between 30,000 and 100,000 miles depending on the belt quality and your driving conditions. If you hear squealing that doesn't go away after a day or two, the belt is probably worn enough to replace. A belt that breaks completely will leave you stranded, so don't ignore the warning signs.
Replacing a serpentine belt is a job most people with basic tools can do in 30 minutes to an hour. You don't need to remove other parts or drain fluids. The hardest part is usually just finding the belt routing diagram, which tells you how to thread the new belt back on.
Key Takeaways
- You'll need the correct belt size for your vehicle, which you can find on a sticker under the hood or by giving your vehicle's year, make, and model to an auto parts store.
- The belt routing diagram shows you how to thread the new belt around all the pulleys; most cars have this diagram printed on a sticker under the hood or in the owner's manual.
- You'll need a wrench or socket to loosen the tensioner pulley, which holds tension on the belt so it doesn't slip.
- Once the old belt is off, the new one threads on the same path, and you tighten the tensioner back down until the belt has the right amount of give.
Gather the right belt and tools
Before you start, you need to know your belt size. Open your hood and look for a sticker on the underside of the hood or on the radiator support — it will show a diagram of the belt routing and list the belt length in inches. If you can't find it, write down your vehicle's year, make, model, and engine size, then call an auto parts store or search online for "serpentine belt [your vehicle]." They'll tell you the exact size.
Buy the replacement belt at an auto parts store. Bring the old belt with you if you have it, or give them your vehicle information. A new serpentine belt costs between $15 and $75 depending on your vehicle. While you're there, pick up a belt routing diagram if you don't have one — many stores print them for free.
For tools, you'll need a wrench or socket that fits the bolt on your tensioner pulley. The tensioner is the spring-loaded pulley that keeps the belt tight. Most vehicles use a 15mm socket, but check your manual or the diagram to be sure. You may also want a belt routing diagram printed out or on your phone so you can reference it while you work.
Remove the old belt
Locate the tensioner pulley under your hood. It's the pulley that has a bolt through its center and is connected to a spring or arm. The diagram will show you where it is. Use your wrench or socket to turn the bolt counterclockwise — this loosens the spring tension and allows the belt to go slack.
Once the tensioner is loose, slide the old belt off all the pulleys. Start from the tensioner and work your way around, noting which direction the belt was running. Take a photo of the belt routing before you remove it completely, or print out the diagram so you have it handy. The belt should come off easily once the tensioner is released.
Inspect the pulleys while the belt is off. Spin each one by hand — they should turn smoothly without wobbling or grinding noise. If a pulley is damaged or doesn't spin freely, it will need to be replaced before you install the new belt, or the new belt will wear out quickly.
Install the new belt
Lay out your belt routing diagram where you can see it. Starting at the crankshaft pulley (the largest one at the bottom of the engine), thread the new belt around each pulley in the exact order shown on the diagram. Work slowly and make sure the belt sits fully in the groove of each pulley — it should not be twisted or riding on the edge.
The last pulley you thread should be the tensioner. Once the belt is routed around all the other pulleys, pull it over the tensioner pulley. The belt should be snug but not tight at this point.
Now tighten the tensioner bolt clockwise. As you turn it, the spring will pull the tensioner arm and create tension on the belt. Tighten until the belt has about a half-inch of give when you press on it with your thumb midway between two pulleys. Don't over-tighten — too much tension will wear out the belt and bearings faster.
Check your work and test the belt
Before you close the hood, spin the engine by hand (using the crankshaft bolt) to make sure the belt moves smoothly around all the pulleys without slipping or rubbing. Watch it go around at least twice to confirm it's tracking straight and not hitting anything.
Start the engine and listen for squealing. A new belt may squeal slightly for the first few minutes as it settles into the grooves, but it should quiet down quickly. If it squeals loudly or continuously, the tensioner is too loose — turn the bolt a bit more clockwise to tighten it.
Let the engine run for a few minutes, then turn it off and let it cool. Check that the belt is still in place and hasn't shifted. Once it's cool, you can close the hood. The belt will continue to settle for the first 50 to 100 miles of driving, so listen for any unusual noises over the next few days.
What to do if the tensioner is stuck or broken
If the tensioner bolt won't turn or turns but doesn't move the arm, the tensioner itself may be worn out or seized. A stuck tensioner can't hold the new belt tight, so the belt will slip and squeal no matter how hard you try to adjust it. In this case, you'll need to replace the tensioner as well as the belt.
Replacing a tensioner is similar to replacing the belt — you unbolt it from the engine and bolt a new one in place. The bolt is usually 15mm or 18mm. If you're not comfortable doing this, a mechanic can replace both the belt and tensioner in about an hour for $100 to $300 in labor, depending on your vehicle.
Frequently Asked Questions
How do I know if my serpentine belt is actually worn out?
A squealing noise when you start the car or turn the steering wheel hard is the most common sign. You can also look at the belt itself — if the rubber is cracked, frayed, or shiny and smooth (instead of textured), it's time to replace it. If the belt is visibly damaged or squealing doesn't stop after a day, don't wait.
Can I drive with a squealing serpentine belt?
You can drive short distances, but not safely for long. A squealing belt means it's slipping, so your alternator may not be charging your battery, your power steering may fail, or your engine may overheat. The belt could break completely at any time, leaving you stranded. Replace it as soon as you can.
What if I can't find the belt routing diagram?
Check your owner's manual first — most include the diagram. If you don't have the manual, search online for "[your vehicle year, make, model] serpentine belt diagram" or call an auto parts store with your vehicle information. You can also take a photo of the old belt before you remove it, which shows you the routing.
Do I need to replace the tensioner every time I replace the belt?
No. Replace the tensioner only if it's visibly damaged, won't tighten, or doesn't hold tension. A tensioner usually lasts much longer than a belt. However, if your tensioner is old and you're replacing the belt, it's often worth replacing both at the same time to avoid a second job soon after.
How tight should the new belt be?
The belt should have about a half-inch of give when you press on it with your thumb midway between two pulleys. This is tight enough to transfer power without slipping, but loose enough to avoid wearing out the bearings. If you're unsure, err on the side of slightly looser — you can always tighten it more if it squeals.