Replacing a Car’s Serpentine Belt: A Step-By-Step Guide

Determine if you need to replace your serpentine belt

According to Peter Meier, ASE Master Technician and automotive trainer, “inspecting the wear of the ribs and grooves [of a serpentine belt] is much like checking the tread depth of a tire.” Meier added, “Just like a balding tire, rounded belt teeth cannot shed rainwater, resulting in belt slippage.” The ribs on a new belt come to a point allowing the belt “seats” to fit into the pulley groves fully. As the belt wears, the rib seats become rounded, and the belt “teeth” begin to narrow, causing the belt to sit lower in the accessory pulleys.

Use your smartphone to take a high-resolution photo of the belt. Check rib and seat wear using the Gates Tech Tip Bulletin TT002-15. Replace it if the ribs are rounded over or flattened out. Check for cracks in the belt ribbing. Small cracks are OK, but spider web-like cracking on the surface signals extreme wear. Also check if pieces of the belt ribbing are missing or frayed, or if the smooth side of the belt is glossy or shiny (a serpentine belt wears on BOTH sides); has rounded edges or chunks missing; is glazed, shows large cracks, or is peeling or fraying.

• SAFETY TIP: Always ensure the engine is off and cool before replacing a serpentine belt.

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Serpentine belt tool and belt placement tools

Specialty tools greatly simplify changing the serpentine belt. You can replace a serpentine belt with ordinary hand tools, but I don’t recommend it. Space around belt-driven accessories can be tight and difficult to reach. Using a serpentine belt tool to loosen the tensioner and a belt placement tool allows you to cleanly remove and properly place the new belt onto the pulleys.

With these two tools, you should be able to complete the entire job in about 30 minutes without breaking a sweat. Look for serpentine belt tools that come with an assortment of sockets, extension bars and extra-long handles that you can configure to reach the tensioner at the proper angle. These tools provide maximum leverage, so you can easily release tensioner pressure, making them the perfect tools for all the cars in your family. Unless you’re a tool nerd like me, who buys tools even though, in all probability, I will use them only once, most major auto parts stores will lend you these tools free of charge.

Check the tensioner

Tensioners maintain proper pressure (tension) on the belt to effectively drive all of the accessories and absorb (dampen) impulses from the engine when each cylinder fires. Before removing the belt:

• With the engine running, visually inspect that the tensioner is not vibrating and the belt is running smoothly with no visible vibrations.
• If the tensioner arm exhibits a jerky motion, the belt vibrates, makes noise, or the tensioning arm moves 1/4 in. or more, the tensioner is bad.

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Check and inspect pulleys and idlers

Often overlooked even by pros, with the engine off, check the built-in tensioner belt wear indicator marks—small arrows or marks on the tensioner that move as the belt wears and the tensioner self-adjusts, maintaining proper belt tension. Then use the serpentine belt tool to loosen, but do not remove the belt and:

• Check that the tensioner swings freely and doesn’t squeak, bind, click or rattle from a weak, rusted or failed tensioner spring.
• Spin all drive accessories and serpentine belt idler pulley and pulley roller bearing assembly components to ensure they rotate smoothly, are not binding or noisy, and do not rock from side to side.
• Inspect the idler pulley and pulley roller bearing assembly surface for scoring, glazing, grooves, overheating or rust, and the crankshaft harmonic balancer for stone or wear damage.
• Check for oil, antifreeze or power steering fluid leaking onto the belt, which will quickly ruin a new belt. Repair any leaks and thoroughly clean all pulleys, grooves and idler surfaces before installing a new belt.

In my experience, replacing the belt tensioner and idler pulley and pulley roller bearing assemblies is wise when installing a new serpentine belt. Frank Woodson, from Gates North America, also “recommends belts, tensioner and idler pulleys be replaced at 90,000-mile intervals.” He added that premature belt failures are due to “worn tensioners, damaged—or out-of-alignment [and the reason for the laser level] accessory pulleys. Meier added, “excess heat also means premature—and potentially costly—failure of these components.” Joe Simes, ASE and Toyota Master Technician suggests using “a thermal imager to see if the tensioner or idler pulley bearings or driven accessories are overheating.”

Many tensioners and idler pulleys are readily accessible and attach to the engine with a single bolt. After removing the serpentine belt, remove the mounting bolt. Pull off the old components, noting the location of the locking pin on the back of the tensioner. Then, slide the new unit into place, ensuring the locking pin sits squarely in the hole in the engine. Hand-tighten all fasteners and then tighten them with a torque wrench to the factory specifications shown in your shop manual.

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Time to replace the serpentine belt

Put on your eye protection and gloves. The right tools make it a do-it-yourself project. First, locate the belt routing diagram, which is usually found under the hood. Examine the label that shows the belt routing. If your vehicle doesn’t have one or cannot read it, draw the belt’s path on paper before you start the job. You can also take digital photos of the belt, including where it goes over and/or under pulleys and accessories. Also, the belt routing diagram may be found in the owner’s manual.

FAMILY HANDYMAN

Remove the belt with the serpentine belt tool

Rotate the tensioner to release tension from the belt. Slide the belt off any smooth pulley, remove the old belt with the serpentine belt placement tool, and slowly release the tensioner. Thoroughly clean gunk, rust, and old belt fragments from accessory pulley grooves and idlers before installing the new belt.

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Place the new belt

Using the serpentine belt placement tool, route the new belt following the belt routing diagram path. Rotate the tensioner again while you load the belt around it. Wrap the new belt around the crankshaft pulley, then around the grooved pulleys. Finish the job by sliding the belt onto a rounded, non-grooved roller. Double-check to ensure the belt is sitting squarely in all the pulleys and follows the correct path. Slowly release the tensioner. Check that the tensioner belt wear indicator marks are in the correct position. This ensures you have installed the proper length belt. Use a straight edge or laser alignment tool to check that the pulley alignment is parallel. If the pulleys are misaligned, reposition them until the laser beam is centered.

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Check belt operation

Start the engine and check the belt for vibrations or noises to ensure that it is running smoothly and in the center of the idlers, rollers, and tensioner. Shut the engine off and let it cool down, then check that the serpentine belt is taut. See FAQs below.

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FAQ

How long do serpentine belts generally last?

According to Woodson, depending on driving conditions (highway driving versus stop-and-go city driving), the quality of the belt (Ethylene Propylene Diene Monomer—EPDM) belts can last between 50,000 and 100,000 miles. Older (neoprene) engine drive/accessory belts typically last about 40,000 miles. Unlike EPDM belts, neoprene is vulnerable to heat and cold and environmental contamination, causing them to wear and age faster. Simes added, “You should inspect your belts every three months or every oil change.” Like tires, serpentine belt wear slowly and may not show signs of wear.

When should I call a professional?

If you need to remove an engine mount or other components to remove the serpentine belt, or if the belt is nearly impossible to reach, it’s time to see the pros. Depending on where you live, who does the repair (independent repair shop, auto repair chain or dealership), and your vehicle’s make, model, year and engine, expect to pay your mechanic between $50 and $250 in labor, plus the cost of the belt, to replace a serpentine belt.

How tight should a serpentine belt be?

A serpentine belt should have about a 1/4-inch to less than 1/2-inch deflection when pressed with your thumb along its longest run between pulleys. Too loose, and the belt will slip and squeal. Too tight, it can overheat and damage drive accessory bearings.

The Experts

• Peter F. Meier, Dorman Training Center Training Manager, has worked as a technician, service manager and automotive repair educator for fifty years. He has held ASE Master Technician status, taught in vocational schools, trained technicians at regional and national events and served as the technical editor for Motor Age magazine.
• Frank Woodson is the Gates Corporation Training Manager for the N. America Aftermarket
• Joe Simes is an ASE and Toyota Master Technician, PA certified emissions and safety inspection instructor and inspector and a Pennsylvania Department of Education certified automotive technology instructor at North Montco Technical Career Center in Lansdale, PA.

Resources

• ASE Education Foundation, Webinar, “Better Belt System Diagnostics (Dec 3, 2024)
• Car Talk, How Much Does It Cost to Replace a Serpentine Belt? (Jul 11, 2024)
• Dayco, “How to Tell when an Idler or Tensioner Pulley Needs to be Replaced, Top Eight Pulley Warning Signs” (Dec 11, 2024)
• Dorman Shop Press, “How do you inspect a serpentine belt for wear?” (Mar 28, 2024)
• Gates Corporation, Technical Service Bulletin; Bulletin TSB_052_US, Accessory Belt Drive System, Belt Inspection Changes (2020)
• Gates, Tech Tip, Bulletin TT002-15, Accessory Belt Drive System, Belt Inspection Changes (2020)
• Mighty Parts of Eastern Pennsylvania, Phone interview (Dec 13, 2024)