One belt drives your alternator, water pump, power steering and air conditioning. When it snaps you lose all of them at once, usually miles from anywhere.
- Time: 30–60 minutes
- Difficulty: intermediate — simple in principle, awkward in a tight engine bay
- Shop: $90–$250. Part: $20–$45. Saves: $70–$205
When it needs doing
- A squeal on a cold start that fades as things warm up. The classic first sign.
- Cracks across the ribs. Bend a section back on itself with the engine off — a few fine cracks are normal on an older belt, but several deep ones across the width mean replace.
- Glazed, shiny ribs, missing chunks, or fraying at the edges.
- Age. Most manufacturers say 60,000 to 100,000 miles. Check the manual.
This is a cheap part that strands you when it fails, and on many engines the water pump is belt-driven, so a snapped belt also means the engine overheats within minutes. It is worth replacing on age rather than waiting.
A serpentine belt is not a timing belt. The serpentine belt is on the outside of the engine, visible as soon as you open the hood, and drives the accessories. A timing belt is behind a cover, keeps the internals synchronised, and is a major job — $400 to $1,000 — that is not a DIY project. If someone quotes you a timing belt, that is a different and far more serious item.
What you need
- The correct belt for your exact vehicle — length matters to the millimetre
- A long ratchet or breaker bar, usually 3/8″ drive, and a socket to fit the tensioner — often 15 mm or 3/8″ square
- A flashlight, and a phone for photographs
- Optional but excellent: a serpentine belt tool, about $25, which is a long handle designed for exactly this
The job
- Engine off, cold, key out. Nothing spins up while you are in there, but a hot engine will burn your forearms.
- Find the routing diagram. There is usually a sticker under the hood or on the radiator support showing exactly how the belt snakes around the pulleys. Photograph the belt as it sits, from two angles, before you touch it. If there is no sticker and no photo, getting it back on correctly is guesswork.
- Find the tensioner. A spring-loaded pulley on an arm, with a square hole or a hex boss on it for a wrench. It is the only pulley that moves.
- Put the ratchet on the tensioner and lever it in the direction that releases belt tension — usually clockwise, but it varies. It takes real force. The belt goes slack.
- Slip the belt off the easiest pulley — usually a smooth idler — while holding the tensioner. Then let the tensioner back gently. Do not let it snap.
- Work the old belt out. Sometimes it has to come past the fan or a mount. Note how.
- Compare old and new side by side — same length, same number of ribs.
- Spin each pulley by hand. They should turn smoothly and silently. A rough or noisy idler or tensioner pulley is best replaced now while the belt is off, and they are cheap.
- Route the new belt following your photo, leaving the tensioner pulley until last. The ribbed side goes on the grooved pulleys; the flat back runs on the smooth ones.
- Lever the tensioner again and slip the last loop on. Release slowly.
- Check every pulley. The belt must be fully seated in the grooves everywhere — a belt riding one rib off will shred itself in seconds. Go round twice with the flashlight.
- Start it and listen. A brief chirp as it settles is fine. Continuous squealing or a belt that wanders means it is not seated or not routed correctly — shut it off immediately and look again.
If it still squeals afterwards
A new belt should be quiet. If it is not, the usual causes are a weak tensioner that is no longer holding proper tension, a misaligned or seizing pulley, or oil or coolant dripping onto the belt from a leak somewhere. Do not use belt dressing spray. It quietens the noise for a week and gums up the pulleys.
The mistakes that catch people
- No photograph of the routing. By far the most common way this job goes wrong.
- A belt riding one rib off a pulley. Check every single one before starting the engine.
- Letting the tensioner snap back. It can damage the tensioner or your hand.
- Ignoring a rough idler pulley while the belt is conveniently off.
Information only, not professional advice, and no substitute for your vehicle’s service manual. Work on your own vehicle is at your own risk. See the disclaimer.