Battery light on, gauge climbing or heavy steering? Pull over
A loud slap followed by silence can mean the belt has broken. If the battery light comes on, the temperature gauge climbs or the steering turns heavy, pull over somewhere safe and switch off. On many engines that one belt runs several parts at once, including the water pump, so the engine can overheat quickly. Never open a hot radiator cap. If you're a roadside assistance member, now's the time to ring them. Anyone hurt or in danger: call 000.
What's the belt doing, and why does it squeal?
Most modern engines use one long ribbed belt, called a serpentine belt, turned by the crankshaft. It can run the alternator, power steering pump, water pump and air conditioning compressor all at once. Older engines use several thinner V-belts for the same jobs.
It squeals when its grip isn't enough for the load. That can be a worn or glazed belt, a tensioner (the spring-loaded arm that keeps the belt tight) that's gone weak, a pulley that's dragging, or something wet on the rubber.
When does it squeal?
| What's likely | How soon to act | |
|---|---|---|
| Cold start, gone within a minute | A worn belt, or a tensioner losing its spring | At your next service, or sooner if it's lasting longer |
| In the rain or after a car wash | Water on the belt, reducing its grip | No rush, but mention it. A healthy belt usually copes |
| With the air conditioning on, or the steering at full lock | A bigger load than the belt can grip for | In the next few weeks |
| Constant, and rising with the revs | A worn belt, or a pulley bearing starting to drag | This week. A seized bearing can take the belt with it |
| A chirp more than a squeal | A pulley slightly out of line | Soon, because it wears the belt's edges |
| A wet belt with oil or coolant nearby | A leak reaching the belt | Now. The leak needs fixing as well as the belt |
Is it definitely the belt?
Go by where it's coming from. Under the bonnet points to the belts. From the wheels as you brake, NRMA says it's more likely worn pads or another brake problem, so see squealing brakes. A steady whine that rises with the revs can be the power steering pump, which is on RAC WA's list of parts that whine.
It isn't the timing belt, either. That one sits behind a cover, and RAA says only a mechanic can check and replace it, on the maker's schedule. If you're not sure when yours was done, our timing belt page explains what to ask. Some engines use a chain instead: see timing belt or timing chain.
What will the mechanic look at?
With the engine off, they'll check the belt for cracks, shiny glazing, missing chunks and frayed edges. A belt can lose several ribs and still run, so one that seems fine from above may not be.
Then they'll test what the belt runs around. They'll check that the tensioner moves and holds, take the belt off and spin each pulley by hand to feel for a rough bearing, and look for oil or coolant that's reached the rubber.
That's why a new belt doesn't always end the noise. If a tensioner or pulley is the cause, the squeal comes back, and a seized part such as a power steering pump can stop the whole belt drive. RAA says a mechanic should check the drive belt at every service, so ask about it at yours.
What a good estimate covers
- The belt itself, and what state the old one was in.
- Whether the tensioner and any idler pulleys (the plain guide wheels) need replacing.
- Which pulley or bearing is the real cause, if the belt isn't.
- Any oil or coolant leak that has reached the belt.
- Whether the alternator or water pump bearing is part of it.
What you can do, and what to leave
You can
- Note when it squeals: cold, wet, air conditioning on or at full lock.
- With the engine off and cool, look at the belt for cracks or fraying.
- Record the noise on your phone.
- Watch the battery light and temperature gauge on the way to the mechanic.
Leave to the mechanic
- Anything near the belt while the engine is running.
- Testing the tensioner and pulleys.
- Fitting a new belt at the right tension and on the right path.
- Fixing a leak that's reaching the belt.
What we'd do about it
A short squeal on cold mornings: we'd mention it at the next service, or sooner if it's lasting longer. A squeal that's constant, loud or getting worse: we'd send the job this week and not wait for it to let go somewhere inconvenient. If the battery light or the temperature gauge joins in, we'd stop. It's usually one of the smaller jobs on a car, so it's an easy one to get ahead of.