A sharp squeal when you start the engine on a cold, damp morning is a sound most drivers recognise. More often than not, it is the auxiliary drive belt. It is usually a simple fix, but it is worth understanding why it happens and why the belt and tensioner are best replaced together.
What the auxiliary belt does
The auxiliary belt, also called the drive belt, fan belt or serpentine belt, takes power from the crankshaft pulley to drive engine accessories. Depending on the car, these can include the alternator, air-conditioning compressor, power steering pump and sometimes the water pump. Most modern cars use a single ribbed belt that snakes around several pulleys.
It is a different part from the timing belt. The auxiliary belt is on the outside of the engine and is usually easy to see. If you'd like to know more about timing belts, read our guide on timing belts and chains.
Warning signs
- Squealing on start-up, when turning the steering or when the air conditioning switches on
- Chirping or ticking, often from a misaligned or worn pulley
- Visible wear: cracks across the ribs, fraying edges, glazing or missing chunks
- A battery warning light if the alternator is no longer being driven properly
- Heavy steering on cars with a belt-driven power steering pump
- Rising engine temperature where the belt drives the water pump
If the battery light comes on or the temperature rises, stop safely as soon as you can. A broken belt can quickly lead to a flat battery or overheating.
Why belts squeal
A squeal means the belt is slipping on a pulley. Common causes are:
- A worn, stretched or glazed belt
- A weak tensioner that no longer holds the belt tight enough
- Oil, coolant or power steering fluid contaminating the belt
- A worn pulley or a seized bearing on an idler or accessory
Spraying belt dressing might quieten things briefly, but it hides the cause and can make the belt slip more in the long run.
Why replace the belt and tensioner together
Most modern engines use an automatic tensioner. It has a spring and a damper that keep the belt at the right tension as the engine speeds up and slows down. Over time the spring weakens and the damper wears, and the pulley bearing can become rough or noisy.
Fitting a new belt to a tired tensioner often brings the squeal back within weeks, and a failing tensioner can throw the belt off altogether. Because the labour overlaps, replacing the belt, tensioner and any idler pulleys at the same time is usually the most cost-effective option. Many manufacturers offer these as a matched kit.
The alternator pulley
Many diesel and some petrol engines use a special alternator pulley with a built-in clutch or decoupler. It smooths out the pulses from the engine and protects the belt. When it fails, it can cause vibration, rattles and early belt wear. It is worth checking whenever you change the belt.
Make-specific points
- Volkswagen Group: belt routing and tensioner type can vary with and without air conditioning, and between engine codes.
- Ford: some engines use a stretch-fit belt with no adjustable tensioner, which needs a special fitting tool.
- Toyota hybrids: many hybrid engines have an electric air-conditioning compressor and water pump, so their belt layout is simpler or may not use a conventional auxiliary belt at all.
The manufacturer's diagrams show the belt routing and part numbers for your exact engine. Our guide to exploded parts diagrams explains how to read them.
How long does an auxiliary belt last?
There is no fixed rule. Some manufacturers give a replacement interval, while others ask for the belt to be inspected at each service and replaced when worn. Heat, oil leaks and short journeys all shorten its life. If your car is approaching high mileage and the belt has never been changed, it is worth having it checked at the next service rather than waiting for it to squeal.
Why genuine belts and tensioners matter
A genuine belt has the correct length, rib count and rubber compound for your engine's pulleys and loads. A genuine tensioner has the right spring rate and damping. Small differences in length or tension can cause slip, noise and early wear, so it pays to match the original specification. Check the part number on your VIN with our guide to using your VIN.
Browse our Volkswagen, Ford and Toyota parts, or search by part number for genuine belts, tensioners and pulleys.