The auxiliary (serpentine) belt drives the alternator, power steering pump, and air conditioning compressor.
When it slips, it squeals.
Common causes
- ▸Worn or glazed belt
- ▸Low belt tension
- ▸Worn tensioner or pulley bearings
- ▸Oil or coolant contaminating the belt
- ▸Damp weather
Why it matters
If the belt breaks, you may lose charging, power steering, and air conditioning. On some engines it also drives the water pump, risking overheating.
Inspection
Look for cracks, fraying, or shiny surfaces on the belt. A mechanic can check tension and pulleys.
Do not confuse with the timing belt
The timing belt is hidden behind covers and usually makes no noise before failing. See our timing belt guide.
Replacement intervals
Auxiliary belts are wear items. Many manufacturers recommend inspection at each service and replacement at set mileages, often alongside the tensioner. Replacing a belt is usually far cheaper than dealing with the consequences of one snapping, such as losing power steering at speed. If the belt has been contaminated by an oil or coolant leak, fix the leak first or the new belt will slip too.
CAUTION: This is general automotive information, not professional mechanical advice. Always follow your vehicle manufacturer’s specifications and service schedule, and have brakes, steering, tyres, and other safety-critical systems inspected by a qualified mechanic.