The serpentine belt is wound on the belt tensioner pulley. A spring-loaded arm on the tensioner applies pressure to the belt. The whole point of having a belt tensioner on your engine is to keep the serpentine belt tight enough. If the belt is too loose, it will not be able to provide sufficient power to any accessories, including the A/C compressor, alternator, power steering, etc.
Belt tensioners last longer than serpentine belts. However, they don't last forever. Like all moving parts, they wear out. They usually fail in one of two ways:
The tensioner pulley bearing failed, causing the pulley to start wobbling. This makes the belt wobble side to side, causing fraying of the edges. Bearings also sometimes start to squeal. The tension might be ok, so the accessory will keep power, but the belt will fail sooner than it should.
The tensioner spring weakened. This allows the belt to slide over the attachment pulley so the attachment does not get full power. This could mean:
1) The alternator stops fully charging the battery
2) The power steering pump doesn't always have enough power, making the steering wheel difficult to turn at times
3) The water pump stops working, causing the engine to heat up or overheat
Once the tensioner is broken, the wear of the belt will accelerate. If not replaced, sometimes the belt will fail suddenly and the engine will not run. Therefore, if you find a problem with your belt tensioner, it's best to replace it as soon as possible.
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