Auxiliary belts are used to drive the air-conditioning compressor, power steering pump, water pump and the alternator, and are connected to those components from the crankshaft pulley.
In more modern cars one or two belts - known as the serpentine belt because of the way they twist around multiple pulleys - do all of these jobs.
The auxiliary drive belt should be checked every once in a while to make sure the tension between pulleys is correct. Pick a point on the belt midway between pulleys and push on it with a finger. However, most engines are fitted with an automatic tensioner, which applies constant tension to the belt. The tensioner automatically compensates as the belt stretches with time/use.
Using a ruler, if the belt flexes by more than an inch it may need to be tightened, or the tensioner may be faulty.
Also check the condition of the auxiliary belts on a regular basis. The Haynes website has free step by step instructions on how to do this. With the engine stopped, inspect the full length of the belts for cracks and separation of the belt plies.
Twist the aux drive belt between the pulleys so that both sides can be viewed. Also check for fraying and glazing, which gives the belt a shiny appearance.
Check the pulleys for nicks, cracks, distortion, and corrosion; small cracks in the underside of a V-ribbed belt are acceptable - lengthwise cracks, or missing pieces that cause the belt to make noise, are cause for replacement.
What's that squealing noise under the bonnet?
Occasionally you’ll hear a squealing or screeching noise coming from under the bonnet, especially when you start the engine from cold and when pulling away.
Sometimes the squealing will be accompanied by the alternator light illuminating on the dashboard, indicating that the battery isn’t charging.
This is likely to be caused by a slipping auxiliary belt, either because it has oil on it or it is damp and there’s not enough tension, so the belt needs to be tightened or replaced.