The engine’s fan belt supplies power to the car’s air conditioning, compressor, power steering, and alternator, among other accessories. Here are simple directions for performing the most common fan belt maintenance.

Testing fan belt tension by measuring deflection

Proper tension is required to keep the fan belt from slipping, binding or breaking. Belt tension should be checked regularly and after installing a new belt to make sure it falls within the manufacturer’s guidelines.  (When you install a new belt, check it again after a week to make sure it hasn’t lost tension.)

Here’s how to test the belt tension by measuring the deflection.

• Wherever it’s easiest to reach, lay a straight edge flush along the fan belt.
• Press down firmly on the belt with your finger or the end of a ruler, and measure the distance between the belt and the straight edge.
• Compare this measurement with recommended deflection measurements in the owner’s manual or with your dealer’s or mechanic’s guidelines. Usually it’s between 8 and 12 mm.

How to troubleshoot noise from the fan belt

Failing or loose fan belts can be mistaken for a failing alternator or a knocking compressor. Here’s what to look for in your fan belt if you suspect that noise is coming from that direction.

• Fraying or cracks on the belt. Loose fabric touching other parts of the engine can cause noise, as can small pieces breaking off into the engine. Be sure to replace the belt if you see this type of damage.
• Wet fan belt. A wet belt that doesn’t sit flat against the pulley will make noise.
• Bad bearing in the pulley used for the serpentine belt. To determine if this is the problem, put chalk or whiteout around the edge of the pulley and turn the engine on. If the noise occurs at the same time the chalk appears on the pulley — that is, when it rotates — it’s likely that the pulley has a bad bearing.

If nothing is immediately visible, ask your mechanic to check to see if the belt tensioner is causing a loose belt.

How to determine the proper fan belt size

If you need to replace a fan belt but can’t find the specific dimensions in the manufacturer’s manual, you can measure it yourself.

• For the length: wrap a piece of string around the fan belt gears, pull tight, and mark off the place when the two ends meet with a pen. Measure that length with a tape measure.  (Another option is to mark the route of the belt around the pulleys with duct tape, and lay rope on top of the tape to measure the path.)
• For the width: this is the same as the width of the groove that goes around the gears.

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