What are the idler and tensioner pulleys and where are they located? 

The idler and tensioner pulleys help keep your engine’s drive belt or belts on track.  Your engine has many accessories that are driven by the crankshaft.  These include the alternator, water pump, and power steering pump, among others.  In older cars, these would be connected in small groups by a number of different belts.  Each of these belts would be set up in a more or less triangular shape, leading many to call them V-belts.  Today, engines tend to use one belt that snakes around all the accessories.  This is called a serpentine belt. 

Both kinds of belts sometimes need pulleys to help keep them in line.  A belt that spans a long distance without support tends to vibrate and flap around.  Tensioner pulleys and idler pulleys keep the belt taut and running smoothly.  The accessory pulleys have to spin a certain direction to power their respective parts.  This determines whether they “ride” on the inside or outside of the belt.  Since idler pulleys and tensioner pulleys spin freely, they can ride either on the inside or the outside of the belt depending on their position.  Pulleys that ride on the inside, or grooved part of the belt, have a grooved surface; pulleys that rid on the outside, or smooth part of the belt, have a smooth surface. 

The tensioner pulley is, of course, part of the tensioner.  The tensioner has a spring loaded arm that pushes the pulley into the belt.  The belt will loosen over time as it begins to wear.  The tensioner pushes against it to tighten it up.  Idler pulleys simply help keep the belt stretched taut around its path.  Some very long belts might require more than one idler pulley. 

How do I know if my ­­­­idler and tensioner pulleys need to be replaced? 

The constant spinning of idler and tensioner pulleys wears on their central bearings over time.  Eventually, the pulley can become loose.  When that happens, the belt will move more loosely over the pulleys.  Corrosion or dirt in the bearing can also make the pulley move less freely, meaning the belt may slip and jump across that pulley.  If your belt is slipping or squealing, the problem might be with the belt or pulleys.  Listen for a squealing sound when you start the engine. 

You can check the pulleys by opening the hood and watching the belt with the engine idling.  If you see any wobbling in the belt, then the idler and tensioner pulleys aren’t doing their job.  Pulleys usually last longer than belts, which require regular changes.  Still, it’s a good idea to inspect the pulleys any time you have to replace a belt.  Try spinning the pulleys by hand.  If they feel loose or wobble, or, conversely, they are hard to spin, they will need to be replaced. 

Can I replace the idler and tensioner pulleys myself?  

Replacing an idler or tensioner pulley is pretty easy.  Depending on the model of your car, it may be easier to access the pulleys under the hood or from underneath the vehicle.  If you need to reach the pulleys from underneath, lift and support the vehicle and remove the splash shield. 

You’ll have to take the belt or belts off before you can access the pulleys.  Before doing anything with the belt, you’ll want to make sure you know how it’s routed so you can put it back on the exact same way.  There should be a belt-routing diagram under your hood.  If you can’t find the diagram, draw one yourself, or take photos.  Loosen the tensioner to slacken the belt and pull the belt off.  Then remove the central bolt from the pulley in question and pull it right off.  To put the new one on, it helps to thread the belt through the pulley by hand before you put it into place.  Then you can tighten the bolt, and put the belt back on, following the diagram.  Replace the splash shield and lower the vehicle on the ground if need be.  Check that the belt is routed correctly, and then run the engine at idle to make sure the belt and pulleys are working correctly.