WHAT DOES AN IDLER DO?
                     WHAT DOES AN IDLER DO?
Paul Norman Jasper
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An idler pulley is part of the accessory belt routing for your vehicle’s engine. 
Since engines don’t always have accessories in exactly the same place, the 
idler pulley allows you to route the belt so that it aligns with all of the 
accessory pulleys. They rarely help tension the belt – manufacturers use a 
tensioner pulley for that purpose. Some engines may not require an idler 
pulley if the belt routing lines up with all of the accessories.
THE IDLER PULLEY DESIGN
The idler pulley is a simple pulley that has a wheel with a wide groove in it. It 
keeps the belt aligned with the tensioner and the accessory pulleys. It’s a 
simple design, but without it – because of the location of the accessories on 
the engine – the serpentine belt might not align properly.
WEAR AND TEAR
The idler pulley is a maintenance item, which means that it does wear out 
over time. It contains bearings that could wear out and squeak or grind. The 
bearings could eventually freeze up. In some cases, the wheel could also 
suffer damage from a belt that is misaligned. If the idler pulley is made from 
plastic, the surface of the pulley could also wear out.
WHY AN IDLER PULLEY IS DIFFERENT FROM A DRIVE PULLEY
The idler pulley is aptly named because it does just that: idles. Unlike drive 
pulleys, which are driven by a motor and a shaft, the idler pulley doesn’t turn 
on its own power. The crankshaft pulley, alternator pulley, power steering 
pump pulley, and the air compressor pulleys are all drive pulleys. Although, 
the crankshaft pulley is the only one that turns the belt. The belt turns the 
motors in the accessories.
Thus, if the belt is slipping, the alternator won’t charge the battery, the air 
compressor won’t compress and move refrigerant through the system, and 
the power steering pump won’t pump power steering fluid, which then 
causes the steering to be stiff.
The idler pulley’s function is to make sure the belt stays aligned so that the 
belt stays on the crankshaft and accessory pulleys. The idler also changes the 
direction of the belt when the belt cannot be routed to all of the accessories.
REPLACING AN IDLER PULLEY
When you need to replace an idler pulley, you’ll need a few tools, including a 
tensioner pulley wrench. In most cases, replacing the idler pulley is a simple 
job as long as you can easily access the idler. In some cases, the idler is in a 
tight place, and you might have to remove other parts to get to it.
You must loosen the tension on the belt to replace the idler pulley. Locate the 
tensioner and rotate it toward the center of the engine. Remove the belt 
from the idler pulley. You do not have to remove it from the other pulleys 
unless you need to replace the belt or it’s in the way.
Unbolt the idler from the block. Check the new pulley to make sure it’s not 
defective. Simply spin the pulley. It should spin smoothly and should not 
squeak or grind. Bolt the new idler to the block. While the belt is loose, check 
the tensioner pulley. It’s better to replace it at the same time if it is bad.
Rotate the tensioner toward the center of the engine and slide the belt on 
the idler pulley. Make sure the belt is properly aligned on all of the pulleys.
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