The physical difference between an idler pulley and a tensioner pulley is a simple adjustable bolt. Tensioner pulleys are mounted on this bolt, and idler pulleys are not. They also differ in purpose. Idler pulleys spin in order to move belts to different places. Tensioner pulleys, which are spring-loaded, provide pressure to a belt that drives other pulleys while simultaneously easing the strain on that belt.
Idler and tensioner pulleys need to be replaced when bearings fail. The sound of squealing or grinding or the appearance of wobbling indicates the need for replacement. When a tensioner pulley is failing, belts may loosen, so immediate replacement is key. Confirm malfunctioning pulleys by checking their bearings. Remove the belts attached to the pulleys, and spin them. If the pulleys do not move freely and without wobbling or hesitation, they need to be replaced.
Check the alignment of the pulleys as well. Improperly aligned pulleys indicate problems mounting the belts and often leave abrasions on the belts themselves. Check the alignment by holding a straight-edged ruler across the pulleys or using a laser alignment tool.
To change a broken pulley, first remove the attached belt. While holding the pulley, remove the center bolt using a 13-millimeter socket wrench. Replace the pulley, and retighten the bolt. The torque should be 18 feet per pound.