Both Tensioner Pulley and Idler Pulley should be replaced by 100k miles at the very latest. They have a tendency to come apart and in cases disintegrate completely.  This is always bad and in particular if you are far away from a source of OEM parts like Mexico can be very bad.  These pulleys are required to keep the serpentine belt aligned and tensioned properly. The belt runs the water pump, alternator, air conditioning, power steering and more.  These parts are cheap insurance against being completely stranded.  Unlike the original OEM Mercedes-Benz plastic or pressed sheet metal pulley, the all aluminum belt tensioner pulley is precision machined from aerospace aluminum, and features a serviceable ball bearing which is retained by a snap ring and can be replace if needed.

The aluminum belt tensioner pulley is a much more durable part, as embraced by the Baja 1000 racers as well as by the Overland Traveler.

Features:

• Stronger replacement than the original
• High quality and light weight aluminum
• Black anodized finish
• High quality bearing
• Serviceable / replacable bearing
• Utilizes existing bolt and tensioner body
• German Engineered, made in USA