2015 Mercedes-Benz ML400 Drive Belt Tensioner Replacement Cost

As drive belts are installed the belt tensioner is tightened to manufacturer's specifications, placing the proper amount of tension on the belt. It is bolted to the front of the engine, and the flat side of the drive or serpentine belt rests tightly against the belt tensioner pulley. When the belt is driven, the belt tensioner pulley constantly rotates, keeping tension on the belt.

If the drive belt tensioner fails, there will be a very audible chirping, grinding, or rattling noise from the front of the engine. This is because the tensioner pulley bearing is no longer smooth, and rotating causes metal to grind at high speeds. If it is not changed immediately, there is a chance it will become dislodged, or allow the drive belt to lose tension, or break. If the belt falls off, the accessory that it drives will no longer function. Also, if the tensioner spring fails, allowing slack in the belt, it may make an extremely loud squeaking or screeching noise combined with a rattle as the tensioner bounces back and forth under load. The squealing noise is the belt slipping on the crankshaft pulley, and will require belt replacement once the belt tensioner is replaced.

If the belt tensioner is totally destroyed, driving may be hazardous, or impossible. If the tensioner has just begun to chirp or rattle, the vehicle can usually be driven to a repair facility. Keep in mind that once the tensioner fails, the vehicle may only last a few short minutes before the engine stalls due to loss of electrical power from the alternator.

Drive belt tensioner pulleys are only replaced due to suspected failure. They are not considered a wear and tear item, and are generally intended to last the life of the vehicle. However, the majority of vehicles will see a drive belt tensioner replaced before the 125,000 mile mark has been reached, with some replacements occurring as early as 50,000 miles.

Diagnosis of a failed or failing tensioner requires removal and inspection. This hands-on inspection will allow the servicing technician to check the pulley by hand for signs of grinding, looseness, chirping, and rattling. Upon inspection of the belt tensioner, a well-trained technician will notice these flaws, and may recommend immediate replacement of the tensioner. If the engine is running and the technician notes that the tensioner is bouncing back and forth due to a failed spring, they will recommend replacement.

Removal of the drive belt by releasing the tensioner is the first step. On spring tensioners, removal may require the tensioner be pinned in place to prevent tension from returning as soon as it is released. The tensioner is then removed, and replacement is the reverse order. The tensioner retaining fasteners must be tightened to manufacturer's specifications to prevent damage to the mounting bracket or timing cover, and non-spring tensioners will be manually adjusted. Spring tensioners only need to be released, and they will adjust automatically as the engine is running.

Since this service requires removal of the serpentine belt, it may be recommended to replace the belt unless recently replaced. This is much more cost effective than returning for the same service after a partial service interval has elapsed. Frayed or cracked belts should be replaced without regard to current age or mileage, as they are near the end of their reliable service life.

Replacement may require the removal of other engine accessories, or even a motor mount. This is because these vehicles will have reduced working room, or the retaining bolts may be located behind other components.

The novice DIYer would be perfectly comfortable replacing the belt tensioner and pulley, but a torque wrench with small enough increments must be used to avoid damaging the engine accessory mounting bracket, or timing belt cover, whichever the particular vehicle uses. If a torque wrench is unavailable, do not attempt this repair, as the damage caused by over-tightening, or under-tightening can be very costly.