2009 BMW 135i Serpentine Belt Replacement Cost

As the engine's crankshaft pulley turns it drives the serpentine belt. In turn, the belt drives the pulleys for the other systems including the air conditioning, alternator, water pump, cooling fan, and power steering.

Over time serpentine belts stretch, crack, and the ribs that ride on the accessory pulleys being to wear smooth. A squealing noise that increases with engine speed or as engine driven accessories are turned on may be noted. The noise is caused by the belt slipping on the pulleys, insufficiently driving them. Ineffective air conditioning, a battery warning light, or intermittent power steering assist can result from this. »MORE: Symptoms of a bad serpentine belt

A slipping belt means one or all of the accessories are not being driven properly. Loss of air conditioning, electrical power, power steering, or water pump and fan drive are possible depending on the severity of the wear. If left for too long the belt can shred or come off of the pulleys. The belt should be inspected as soon as is convenient to protect against becoming stranded.

Replacement intervals vary by vehicle and driving conditions. We typically see this part replaced approximately every 100,000 miles.

The technician will visually inspect the serpentine belt for wear or fluid contamination. The belts have grooves that locate them laterally on the accessory pulleys. These will be checked for cracking or chunking. On vehicles equipped with automatic belt tensioners there is usually a gauge on the tensioner that indicates how much the belt has stretched, this will also be checked.

The technician will loosen the tensioner to allow slack in the belt. After removing the old belt, the technician will route the new one over the accessories, then release the tensioner so that it puts tension on the belt. The vehicle will then be started to check that the belt has been properly routed.

High-quality serpentine belts are less likely to squeak and/or prematurely fail. The vehicle may have multiple tensioners and idler pulleys that the belts ride on. This would be a good time to inspect them for bearing wear and replace as needed. It is also important to inspect the accessory pulleys, especially in the case where a relatively new belt was shredded or thrown off. Worn or stuck pulleys can damage a new belt in short order.

Belts can be referred to as serpentine belts, accessory belts, drive belts, fan belts, alternator belts, water pump belts, or V-Belts depending on vehicle application. It is important to order the correct one for your specific vehicle. Belts can also differ by optional equipment, such as air conditioning. Such cases will provide a range in belt size. When driving through large puddles, it is possible for water to splash up into the engine bay and get onto the belts and pulleys. This will not damage them, but a squealing noise may be noted, along with the alternator or battery light illuminating due to the alternator belt slipping. Additionally, leaking coolant or oil can cause belt issues. Coolant will act as a lubricant and cause the belts to slip, while leaking oil will also damage the belt's rubber.

The vast majority of serpentine belts only require simple tools to replace. However, the procedures vary wildly from car to car. Before ordering parts or attempting this repair yourself, look online for guides on how to replace the belts in your specific vehicle. Vehicles with transversely mounted engines are often more difficult to service due to lack of space between the engine and frame. Special tools to release and lock the belt tensioner may be required to perform this procedure.