When should a drive belt be replaced in your car?

Updated: March 17, 2020
Your engine has a number of mechanical accessories attached to it, such as an alternator, water pump, power steering pump and an air conditioner compressor. A drive belt is the part that drives all these accessories. The belt itself is driven by a pulley attached to the engine crankshaft. Most cars have one or two drive belts. When a car has only one drive belt, it might be called a serpentine belt.
Drive BeltEngine with one 'serpentine' belt. Fig1, Fig2.
Drive Belts The Mazda has two drive belts. Fig3.
A drive belt is made of long-lasting rubber-based material. In most cars, a drive belt is replaced between 40,000 and 70,000 miles, however, we have seen cases where the drive belt lasted over 100k miles.
What makes the belt wear sooner? Water, road dirt, excessive heat and oil leaks can cause the belt to deteriorate prematurely. For example, if the splash shield underneath the engine (engine undercover) is broken, the belt will wear out sooner. Oil leaks can also damage the drive belt. Eventually, a worn drive belt will break.
Broken drive belt Broken drive belt.
If a drive belt breaks, you won't be able to drive your car. This is because the engine will overheat if the water pump is not running. Also, if your car has hydraulic power steering, you will lose the steering assist, as the power steering pump will be disabled. In addition, the alternator will stop supplying electric power that is needed to recharge the car battery and run the vehicle electric systems.
 Worn out versus new drive belt Old vs new drive belt. See larger photo.
When should the drive belt be replaced? Most car manufacturers don't specify the exact mileage, but recommend having the belt inspected during your regular maintenance visits.
How can you tell if the drive belt needs a replacement? Your mechanic will look for cuts, cracks, missing chunks and other signs of wear; usually, it's fairly easy to see if the belt is worn out. For example, the old belt in the photo above shows cracks.
Worn out drive beltDrive belt showing cracks and missing chunks. Fig 2.
This photo shows the belt with missing chunks of the tread material on the ribbed side. The belt may also need to be replaced if it makes chirping or squeaking noises or if it's stretched. One of the early signs of a worn out drive belt is when it starts squeaking in rainy weather.
Another common reason to replace a drive belt is when it's soaked in oil. Oil damages the rubber material, like in this photo.
Drive belt damaged by leaking oil Drive belt damaged by leaking oil.
If your mechanic found that a drive belt is saturated in oil, he or she may recommend repairing the oil leak first. This is because if you install a new belt and the oil leaks onto it, the belt will not last long.
To work properly, a drive belt must be under proper tension. In older cars, the belt tension had to be adjusted manually in regular intervals. One of the symptoms of a loose drive belt is a loud screeching noise that lasts for a few seconds after the engine was started.
Modern cars have an automatic spring-loaded or hydraulic belt tensioner that doesn't need to be adjusted.
Drive belt tensioner Automatic spring-loaded drive belt tensioner. Fig1, Fig2.
If the automatic spring-loaded belt tensioner is sticking or shows signs of wear, it makes sense to replace it together with the drive belt; it takes little extra labor. A hydraulic belt tensioner needs to be replaced if it's leaking oil.
Replacing the serpentine belt may cost from $75 to $250, depending on the car. Replacing the belt tensioner together with the belt will add $45-$155 to the total repair bill.
Is it easy to replace a serpentine belt as a DIY project? If you have sufficient mechanical skills, proper tools, belt routing diagram and follow the service manual instructions, it's not very difficult. Your parts store may help you with the routing diagram and tools.
If a car has two drive belts, do both have to be replaced at the same time? No, it's not necessary. However, in most cases there is little extra labor involved in replacing the second belt. If you only have to pay $20-$40 extra for the second belt, why not get them both done at the same time?