What Is A Belt Tensioner And Why Is It Important?

Every engine has a belt tensioner. In this post, we’ll talk about what a belt tensioner is and why it’s important.

What Is A Belt Tensioner?

If you follow the path of the serpentine belt on your engine, you’ll find it passes by several accessory pulleys. It also passes by a small pulley on the belt tensioner. A belt tensioner is a small component with four main parts:

  1. Base: A metal part that holds all the other parts
  2. Tensioner arm: Located at the bottom of the tensioner; applies pressure to the belt, and allows enough slack to help you adjust or remove the belt
  3. Spring: Provides pressure to keep the tensioner tight
  4. Pulley: Rotating part that allows the belt to move

If you need visual aid, here’s an illustration of a belt tensioner for select 2012-2020 Mazda models:

Mazda belt tensioner

For the most part, all belt tensioners look the same. Some are bigger, and others are smaller. Yet, the design more or less remains the same.

The belt tensioner is always on the serpentine belt's path. Its pulley is smaller than the other pulleys on that path, so it's easy to identify. If you look at a Mazda CX7 engine, you'll find the belt tensioner on the right side.

Why Are Belt Tensioners Important?

The serpentine belt wraps around the belt tensioner pulley. The spring loaded arm on the tensioner applies pressure to the belt. The whole point of having a belt tensioner on your engine is to keep the serpentine belt tight enough. If the belt is too loose, it won't fully power any of the accessories, including:

  • AC compressor
  • Alternator
  • Power steering
  • Etc.

The belt tensioner also keeps the serpentine belt correctly seated in all the other pulleys in the path.

What Happens When A Belt Tensioner Fails

Mazda tensioner

Image Credit: YouCanic

Belt tensioners last longer than serpentine belts. Yet, they don't last forever. Like all moving parts, they can wear out. They usually fail in one of two ways:

  • The tensioner pulley bearing fails, which allows the pulley to start wobbling. This allows the belt to wobble side to side, causing edge fraying. The bearing also will sometimes begin to make a squealing sound. Tension may still be ok, so accessories stay powered, but the belt will fail earlier than it should.
  • The tensioner spring gets weak. This allows the belt to slip over the accessory pulleys, so the accessories don't get full power. This can mean that:
    • The alternator stops fully charging the battery
    • The power steering pump doesn't always have enough power, making the steering wheel hard to turn at times
    • The water pump stops working, which causes the engine to run hot or overheat

Once the tensioner goes bad, the wear on the belt will accelerate. If not replaced, sometimes the belt will suddenly fail, and the engine won't be able to run. So if you notice issues with your belt tensioner, it's best to replace it ASAP. They are really easy to install. First, you need to get a replacement belt tensioner. You'll save some money by ordering a genuine OEM belt tensioner from us. We offer wholesale pricing and fast shipping. Feel free to contact us if you need help finding the right part number for your Mazda model.