How to Replace Your Car's Alternator Belt

A serpentine belt (aka alternator belt) sounds cool, right? All slithery and snaky. Well, it isn't called that for nothing. This long belt in your car's engine has vertical grooves running its length and it snakes around several pulleys, connecting them to the motion of the engine's crankshaft. As that turns, the belt causes the pulleys to turn, which powers all the engine accessories, including your car's alternator, air conditioning compressor, power steering pump and sometimes a water pump [source: AutoNation].

Older cars had several belts to run all these systems. They wore out faster, and the mere fact of just having more belts meant they took up more space. A single serpentine belt is lighter and makes for a more compact, efficient engine.

But it also means having to thread a new belt around all those pulleys when it needs replacing. With some preparation and care, it's not all that difficult to do.

Serpentine Belt Prep Work

There are only a few things you'll need to replace a serpentine belt: a new belt, a belt tensioner tool and maybe a socket wrench. These things should be easy to find at an auto parts store. Gloves and eye protection never hurt either.

Every car engine is different, even engines from the same auto manufacturer. That means the serpentine belt will snake around the pulleys in a different order too. Newer cars might have a plastic covering over the engine that will need to be removed in order for you to see what's going on with the belt. When your engine is cool, open the hood and get your bearings.

A few tips for making sure you get the replacement positioning right:

Check your car's owner's manual for a schematic of the serpentine belt.
Search online for schematics of the belt too. (We found tons.)
Take a lot of pictures from several angles of your engine with your phone.
There might even be a belt winding diagram under the hood.

You'll want to refer to these as you remove the old belt and as you thread the new one into place.

Removing the Old Belt

After you've taken note of the belt's placement, you're ready to take off the old belt.

First, find the belt tensioner. It keeps the belt from being too loose, which would allow it to slip. It usually has a square-shaped hole in it, and that's where the belt tensioner tool slots in. This is the most common kind of belt tensioner in modern cars, but some cars just use a bolt that can be loosened with a socket.

When you've got some slack in the serpentine belt, you can carefully remove it from the pulleys. You'll probably notice that there's one pulley pressing on the smooth back of the belt. That's the idler pulley. It doesn't power anything, but it does help keep the belt in place.

Now's your chance to do a little maintenance. You can carefully clean the grooves of the pulleys with a wire brush to get any old rubber bits and grime out of the grooves where the belt sits. You can also inspect the belt for wear [Source: AutoZone]. Most belts will need to be replaced over time, so you're looking for cracks, fraying or shiny spots (known as glazing) from misalignment or other issues.

Also check the pulleys for oil, as that can be a sign of a leak. In addition to not being great for the engine, oil can break down rubber, meaning you'll have to do this all again sooner rather than later.

A diagram of detailing the path of an alternator belt. — This a diagram shows a serpentine belt for a 2016 Mercedes Benz S550 and how it should be threaded through the pulleys. 1) serpentine belt; 2) tensioner; 3) top pulley; 4) side pulley left; 5) side pulley right lower; 6) side pulley right upper.
mbworld

Installing the New Belt

Now is when you'll be glad to have diagrams and photos. Snake the belt back onto all the pulleys in the correct order for your vehicle.

Remember that any pulley that has grooves is meant to be driven by the grooves in the serpentine belt. The idler pulley, for example, doesn't have grooves because it rests on the back side of the belt. But the power steering pump pulley does have grooves. You literally need to get your groove on here for the power of the crankshaft to turn these accessories.

Hold the belt tensioner slack as you thread the last pulley. When everything is in place and the grooves are seated, tighten the tensioner pulley with your belt tensioner tool or socket.

It's time to give it a try. Gentlereaders, start your engines! Let it run for a minute or two to make sure it's in place and everything is working as it should.

How Often Does a Serpentine Belt Need to Be Changed?

Serpentine belts can last a long time. Manufacturers usually recommend changing them at 60,000 miles (96,560 kilometers), but some recommendations say go as long as 100,000 (160,934 kilometers). The key is to change them before the belt breaks.

It might not seem so terrible to drive without air conditioning, but the alternator charges your car's battery and electrical system, and driving will become a workout if you're suddenly without power steering.

Here are a few signs that your serpentine belt might be on its last slither:

You hear a loud squealing, especially when cranking the steering wheel all the way to one side or the other. This is the most common sign people notice.
The battery light in the dashboard comes on if the alternator isn't getting power.
The temperature light in the dashboard comes on if the belt is supposed to be powering the water pump.
The check engine light comes on.
The steering feels heavier or more sluggish than usual.

While this isn't an expensive repair or terribly complicated, it is tricky to snake a serpentine belt into place. It's definitely helpful to know what this belt is and what it does, but it's also sometimes best to leave engine repairs to the professionals. Your mileage, as they say, may vary according to your expertise and confidence.