What Does an Auxiliary Belt Do?

Does your owner’s manual recommend having the auxiliary belt on your vehicle changed during your next regular maintenance visit?  It can be tempting to skip what may seem like an unnecessary job. That is until you understand how the auxiliary belt works.

The auxiliary belt in a modern truck, SUV, or sedan provides power to the air conditioning system, the water pump, the power steering pump, and the alternator.

What happens if it fails? What if your manual calls it something else? We’ve got all the answers and tips for tackling this routine part replacement.

Are the Auxiliary and Serpentine Belts the Same?

Yes. The auxiliary belt is an old-school term as the first ones installed in vehicles would run just one system like the alternator or water pump. The auxiliary belt ran in a single loop from the crankshaft pulley. Many modern vehicles refer to it as a serpentine belt as it now runs all the extra power equipment while winding its way around multiple pulleys. 

Does My Auxiliary Belt Need the Tension Adjusted?

Simple auxiliary belts may operate off a manual tensioner pulley. If you run your finger between the belt and the pulley and have more than an inch of play, you need to adjust the tensioner so that the belt stays in the optimum path for power transfer. A loose belt can become frayed and even snap.

Some cars use an automatic tensioner that constantly adjusts its position to maintain belt tension.

Is the Auxiliary Belt Different from the Timing Belt?

Yes. If you see two different belts under the hood, one may be your timing belt. The timing belt generally does not work to keep any of your auxiliary systems working. Its sole function is to keep the camshaft and crankshaft in sync so that the combustion process can continue unimpeded. The engine cannot operate without the timing belt. The engine can run without an auxiliary belt, but not for an indefinite amount of time.

Identifying Wear and Tear on Your Serpentine Belt

Modern serpentine and auxiliary belts have grooves that help them stay in alignment while running through the pulleys. Newer belts will not display the stretching and cracking of older belt types. A belt depth gauge tool will indicate if the grooves are worn down or if the belt is too thin. Check your owner’s manual for the recommended replacement interval, but most belts last between 60,000 and 100,000 miles.

What Happens if the Auxiliary Belt Fails?

While you can technically keep driving a car with a snapped auxiliary belt, it can be difficult and is not recommended. Once the belt gives way your power steering will become near impossible to use. The water pump stops which results in rapid overheating of the engine. The alternator stops providing electrical power which will drain your battery. And yes, your climate control will go out.

Also, a broken auxiliary belt can fly around under the hood, and become entangled in other moving parts causing even more damage.

Changing Out the Tensioner when You Replace the Auxiliary Belt

Your tensioner pulley works nearly as hard as the serpentine belt, constantly adjusting tension over every single mile. It is not a part designed to last the lifetime of the car. For this reason, when you replace your auxiliary belt, it is a good idea to also replace the tensioner and any other pulleys that have worn or rounded edges. If your mechanic is doing the job, specify that you want all the pulleys replaced or they may just swap out the belt.

Failing to change the tensioner can result in a pulley failure which can destroy the new serpentine belt. You now have to pay double the time and money for a routine maintenance job.

Is Changing the Auxiliary Belt a Good DIY Job?

If this is the first time that you are changing a serpentine belt, leave a few hours open. Next time you should be able to do it in about an hour. You only need a few tools like a socket wrench set, a torque wrench, jack stands, and the new belt and tensioner. The parts will run you about $40 to $100 depending on your vehicle.