How to tell if Your Alternator Belt has Become Loose

alternator and belt on a car

The alternator generates charge from the movement of your engine. The engine turns, the alternator turns, and this in turn creates power, which is used to keep the battery fully charged.

If the alternator fails, the battery will rapidly lose charge, and be unable to keep powering your vehicle’s electrical systems, causing some of them to start failing and worse still, they could shut down completely.

The alternator needs to rapidly rotate to power the vehicle. This rotation is created by the alternator belt (also known as the serpentine belt), which connects the alternator to the crankshaft via a system of pulleys. One end of the belt is wrapped around the alternator – the other end around the crankshaft.

Therefore, if your alternator belt becomes loose, then it will not be able to fully transfer the rotational energy from the crankshaft to the alternator, resulting in a loss of charge. What are the signs that your alternator belt may be becoming old and no longer supplying the required energy to your battery?

Some of the Most Common Symptoms

Alternator belts don’t last forever. Older neoprene rubber belts need to be replaced roughly every 5 years or somewhere around 25,000 to 50,000 miles, whilst newer ethylene propylene diene monomer (EPDM) belts can last for around 100,000 miles. Over time, the stretch material that makes up either type of belt will crack, stretch, gradually loosen, and eventually break, leaving the alternator struggling to create enough charge to fulfil your battery’s energy consumption needs.

However, this degradation doesn’t take place overnight. As the belt wears out, you’ll experience a number of symptoms. As with many problems in your car, because all the systems are connected, you’ll want to attend to an issue with the alternator belt as soon as possible, so that it doesn’t lead to any more serious faults.

serpentine belt on a car

Battery Warning Light

One of the earliest indicators of a loose or slipping alternator belt is the battery warning light. A belt that has not been fully tightened will cause the alternator to spin at sub-optimal speeds, which means the battery won’t get the required charge to top it up. Of course, issues with the alternator belt are not the only problem that could cause the battery warning light to illuminate – but don’t take a chance, and get the belt checked out to be on the safe side.

Strange Noises

A loose alternator belt will often make squealing noises for a few moments when you start the car, with the sound reducing as the car gets going. Such noises are due to the more rapid motion of the alternator as it charges the battery up as the car starts. A loose belt won’t be seated correctly in the grooves of the pullies – creating noise from friction. You may also find that the problem becomes more pronounced in the winter, as the belt slips even more due to the cold weather. If you’re experiencing this problem, it will only get worse with the belt falling off the pulley or breaking completely; replace your alternator belt to avoid your car breaking down or failing to start altogether.

Issues with Lights

Have you noticed that the strength of the beams from your headlights has diminished? Perhaps the cabin lights inside your car are not as bright as they once were? One of the main reasons for this could be an issue with the battery or alternator – and very possibly a slipping alternator belt. When there’s a lack of proper contact with the alternator, a loose belt can cause peaks and troughs in the voltage, eventually leading to the lights dimming and even going out altogether due to the lack of electrical power. This may not just be a sign of a problematic alternator belt – it could be an indication the alternator is nearing the end of its life, especially if it’s getting old.


If you’ve been out on the road for a while with a loose alternator belt, then you could find that the car starts draining so much power from the battery that the car shuts down – effectively stalling. You’ll have to find a place to pull your car over, and with no energy in your battery, you might even struggle to get your car started again, leaving you stranded. Such a situation could put you in danger if you stall on a busy road. Avoid this situation by paying attention to this and other warning signs on our list.

Dead Battery

A loose alternator belt may still provide charge for the battery; the trouble is however this may not be enough, and your car could be running at an energy deficit. Because of the high levels of power needed to get the engine firing, this may be noticeable when starting your vehicle. If you repeatedly drive the car around, this means that the battery will use its own electrical reserves to power the car, which will eventually leave it drained. Once the battery is out of power, you may not be able to start your vehicle again without recharging the battery.

Power Steering

Nearly all modern cars have power steering. Whilst some models might have a separate belt for the power steering pump, some vehicles use the alternator belt to power the pump. This means that when the alternator belt becomes lose, you might find that your power steering starts to fail; steering becomes more difficult. A very dangerous situation if you’re out on the road.

Our Final Word

Regardless of the problems you might be having with your car because of the alternator belt – get it changed quickly. Problems with the alternator belt can lead to other issues in the car as we stated and could even leave you needing a replacement battery. You can either replace it yourself or get to a garage – but make sure you don’t leave it too long.

This entry was posted in Maintenance on by Justin Smith.

About Justin Smith

As the man at the helm of BreakerLink, it is no surprise that its Director, Justin Smith, has always had a keen interest in cars, bikes and most things wheeled. Having spent over two decades in the car parts industry, Justin combines his passion that since 2002, has successfully united those looking for new and used car parts with the breaker that supplies them. Follow Justin on LinkedIn.

Disclaimer: These articles are for guidance purposes only. If you have any questions regarding any matter relating to your vehicle we would recommend that you seek the advice of an appropriate professional. We accept no responsibility or liability should you suffer financial or personal damages in relation to the advice stated on this website.