What Does an Alternator Do?

  • Aug 3,2020
Close up of a belt on an alternator Close up of a belt on an alternator

When it comes to powering your car's radio, headlights, and other electronic components, you may think it's the battery that's putting in all the work. In reality, it's your alternator that keeps things up and running. But what exactly does an alternator do and how does it work? Read on to find out what makes your alternator so important and how to recognize car alternator problems before they become a major issue.

What Does an Alternator Do?

While the battery is essential for starting your car when it's off, the alternator keeps your car alive when the engine is running. The alternator powers most car's electronic components while you're driving around or idling, including your headlights, electric steering, power windows, windshield wipers, heated seats, dashboard instruments, and radio. The alternator supplies all of them with direct current (DC) power. Your alternator is also responsible for charging your car battery while driving.

The alternator works by turning mechanical energy into electrical energy. When your engine is on, it powers a drive belt that rests on a pulley attached to the alternator. The pulley turns the alternator's rotor shaft, which spins a set of magnets around a coil. These spinning magnets generate alternating current (AC) around the coil, which is then channeled to the alternator's rectifier. The rectifier converts that AC power into DC power, which activates your car's electrical systems.

Alternators typically last the lifetime of your vehicle, but that doesn't always happen. General wear and tear, heat damage, overuse, exposure to water, faulty parts, or frayed wires can put your alternator out of commission before your car heads to the scrap yard.

Warning Signs of a Bad Alternator

Without a working alternator, your car won't start in the near future or stay on for more than a few minutes. Yet the typical signs of a bad alternator are often mistaken for problems with the battery or other car parts that display similar symptoms. In other words, if you're experiencing only one of the issues below, your alternator may not necessarily be the issue. However, any of the following warning signs may indicate a potential issue with your vehicle’s electrical system. Bring your car to your local Firestone Complete Auto Care to have your electrical system inspected so we can get to the root of the issue.

Dim or Overly Bright Lights

When an alternator begins to fail, it provides inconsistent voltage to your electronic accessories. Generally, that takes the form of under- or over-performing equipment, like headlights that are either too dim or extremely bright. You may also experience flickering lights or lights that erratically go from bright to dim and vice-versa.

Dead Battery

Sometimes a dead battery is just a dead battery — it's reached the end of its life after a few years of use — or maybe you accidentally left the headlights on all night. Other times, however, a dead battery could be a sign that your alternator is malfunctioning.

A bad alternator won't sufficiently charge the battery while the engine is running, causing the charge in the battery to deplete faster than usual. One way to test whether the issue is battery- or alternator-related is to jumpstart the car. If you jumpstart your car and it stays running, your battery may need replacing soon. However, if you jumpstart the car and it dies again shortly after, it may mean your alternator isn't getting enough power to the battery.

Slow or Malfunctioning Accessories

An alternator that isn't supplying enough power to your car's electronics often results in slow or non-working accessories. If you notice your windows taking longer-than-usual to roll up or down, or if your seat warmers aren't heating quickly, or even if your speedometer and other instruments start going haywire, you may have an alternator problem.

Many modern vehicles also have a priority list of equipment programmed into the car that tells the onboard computer where to cut power first if the alternator isn't supplying enough electricity. That way, if you're driving with a failing alternator, you'll lose power to your radio (or other nonessential accessories) before losing power to your headlights.

Trouble Starting or Frequent Stalling

As previously mentioned, trouble starting your engine might mean that your alternator is failing to charge the battery. So when you turn the key in the ignition, all you hear is a concerning clicking sound — not the purr of your engine.

On the other hand, if your car is frequently stalling out while driving, it may be a sign that the spark plugs and coils aren't getting enough power from the alternator to keep the engine running.

Growling or Whining Noises

Cars make a ton of odd sounds — some are harmless, while others can indicate serious mechanical problems. When it comes to bad alternators, you're most likely to hear either growling or whining under the hood.

This growling or whining sound happens when the belt that turns the alternator's pulley becomes misaligned. You may also hear this sound if the bearings that spin the rotor shaft are going bad.

The Smell of Burning Rubber or Wires

A foul odor of burning rubber or wires may indicate that parts of your alternator are starting to wear out. Because the alternator's drive belt is under constant tension and friction — and because it's close to the hot engine — it may wear out over time and emit an unpleasant burning rubber smell. A seized alternator pulley bearing will also emit a burning rubber smell as the belt rubs the seized pulley when the engine is on.

Similarly, if your alternator is being overworked or has frayed or damaged wires — you may smell a burning odor comparable to an electrical fire. An overworked alternator tries to push too much electricity through its wires, causing them to heat up unsafely. Damaged wires also create resistance to the flow of electricity, causing the wires to heat up and emit a foul odor.

Battery Warning Light on Dash

When the battery warning light pops up on the dashboard, it's commonly mistaken to be a battery-specific issue. However, the battery warning light indicates that there could be a problem within your car's wider electrical system, including the alternator.

Alternators are designed to work at a specific voltage, typically between 13-14.5 volts. If your alternator is failing, its voltage may drop below capacity, causing the battery warning light to appear on your dash. Similarly, the battery light will also appear if the alternator is exceeding its voltage limit, depending on how much stress it is under.

Depending on the electrical load from your car's accessories (headlights, wipers, radio, etc.), you may see the battery warning light flicker on and off as the alternator fluctuates in and out of its intended voltage capacity. While this may seem like a minor annoyance, it's better to bring your car in for an electrical system inspection rather than wind up on the side of the road.

Keep Your Car Charged Up

Difficulties when starting your vehicle or charging your battery could be due to a faulty alternator! For professional diagnostics and transparent service recommendations, schedule an electrical system inspection or an alternator service at your nearest Firestone Complete Auto Care.

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