Top 10 Signs of Alternator Problems

By: Eric Baxter & Cherise Threewitt  | 
alternator problems
The alternator is a relatively simple component containing only a few parts, but it plays a critical role in the operation of any gasoline-powered vehicle. Phuchit/ThinkStock

It all starts with a few telltale problems: A dash light coming on for a brief moment, maybe dimmed headlights and a few flickering gauges. Perhaps your car even gives off an odd smell, or you hear a growling sound coming from under the hood.

Is this a case of your auto being possessed? No. Most likely it's one of many possible alternator problems, and without some attention, the alternator can cause car trouble ranging from slow starts all the way up to a dead car.


The alternator is a relatively simple component containing only a few parts, but it plays a critical role in the operation of any gasoline-powered vehicle. Essentially, the alternator turns the mechanical energy of the engine's rotating crank shaft into electricity through induction. Wires within the alternator cut through a magnetic field; this in turn induces electrical current. That current is used to power your car's accessories, which can be anything from headlights to the electro-hydraulic lifts on a snow plow. The alternator also keeps the battery fully charged, providing the power it needs to start the car. (Electric vehicles don't need alternators since they are driven by an electric motor and battery combination that provides plenty of energy for the vehicle's normal operations. EVs and hybrids, which do have alternators, often use regenerative braking systems to give the battery an extra boost.)

Because the alternator is connected to (and critical for) other vehicle systems, any mechanical auto problems can have an effect on its function and have an effect on diagnosing car problems. Paying attention to the 10 trouble signs on this list can make it a little easier to diagnose car trouble caused by the alternator.

Keep reading to find out about car problems potentially caused by the alternator.

10: Warning Light

alternator problems
If the alternator's output goes below or above a pre-set limit, then your car's warning dash light will come on. In most vehicles, it looks like a battery. Baloon111/ThinkStock

Within the instrument cluster of most cars built in the last decade is a warning light dedicated to signaling an alternator issue. In most cases the light is shaped like a battery, though some show "ALT" or "GEN," meaning alternator or generator, respectively. (In some older cars, the alternator is referred to as a generator, which may be helpful to know if you're looking through the owner's manual or a shop manual.) Many people see this light and instinctively think they have a battery problem, which is symptom that will be covered later, but that's not really why the light goes on.

This light is linked to computer systems within the car monitoring the voltage output of the alternator. If the alternator's output goes below or above a pre-set limit, then the dash light comes on. Once the output is within range, the light remains unlit. In the early stages of alternator problems the light can seem to flicker — on for just a second and then off again. Or maybe it lights up only when accessories are activated.


For instance, let's say it's nighttime, your headlights are on, and everything is working just fine. Then it begins to rain. As you turn on the windshield wipers, the warning light comes on. You turn off the wipers and the warning light goes away. While that may initially seem like an aggravating problem, the warning light is doing its job exactly as it's intended.

Most alternators have an output between 13 and 14.5 volts that they try to maintain at a constant level. As more power is demanded your car's headlights, the windshield wipers, your radio, the heated seats, the rear window defroster, and so on, the alternator needs to work harder to maintain the necessary voltage. If your car's alternator is not working to its full potential, or demands are placed on it that it can no longer meet, the voltage will either go above or below the set level and switch on the warning light.

9: Dim Lights

alternator problems
A failing alternator could create electrical probems that could lead to erratic headlight issues, like frequent dimming or extreme brightness. Zunnie/ThinkStock

Since the alternator supplies your vehicle's electrical needs, when it begins to lose its potential, so do the car's accessories that draw on that electricity. Your car may begin to experience erratic symptoms ranging from dimming or extremely bright headlights and dash lights, to speedometers and tachometers that simply stop working for no apparent reason.

Your exact experience is usually dependent on a number of factors. The first is how well the alternator is still producing power and also where it is in its death cycle. The second is how your car is programmed. In most new vehicles, auto manufacturers have a sort of preprogrammed priority list for where electricity will be sent just in case an alternator problem arises. This is usually based on safety considerations. For example, your car's heated seat will turn off or the radio will quit before the headlights dim and fade away. That's because you need to be able to see in order to safely pull over and stop if your car suddenly dies — your radio and heated seats working simply become secondary at that point.


8: Flickering Lights

On the previous page, you learned that dim exterior and interior lights are common symptoms of alternator trouble. If your lights aren't actually dim, they might be flickering instead. These two symptoms often go hand in hand, but not always. If your lights are bright, then dull, then bright again (either on a regular beat or seemingly at random) then you definitely have a problem with your electrical system, and the alternator is the most likely culprit. The reasoning is the same as if the lights were simply dim — the alternator just isn't generating enough juice to keep the lights going at full strength, and instead of dimming all of the lights in response, your car is trying, and failing, to maintain the proper lighting output.

If you want to be sure that the problem is the alternator, rather than something else in the electrical system, simply look for a pattern. Can you pinpoint that the lights flicker when you do something else that draws electricity, such as turning on the radio, adjusting the climate control or using your power windows? That will be the key to helping you narrow down the possibilities, since it demonstrates your lights are flickering in response to the extra load on the system. As you continue to read through this article, you'll see that a failing alternator simply can't keep up with all of the things you ask your car to do during a typical drive.


7: A Dead Battery

alternator problems
A bad alternator can killing your car's battery, even if the battery wasn’t the problem to begin with. Natnan Srisuwan/ThinkStock

While not technically an alternator problem, a car that won't start is a definitive sign that there's a problem in the starting system. When the key is turned, you'll hear a tell-tale clicking of relays ticking over but nothing else happens. After a time, even that noise stops. The immediate problem is usually a dead battery, but you need to ask "Why is it dead?"

When an alternator begins to fail — or fails outright — the car's battery begins to take up the slack rather than acting as a capacitor for the system by receiving a constant infusion of electrical power from the alternator. However, even the best car battery will run down eventually, leaving you stranded in the driveway or worse, on the side of the road. Car batteries are not designed for long-term power use. They're designed to provide your vehicle with enough electrical oomph to start by juicing the starter motor with a surge of power and getting the whole works spinning. In other words, a bad alternator can get your attention by killing your car's battery, even if the battery wasn't the problem to begin with.


Diagnosing a dead battery versus a dead alternator is relatively easy. Simply jump-start your car and then remove the jumper cables as quickly as possible. Then wait. If the alternator is failing to charge the system, the car will soon die again and you've pinpointed the problem. However, if the car runs and continues to run, then the likely problem is with the battery.

Use caution, however, as a dying battery can only receive a charge for so long and may go completely dead at a later point despite the best efforts of the alternator. You can test the battery with a voltmeter, and most auto stores have the facilities to check a suspect battery, usually free of charge.

6: Broken or Loose Connections

In this case, everything with your car's alternator seems to check out OK — no belt issues or other visible signs of trouble — but the car's battery is dead, as is most of the car's electrical equipment. That might mean that the alternator is producing electricity, but it's either not going anywhere or it's not the right type.

Alternator electricity is piped through large cables and smaller wires. Any problems within the wires, cables or connections at either end can reduce or stop electricity from getting through. Occasionally, a symptom of this issue might be brighter lights, as the alternator produces more energy to overcome the resistance in a bad wire or a broken or loose connection. This symptom is usually accompanied by the smell of hot wires, too. Higher resistance within a wire creates heat, similar to the way an electric burner on a stove heats up by resisting the flow of electricity.


Another problem could be the alternator's diode rectifier. Alternators produce alternating current (AC) electricity in three phases, but a car's accessories require direct current (DC) to operate. The rectifier changes the current from AC to DC. Without that critical component operating properly, the electricity the alternator produces can't be used.

5: Bad Belts

alternator problems
A loose or bad belt can cause the alternator to run incorrectly, so a quick inspection of the belt can diagnose the problem. simazoran/ThinkStock

The next three symptoms are less obvious than a signal from a warning light; however, these signs of alternator trouble can tip off a driver that there's something going on that is out of the ordinary.

Mechanics are used to customers complaining of classic alternator issues, and then opening the hood to find the belt missing, hanging off the engine block, or so loose it's flapping around the engine compartment. Since a broken or loose belt is pretty obvious, take a peek under the hood to see if your car has any of these issues, even if you aren't skilled enough to fix them yourself.


If you're comfortable with basic maintenance and you're familiar with your car's components, you're in good shape to catch a problem before it leaves you stranded. A quick visual check of the belt for cracking, excessive wear, and other age issues can give an indication of a future problem. Keep in mind that the belt must have the proper tension to run the alternator correctly; too much tension is just as bad as not enough. A quick check of the belt tension is usually enough to determine if a problem exists. Make sure to let your car cool off before you start touching anything under the hood.

4: Strange Sounds

If the sight of a failing alternator isn't obvious enough for you (and it's totally understandable if it's not), the sound might clue you in. Drivers will sometimes complain about hearing a "growling" or "whining" noise before an alternator gives out. The alternator is driven by either an accessory belt or a serpentine belt in conjunction with the crankshaft pulley. The alternator pulley typically spins about two or three times faster than the crankshaft pulley to produce the power necessary at lower engine speeds, such as at idle. The alternator pulley spins on a shaft, which in turn is supported by either bearings or bushings. If the pulley is not in correct alignment with the belt, if it's canted on the shaft or if the bearings and bushing are worn out, the growling or whining noise will let you know there's a problem.

Since lots of car problems cause strange noises, this symptom is most useful if it is paired with other symptoms, especially since it can be hard to identify the source of a sound while you are driving.


If the alternator is so far gone that it has caused damage to your engine, you might hear rattles from inside the engine caused by failed engine bearings. Engine damage is one of the worst-case scenarios of alternator trouble and it is definitely a possibility, but it's so difficult and inconvenient to keep a car going with a bad alternator that you'll probably get it fixed long before it causes damage to your engine.

3: Strange Smells

Smell is the third sensory sign of alternator trouble, and it comes from the fact that your alternator is working way too hard to the brink of overheating.

Have you ever been near an electrical fire? An overworked alternator might smell kind of like that. The hot wire scent can be caused by an overheated alternator, one pushing too much power through the rotor and stator.


Or burning rubber? Yeah, that's another possibility, thanks to the rubber belt that keeps the alternator going, as well as the rubber sheaths on the wires that connect everything in your electrical system. A pulley that isn't in alignment or not turning freely will cause more friction on the belt, which creates heat and then the smell of burning rubber. If those get too hot, your nose will probably know.

Not all bad smells indicate potential alternator trouble. The aroma is distinct from other attention-worthy car smells, like the smoky sting of burning oil, the sweet stench of overflowing coolant, or the singe of overheating brakes. Of course, none of these things are good, so if you smell any of them, get them checked out. And keep in mind that the absence of a bad smell doesn't mean everything is okay. In other words, a bad electrical smell will help you narrow down the source of your trouble, but it's possible the alternator could be failing without any of these accompanying odors.

Keep reading for more hints that your alternator is on its way out.

2: Slow Accessories

alternator problems
Trouble with power windows or power sunroofs could be a sign of a failing alternator. Slobo/Getty Images

Are your power windows whining at your simple command to open or close? Is the stereo swallowing CDs or did the power sunroof get stuck open? Modern car features are great right up until the moment they're not, and the complex computers and wiring systems that operate our power accessories lend more opportunity for something to go wrong.

It's possible, of course, for any of these features to break at any time. Motors go bad, wires get crossed, and all manner of glitches can happen. If you notice one electrical quirk, you'll want to get that specific feature checked out. If you're experiencing several issues at once, though, it's probably not a coincidence. Your alternator is one of the most likely sources for electrical problems, and you might not even notice that your car is running with reduced electricity until you make additional demands on the system, such as turning on an interior light or trying to adjust your windows. Avoid using these features as much as possible until you can get your car checked out, to reduce the chances of actual alternator failure.


1: Regular Stalling or Difficulty Starting

alternator problems
Difficulty starting your car or keeping it running is likely to crop up as one of the first symptoms of alternator trouble. lzf/ThinkStock

Because every car is different, difficulty starting your car or keeping it running is likely to crop up as one of the first symptoms of alternator trouble. However, there are a lot of other mechanical issues that can cause stalling or hard starts. It's important to look at everything else going on with your car to pinpoint the problem.

If your hard starts are, in fact, caused by alternator trouble, the reason for this is because the car starts from juice stored in the battery. As we've already discussed, if the alternator is not recharging the battery properly, the battery won't be strong enough to confidently and smoothly turn on the car. If you're stalling even after the car has been running, the spark plugs might not be getting enough power from the alternator to keep the engine going. In other words, your battery isn't dead yet, but it's well on the way.

Sharp-eyed readers might remember that a dead battery was mentioned much earlier in this list as a separate symptom, even though the mechanical factors are similar. Is it really worth bringing this up separately, you might ask? The answer is yes — because it is much easier to ignore or overlook a dying battery than a dead one, and if you're aware of what this symptom means, then you have a chance to get your alternator checked out before you actually get stranded.

For more information about alternators and other related topics, follow the links on the next page.

Alternator Problems FAQ

What does a bad alternator sound like?
A grinding or whining sound is a tell-tale sign of a bad alternator. Even a small rattling sound coming from the engine can be an indicator that your alternator is failing. Most sounds from the alternator are caused by a loose bearing.
Can a car run with a bad alternator?
Yes. However, a car will only be able to run on a bad alternator for a short period of time. Since the alternator is responsible for charging the battery when the engine is running, once the battery is drained, the vehicle won’t start.
How do you test a car alternator?
Open the hood of the car and start the engine. Remove the negative cable from the battery while the engine is on. If the vehicle dies or stalls,the alternator probably needs to be fixed or replaced.
Do alternators fail suddenly?
Once an alternator starts to fail, it causes multiple electrical issues before the car fully breaks down. That said, an alternator can take time to fail or can stop working suddenly. Usually, people recognize the whining sound that comes from a bad alternator and get it fixed before it breaks down completely.
What happens when alternator stops working while driving?
The car tends to slowly lose power if the alternator has gone bad while the car is being driven. You’ll notice the headlights start to dim shortly before the car loses power and dies.

Lots More Information

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