How To Test An Alternator

Your car's alternator must work for all its electricity-powered components to work. Find out here how to test your alternator for issues.

The alternator makes sure to keep the battery of the car charged and power your vehicle.

If you suspect that your alternator is faulty, keep reading to find out how to test an alternator.

We’ll start with the basics so you get a better understanding of car alternators.

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What Is An Alternator?

To fully understand why an alternator should be tested, it is important to understand what an alternator is and what it does.

People usually associate the car’s electricity with the car battery, which is intuitive and correct. However, to make all the components that are powered by electricity work, the alternator must work.

In short, the function of an alternator is to generate electricity to recharge the battery and power the electrical components. It does so by converting mechanical energy (from the engine) into electrical energy.

The battery stores energy, enabling you to start the car. If it were not for the alternator, your battery would be drained within an hour as it would be the only source of electricity.

How Does An Alternator Work?

alternator repair cost

The alternator is connected to the serpentine belt of the car. The belt is powered by the engine, or more specifically, the crankshaft.

As the belt moves, it also moves the rotor, which is located inside the alternator. When the rotor is moving, it produces a voltage through electromagnetic induction.

In short, to summarize and simplify, part of the alternator moves as the engine is on, which converts the energy from the moving belt to electricity.

BBC has a great and short illustrated article showing the basic principles of an alternator.

Signs Of A Broken Alternator

Modern Car Alternator and Engine

The signs of a broken alternator can be many. The good news is that a broken alternator is very rare as they are expected to work for the lifetime of the car.

There are of course exceptions where the alternator actually does break down. Some signs that an alternator is faulty are the following:

  • Flickering lights
  • Smell of burning rubber
  • Electrical malfunctions
  • Starting difficulties/Frequent stalling
  • Dead battery
  • Battery warning light
  • Abnormal noise

Each of these signs will be discussed further. As stated above, it is very rare for the alternator to fail, but it does happen, it is a relatively easy and cheap fix.

Flickering Lights

The alternator works consistently when the engine is one. If for some reason, the alternator does not work properly, there is a good chance that the lights will start to flicker or be very dim.

The reason for this phenomenon is that the alternator does not provide a stable amount of voltage. It may be the case that the voltage is too high, causing the headlights to brighten, or the voltage may be too low, causing the headlights to dim.

In cases where the output of voltage is inconsistent, the headlights will start flickering due to the imbalance of voltage output.

Smell Of Burning Rubber

man and bad smell inside car

The serpentine belt, which spins the rotor in the alternator, is made of reinforced rubber. If the serpentine belt is faulty in any way, it could emit a smell of burning rubber.

There is also a possibility that the alternator in itself is faulty, causing damage to the serpentine belt which leads to the same type of smell. If this is accompanied by strange, whining sounds you may be certain that the alternator is the culprit.

The smell of an electrical fire may also be witnessed. If the alternator is faulty, it may overheat due to either an uncontrolled amount of electricity produced or insufficient cooling.

If you notice the smell of an electrical fire, make sure to take your car to the mechanic as soon as possible. The smell might be toxic, so try to avoid breathing it in. There may be many causes of burning rubber.

Electrical Malfunctions

Almost all electronics such as your electrically powered windows, radio, power steering, wipers and, so on, are powered by your car’s alternator. If it turns out that the alternator is bad, chances are that your electrically powered car components do not work well.

While this might not always be the case, different signs might show up. An example could be that your windows do not smoothly retract, or does so in a much slower manner.

Starting Difficulties/Frequent Stalling

opening the hood of a broken car

If the car has a difficult time starting it is probably due to the battery, however, it might be due to the consequence of a poorly functional alternator.

In case that the alternator does not work as intended, the battery will not be recharged leading to the eventual death of the battery. Keep reading to find out how to differentiate if the problem is in the battery or the alternator.

If you manage to start your car but it keeps stalling, your spark plugs might be the problem. More specifically, your alternator does not provide enough and/or regular electricity to make the spark plugs work properly.

Dead Battery

If your alternator does not work at all, your battery will be dead within an hour or two. As the battery is unable to charge, it will eventually stop working.

Many people assume that the battery is the culprit when it does not work anymore. While that may be the case, and usually is, it is important to exclude the alternator.

If it turns out that the battery does not work and later gets changed, only for the new battery to die within a few days or weeks. The problem lies with the alternator then, not the battery.

Make sure to test the function of your alternator if the battery drains unusually fast. You will save both time and money.

Battery Warning Light

battery warning light

The battery warning light does not only warn when your battery is low but also warns you when the voltage output from the alternator is abnormal.

For the same reasons your headlight might flicker, the warning light can do the same. If the warning light turns on when starting the car but goes away after a drive, your battery is probably getting bad. But hey, at least your alternator is working!

Turning on different functions in the car such as the air conditioner or your high beams might provoke a battery warning light. As those things require electricity, it also requires your alternator to work properly to keep up with the demand.

Abnormal Noise

Sounds in the engine compartment are usually very unspecific for the untrained ear. It does not help that a faulty alternator may produce multiple types of sounds, depending on the core of the problem.

The sound of a defective alternator is usually loud whining and high-pitched noise. The noise might also present as a grinding or knocking noise which may be due to the bearings of the alternator or the serpentine belt.

How To Test An Alternator

Car Alternator

As all the common signs have been listed and you now know what an alternator is, it is time to find out how to test an alternator.

Measure The Volts

Alternators are all about volts. If you have any type of volt- or multimeter, you may test the efficacy of your alternator with it. All you have to do is follow these steps:

1. Measure Your battery

First, make sure that the battery is in good shape. The voltage of the battery should show a voltage level of 12-13 volts when turned off.

If it shows less than 12, charge up your battery until you get to the desired voltage.

2. Turn On The Engine

With the volt- or multimeter attached to the battery terminals, turn on the engine and note the values shown.

If the reading shows a voltage of mid-high 13s to mid 14s, your alternator is in good shape.

3. Turn On Electrical Accessories

If your readings are fine so far, keep the voltmeter on the battery and turn on the AC, headlamps, heated seats, and so on. If it turns out that the voltage drops below 13 while the previously mentioned functions are on, your alternator is defective.


If it turns out that the voltage is higher or lower than the reference range given in the steps above, your alternator is probably due for a change.

Test The Serpentine Belt

car belts

If you assume that a fault might lie in the pulley, due to for example a squeaking noise, you can test it very easily.

Wait for the engine to cool off if it is hot. Simply grab the belt that is connected to the alternator (while the engine is off) and pull it downwards. If you hear the same squeaking sound, the alternator probably needs a replacement.

Inspect The Alternator

The alternator can give off strange noises due to untighten bolts or misaligned belts. Simply give it a close inspection and look for any loose or damaged parts.


The battery usually takes all the credit for powering the electrical aspect of a car, however, the alternator is a critical part of the system.

While it is relatively usual for batteries to go bad, it is very rare for alternators to break. Flickering headlights, the smell of burned rubber, electrical malfunctions are a few signs that your alternator might be defective.

You should avoid driving if you suspect that there is a problem with your alternator. If you have a volt- or multimeter, you can relatively easily diagnose a potential problem.

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Bajro Muric
Bajro Muric

Bajro is a big car enthusiast with a love for writing and teaching. He writes about anything regarding cars - from common problems and fixes to racing.