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An Easy Guide in Testing the Alternator of Your Car

how to spot a broken alternator

The alternator generates electric power in your car and is a major component of your car’s charging system. Every vehicle with an internal combustion engine except from some hybrids has an alternator. When the engine of your car is running, the alternator charges the battery and provides additional electric power for the electrical systems of the vehicle. Because of this vital role, it is important that your alternator should always be in tip top shape. If you are not sure, then you should be testing the alternator of your car. 

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How do you check if your alternator is bad?


It is difficult to determine whether or not your alternator is working if you have no idea what to look for. Using a voltmeter is the easiest way to test your alternator. For those who are well in tune with their car, there are a few other ways that can be tried. Testing the alternator will give you an idea on the general health of your alternator as well as your battery. Some newer cars may come with computer controlled charging systems and would need additional testing.


Voltmeter Method


  1. Purchase a voltmeter. This is available at any auto parts shop for less than $20. No need to get an expensive type as a cheap voltmeter is good enough to serve its purpose. If you have a multimeter, no need to buy a voltmeter since you can use this instead. A multimeter can measure voltage and other electrical properties like current and resistance. Measure the voltage when checking your alternator.


  1. Check the battery first. For a car to start, the battery is needed. When the vehicle starts, it spins the alternator at sufficient speed to continuously charge the battery. If your battery is too low, you cannot start the vehicle and so you will not be able to test the alternator which makes any additional voltmeter-testing totally futile. If there is cold weather or you have an old battery, the battery could be the problem and your alternator might be okay. For this reason, you need to check the battery first by doing the following steps:
  • Raise the hood.
  • Have the voltmeter connected to the battery. Have the red lead of the voltmeter placed to the positive battery terminal and the black lead placed to the negative terminal. Do not let your skin touch the battery.
  • Read the voltmeter. The battery should have more than 12.2 volts so it could start the alternator so it can be tested with the voltmeter.
  • In case it reads below 12.2 volts, you can either get the battery charged and test again, or try a different method for testing the alternator.


  1. Start your car and rev the engine to 2000 RPM. This will get power on your battery, causing the voltage regulator to get the alternator into high gear.


  1. While the engine is running, test the battery again with the voltmeter. The reading should go up to at least 13 volts when you read the voltmeter this time. If changing the RPMs causes fluctuations of the voltage between 13 and 14.5 volts, it means your alternator is in good condition; if, on the other hand, the voltage stays the same or decreases, it means your alternator is not functioning properly. Repeat the same process with the AC, lights and radio on. The alternator is charging if the voltage of the battery is above 13 volts with all accessories on and the engine revving at 2000 rpm.


Monitoring the Alternator of Your Vehicle


  1. Check the alternator gauge. The volt/amp gauge will give you the output of the alternator. To test your alternator, run your engine at 2000 rpm and turn on the headlights, the blower fan for the AC or heater and any other accessories that make use of the alternator too, and check the gauge to determine if it decreases voltage or amperage. The voltmeter should be higher when the engine is running and lower when it is not. If this is the case, you can assume that the alternator is indeed charging.


  1. While the engine is running, listen to the alternator to check for any unusual sounds. You may hear a screeching sound from the front of the vehicle that becomes louder with more electrical accessories using power concurrently if there is an issue. 


  1. Turn on the radio and rev the engine. To make sure you’ll easily catch if there’s an issue, make sure to have your radio tuned in to a low number on the AM band with no music. The alternator is more likely the issue when the radio goes fuzzy or whines each time you hit the gas.


  1. Look for an auto parts shop that tests alternators for free. Many stores will want to gain an edge over their rivals so you would typically find stores offering free testing of alternators so you would buy your alternator from them. Remove your alternator and bring it into the store just to be sure.

How do you tell if it's your battery or alternator?

So you get in your car, put in your key and realize you can’t have your car turn on. If your car is not starting, more often than not, the culprit can either be a malfunctioning battery or a failing alternator. If it is either of the two, determining which is to blame is relatively simple.


To simply start and run an engine, involves three steps. First, the car battery powers the starter by delivering a jolt of electricity to it, starting the engine. When the engine runs, the alternator is put into action. Finally, the alternator charges the battery, completing the cycle.


When your engine starts but dies right away, your alternator is likely isn’t keeping the battery of your vehicle charged. On the other hand, a dead battery is likely the culprit when a jump starts and keeps the car running but the car cannot start again off of its own power. 

Signs that Your Battery is Dead

Here are common signs of a dead battery:


  1. Check the dashboard battery gauge. There might be something wrong with your battery if the dashboard lights are dim. The battery should still be giving a charge even while the car is off. Try to turn on your lights, automatic windows or windshield. Then have them all turned off and try again to start the car.
  2. If you still can’t start your car, carefully wipe away any corrosion on your car’s battery using a rag and have someone jump-start it. Turn off the car after running the motor for a while. If you are unable to get it to restart, this indicates that the alternator is functioning to keep the battery working while the motor is running, however, the battery is not able to retain the charge when the alternator stopped.
  3. An aging battery can also be the reason. As batteries age, their ability to retain a charge also weakens or decreases because the metal inside corrodes. Through time, the level of battery charge reduces to the point where no matter how much power the alternator gives to it, the battery can no longer hold enough power to start the car. But there are steps you can take to lengthen the life of your car battery.

Signs that Your Alternator is Failing

If the steps above indicate that the battery is functioning properly, it is time to check the alternator. Here are some symptoms of a bad alternator:


  1. Note the brightness of the interior lights in your car when the car is running. The alternator is likely the problem if you notice that the dashboard gradually dims.
  2. Another sign is if your headlights are becoming brighter when you accelerate and dims when you stop. This is often due to the alternator not being able to charge the battery enough.
  3. A roaring sound before the problem began can also indicate that an alternator is failing.
  4. Watch out for signs of an overheating alternator such as the smell of hot wires or burning rubber. If this is the case, it means you have to replace your alternator.

How do you check the alternator by disconnecting the battery?

Checking the alternator by disconnecting the battery is a quick and easy process. Here is a step by step guide:


  1. Make sure your car is parked in an open but safe place. You can also use the driveway. If you have pets, keep them at a safe distance.
  2. Open the hood of your car. Make sure that your car is in a stable position.
  3. Go to the driver’s seat.
  4. Start the car’s engine. You most likely will have some difficulties at this point if your alternator has issues.
  5. Get out of the seat once the car is running.
  6. Go to the battery and disconnect it while the car is running.
  7. See if your car is running like before.


If your car is running like it was before, then that’s good news, your alternator is fine. It is possible that some other part of your car has issues. However, if your car stops, then your alternator definitely has some issues. And that is how simple it is to check if your alternator is failing by disconnecting the battery or by removing battery cable. However,this method is super risky and experts these days have expressed concerns on the effectiveness of it.


The problem with this method is it is meant to be effective for generators, not alternators. An alternator is not a generator. An alternator is regulated internally. Hence, when testing an alternator through pulling the battery cable you are increasing the chance of killing that regulator. That was the risk of this method.


When it comes to its ineffectiveness, the car will run if the alternator is producing adequate juice, but a faulty alternator can still produce enough to run an engine. Hence, you can have a false notion that the alternator is good when in fact it is not. Better to just skip this method, as many people have tried this and ended up frying the internal components of their vehicle. 

What does a bad alternator sound like?

A bad alternator produces a grinding sound that may be caused by a worn-out bearing. Your vehicle can also make a squealing noise when the voltage regulator sends signals to the alternator to charge more than what is required. If you hear either of these two types of sounds, best to have your alternator checked. 

Can I drive with a bad alternator?

Yes, you can still drive with a bad alternator. But when an alternator is going bad, it drains the battery quickly, and in most cases the car can be driven for a short period of time and for a short distance, which will allow you to get it to a service station or auto-parts store for a replacement alternator. Here’s what you can do:

  1. Make sure all possible auxiliary items are turned off such as air conditioner or heater, the radio, windshield wipers and lights.
  2. Keep away from driving in highly congested areas. The stop and go traffic can drain your battery more quickly, so find a parking lot to pull into until the traffic is less congested.
  3. Travel to the nearest auto parts store or service station to get a new alternator. Install or have the new alternator installed. Make sure to recharge your car’s battery before leaving.


How long does it take for a bad alternator to kill a battery?

When an alternator goes bad, your car will shut down after 20 to 30 minutes. When the alternator fails, it will drain the battery which will cause all electrical systems to shut off.


How much does it cost to replace an alternator?

The average cost of replacing an alternator with a remanufactured one is $400. A remanufactured alternator on a typical car costs about $300 to $500, inclusive of parts and labor. If you opt for a new alternator, expect the cost to run from $500 to $1,000.


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