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How To Test Alternator By Disconnecting Battery? Step By Step Guide

How To Test Alternator By Disconnecting Battery? Step By Step Guide

You are driving your car while enjoying your high-end music system and other electrical components of your car.

Do you know who provides continuous power to these electrical systems when the engine is running? An alternator is that electrical component or, say, a generator responsible for providing power to the various car components when the engine is running.

However, being a vital component of a car and responsible for the car’s operation, the alternator can suffer internal issues and raise an array of other problems.

The symptoms include no radio, dimming headlights, car starting problems, and much more.

If you’re experiencing any strange issues while the engine is on, the alternator has a severe problem.

You need to test it to confirm.

But how? There are ample ways to test an alternator independently or by a mechanic.

If you are doing it alone, testing the alternator by disconnecting the battery is simplest. Continue reading to know how you can do this.

How to test an alternator by disconnecting the battery?

To test the alternator’s health by disconnecting the battery, you need to follow the following steps. However, I assume that you have sufficient knowledge of electronics before proceeding.

  • Park your car at a safe location and make sure no one is around the car except you.
  • Once the car is in its stable position, open the hood of your car.
  • Go back to the driver’s seat.
  • Turn on the engine, and at this point, you will notice some issues with the alternator.
  • As the car starts running, wait for a few minutes to cross-check whether the car will go off or not.
  • If the car is running continuously, go back to the battery and disconnect it.
  • Now, see if the car is running like before or not.

If the car is running like before, even after disconnecting the battery, congratulations, the alternator is fine. And it’s due to other reasons you’re experiencing the symptoms of a bad alternator.

However, this method isn’t safe. It is because an alternator isn’t a proper generator and merely an electrical regulator.

Hence, you can harm the alternator through the above process if it wasn’t earlier.

Plus, the alternator may have enough juice to keep only the engine running even in bad shape.

It’s also possible that the alternator is in bad shape, yet you assume it is okay. It may cause a more dreadful engine failure later on.


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Do I disconnect the positive or negative to check the alternator?

So now that you know that disconnecting the battery and then testing the alternator isn’t the best resort, you should know another way to do the same.

However, remember that disconnecting the negative terminal won’t do well either, as suggested by many.

Instead, using a voltmeter is the best resort in such situations. So, read on to know how you can use a voltmeter to test the alternator.

  • First, park your car in a safe and stable spot.
  • Open the hood and check the battery first for its charge. Since the battery spins the alternator that, in turn, keeps the engine running, if the battery will have insufficient charge, the test wouldn’t take place. Hence, ensure there is a sufficient charge in the battery before the test.

To check if there is a sufficient charge in the battery or not, follow these steps.

  • Turn off the car and open the hood (if it’s not open).
  • Connect the voltmeter to the battery by connecting its red lead to the battery’s positive terminal and black lead to the battery’s negative terminal.
  • Read the voltmeter reading. If it’s above 12.2V, there is a sufficient charge left in the battery. If the reading fluctuates and reads anything below 12.2V, you’d need to charge the battery.

Now, let’s test the alternator.

  • Start the car and revolve the engine to approx—2000 RPM to kick the alternator to high gear. Keep the engine running and retest the alternator with the voltmeter.
  • If the reading fluctuates between 13V and 14.5V, the alternator is in good shape.
  • However, if the reading shows less or fluctuates between 12V and 13V, the alternator is either in bad shape or discharged quickly.

To confirm whether the alternator is okay or not, test the alternator while keeping the car’s other electrical components, like headlights, radio, etc., in the state.

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What are the symptoms of a failing alternator?

To summarize, an alternator’s job in the car keeps the battery charged, and the battery keeps the car components running.

However, the alternator may exhibit signs of damage. The following are the most common signs of a failing alternator.

The lights are either very dim or overly bright

When the alternator is in bad shape, it will either deliver less voltage or more voltage to the electrical components, say light. Due to this, the lights will either appear dull or overly bright.

In some cases, the lights may even start flickering if the voltage is fluctuating. Or in other cases, the lights will go from bright intensity to dim intensity or vice-versa.

The battery is dead

You know what a dead battery is and what makes a fully functioning battery dead. For example, you left the lights on overnight or left the car unused for over 2-3 months, or due to some faulty internal wiring.

In the simplest of cases, a fluctuating or failing alternator is the reason behind a dead battery. If the alternator is in bad shape, it won’t charge the battery at its optimal rate.

Hence, the battery may discharge faster than it is charging. After some time, when the charge reaches zero, the battery will die, and you’d need to jump-start it.

The various other accessories are malfunctioning

In your car, the alternator provides the required power to keep the various features running like power windows, taillights, headlights, radio, seat warmers, and so on.

If any of these components are behaving abnormally, you’d assume your alternator is failing below its potential.

Car experts suggest that you should refrain from driving with a bad alternator. As a bad alternator won’t charge the battery, the car may come to a stall anytime and anywhere.

Trouble starting the car

If you’re finding it difficult to start the car, your battery is probably short of sufficient charge, i.e., it’s not getting charged from the alternator.

In this case, you’ll either have to replace the alternator or charge the battery manually to start the car.

You get to hear strange noises from under the hood

A car makes many different noises to indicate if everything is okay or there is an issue.

As the alternator is present under the hood, if a strange and loud growling sound comes from under the hood, the alternator is having problems.

It occurs because either the belts that rotate the alternator are misaligned or are rubbing against each other.


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Its alternator charges the battery in an automobile, which powers the car. The alternator is the vehicles’ brain, and similar to the other components of your car, the alternator too can suffer damage or go off.

It results in the occurrence of a multitude of symptoms like the ones discussed above. Hence, you should immediately test the alternator if you feel it’s under-performing.

For this, you can either disconnect the battery or use a voltmeter.

The former method is not a great tactic and may ultimately damage the alternator even if it’s okay. Hence, it would help if you always used a voltmeter to test the alternator.