How to Test Alternator by Disconnecting Battery?

The alternator is one of the most important components of a car. It serves as the brain of the human body. In addition, the alternator of the car runs on the power developed by the vehicle’s engine.

When the alternator starts to malfunction on a trip, it is likely to produce some devastating results. Just like other parts of the vehicle, the alternator tends to break down from time to time.

A broken alternator brings challenges and problems. Therefore, you should take care of the alternator’s health to ensure the smooth running of the vehicle. This way, you will be able to continue driving safely.

It is always requested to check the health of the alternator from time to time. There are several methodologies to test this part of the car.

The most popular way to test the car alternator is by disconnecting the car battery. How to test alternator by disconnecting battery? The following article will discuss the most important points in this context.

Common Symptoms of a Bad Alternator

Before you start testing the alternator, look for various confirmation indicators. Because the problem is always conceivable in different places.

You don’t want to waste time if you don’t get any early indications, as it will certainly show signs of deterioration.

Listed below are some of the most typical signs of alternator failure:

1. Dim lights

The defective alternator symptom that most drivers recognize is dim or flickering lights.

This is really obvious when the headlights are on at night, but it’s also noticeable in the dash lights and interior center light.

If the lights illuminate as the RPM increases, it’s an even safer sign.

2. Service engine light

Another obvious signal is what the vehicle is trying to tell you through the dashboard.

This varies by model, but when the alternator starts to let go, it may light up a “Check Engine,” or battery icon indicator.

3. Strange noises

The serpentine belt may have stretched with age and may not turn the alternator pulley effectively, resulting in a lack of load.

This usually results in a squealing noise.

On the other hand, the internal bearings may wear out, causing a growling or grinding noise.

4. Electrical problems

Electrical equipment in the vehicle, such as power seats or windows, may operate sluggishly.

Without enough power, sometimes the device won’t work or will suddenly stop working, like the radio shutting off.

5. Engine shutdown

If the engine suddenly shuts off while driving, it could be an alternator problem.

Fuel injection needs a good amount of electrical power, and without it, the engine stops quickly.

6. Dead battery

Without a working alternator, the vehicle quickly consumes the battery’s full capacity, leaving it dead.

The hood should be opened and the battery checked.

A new battery and a malfunctioning alternator will leave the vehicle stalled again.

How to Test Alternator by Disconnecting Battery

Testing the alternator of the car by disconnecting the battery is a simple process. Here is your guide in that regard:

Step 1: You should always conduct the test in an open and safe place. For example, you can utilize the driveway for making this test. Before you start, ensure that children and pets are at a safe distance.

Step 2: You should check several times to ensure that the vehicle is in a stable position. If you are satisfied with its position, the testing starts with opening the car hood.

Step 3: Now get to the driving seat.

Step 4: Once you are in the driving seat, you should initiate the car engine. In case there are issues with the car alternator, you will face some issues in starting the car.

Step 5: After the car engine starts, you need to come out of the driving seat.

Step 6: At this stage, you need to disconnect the car battery while the car is still in a running position.

Step 7: Check if the car can run as usual.

Why This Method Is Not Safe?

how to test alternator by disconnecting battery

Actually, it is very easy to test the alternator by removing the battery cable. But this is very risky because where is the problem?

Experts today have shown concern about the effectiveness of this method. This method was effective for generators, however, an alternator is not a generator.

So, when you try an alternator with the method described above (removing the battery cable), it considerably increases the possibility of killing that regulator, and that is the risk.

Now about its ineffectiveness

Well, first of all, the vehicle will work if the alternator produces enough energy.

But, there is also the possibility that the alternator is in poor condition, but produces the least amount of energy to run the engine. As a result, it will give a false positive indicating that the alternator is fine when it is not.

How To Test An Alternator With A Multimeter

The ideal testing process for the alternator involves the application of a voltmeter. It is a much effective and completely safe process to test the alternator for getting accurate results.

Step 1: You must bring your car to a safe position before the test begins.

Step 2: Before you get started, you must have a multimeter handy. Now, bring the vehicle in running condition. You must ensure that you have put off all the car accessories before the testing starts.

Step 3: It would help if you used a voltmeter to get the reading from in between the car batteries.

Step 4: Now, you need to put on all the car accessories. Consequently, take a send reading from in between the car batteries with the voltmeter.

Step 5: To suggest that the alternator is in the right condition, the first reading must give a score of 14 Volts.

The second reading should display at least 13 volts. In case you find some other readings, it is a clear indication of troubles with the car alternator.

It is always better to opt for complete field testing to get the most reliable test result. Even if such testing is time-consuming and intricate, it delivers the most reliable result.

It will make it easier to get an exact overview of the health of the car alternator. Consequently, you can make the most suitable remedial measures as needed.


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

Disconnect the negative battery cable before testing an alternator. Instead, use a voltmeter to perform a basic and safe test.

How do I test to see if my alternator is bad?

First, open the hood and start the engine. Remove the negative battery cable with the engine running.

The alternator is likely to be damaged if the car stalls or stalls. This is because the alternator does not produce enough electricity to keep the engine running on its own.

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

It is determined according to the difficulty of the vehicle and the nature of the activity. However, in most situations, it ranges from one to three hours.

What does a bad alternator sound like?

If there is rumbling or friction, it is most likely due to a defective bearing. On the other hand, if there is a buzzing sound, the regulator may be broken.

What is the cause of alternator failure?

The most frequent cause of alternator failure is old age. A normal alternator will last 5 to 8 years before showing signs of deterioration.

A defective battery can accelerate alternator failure.
A defective battery requires a higher alternator charging current. It is not intended to do so, and the additional heat created can cause the alternator to fail.

How much does it cost to fix the alternator?

The cost of an alternator varies depending on the vehicle. A small alternator may cost about $100, but a large, powerful alternator can cost hundreds of dollars.

The cost of replacement also varies, as some cars are simple to replace. Unfortunately, in other vehicles, it can be difficult and require the disassembly of other components.

The average price per car is expected to be between $150 and $200.

How can I use the test light to check the alternator?

Examining the internal lights is a simple method of discovering possible alternator faults.

When the alternator starts to fail, the interior or dashboard lights may start to flicker.

This is a clear sign that the charging circuit is not functioning properly and should be inspected.

Final Thoughts – How to Test Alternator

You now understand how to test alternator by disconnecting the battery. You now know what the signs of alternator failure are, why this test should be avoided, and what the alternative tests are.

Our main goal is to help you keep your car in excellent condition. Therefore, even if you ask about this specific test, we advise you to avoid it.

Whatever type of test you take, don’t forget to take precautions beforehand. Do not postpone repairs if you have a problem with the alternator. More specifically, entrust the car to a professional.

Liam Dare

As CEO of, my passion for the automotive world motivates me to build online businesses that provide information and entertainment to users. I am proud to contribute in a positive way to the automotive community.