How to Test Alternator by Disconnecting Battery – Guide For Drivers

In this short guide, I’ll show you how to test alternator by disconnecting battery and running the engine.

Loss of power and an inability to crank the engine running are two common signs of a bad alternator. 8 times out of 10, your car won’t start because of a faulty car alternator. Having to take your car to the garage is expensive and time-consuming.

We will show you how to test your car alternator using simple methods that really work.

So gather your stuffs, organize your tools, prep everything, and let’s do this!

What is an alternator?

First of all, let’s talk about what an alternator is, and how it works.

The alternator is a device that converts electricity into direct current, which gives enough power to your car’s electrical systems. In addition to charging the battery, the alternator generates voltage to power the car’s electrical systems.

Methods for checking a faulty alternator

If you have a bad alternator failure, then your engine running won’t turn over when you crank it. The car will also not start, because there isn’t enough voltage to turn on the ignition coil. There are three ways to test your broken alternator:

  1. With a millimeter (which measures voltage),
  2. A hydrometer (which measures water pressure), or
  3. By watching for sparks from wires connected to your battery terminals.

How to know if your alternator needs replacement?

To check for voltage in your vehicle, connect a voltmeter across both terminals on your battery’s negative terminal (the one closest to the engine). If there is no voltage present on either terminal, then your alternator is bad and needs replacement.

If your car is having trouble starting, you may want to get the battery checked under the car hood. If your battery is good, but your alternator isn’t charging it fully, it could be a sign of a bad alternator.

Reasons for bad alternators

When the alternator fails and gets damaged so you cannot trust them for any longer. It is possible that the alternator involves several problems that may be caused by this.

If you’ve ever had a bad alternator, you probably know that it can be a real headache. Your car might not be able to start or run properly and you might need to get a new alternator. But what’s the deal with alternators? What are the common causes of them breaking down?

Here are some of the most common causes of bad alternators:

  • Dirty or corroded battery cables
  • Overcharging your battery for extended periods
  • Bad or weak wiring in the car
  • Poor maintenance of the alternator

How to test an alternator: [Step by Step Guide]

Steps: How to test alternator by disconnecting battery

Before you disconnect the battery you can verify that it is not broken or loose or damaged electrical connections wiring or any loose serpentine cables. Once you are sure that the fault does not originate with an electrical energy component and the alternator works correctly, then you can test alternator by disconnecting battery terminals.

Here’s how to test alternator:

  1. Park on level ground with a good view of the road ahead to safely test it.
  2. Turn off the hot engine ignition and front lights.
  3. Disconnect your negative battery cable from the battery terminal first by loosening it with a wrench while keeping hold of it with one hand
  4. Remove the positive terminal and connect it directly to the car’s engine running block or frame.
  5. Turn on the ignition switch and check for battery voltage between the black/white wires on the harness connected to your dead battery terminals.
  6. Then turn off and reconnect all the car accessories that weren’t on when you disconnected the negative battery cable from your battery (such as air conditioning).

How to test an alternator with a multimeter?

If you’re a car owner, you know that an alternator test is a must. But how do you test alternators using a multimeter?

Here’s how:

  • Start your car and let it run for a few minutes. This car running will help warm up your car battery enough to start charging it up.
  • Set your multimeter to AC volts, and set the range to 20-100 volts DC.
  • This will give you enough voltage for testing purposes, but if you want to make sure everything is working properly, go up to 30 volts DC or even greater!
  • Connect the red lead of your multimeter to one battery terminal on the alternator (positive terminal) and the black lead to the negative terminal.

If something is wrong with your alternator, it should read zero volts when connected this way during the alternator test; if not, then there may be some other battery problems with the converting mechanical energy—maybe even something as simple as a loose connection or bad battery wiring affecting the alternator’s health!

If this happens when connected this way, don’t worry: just disconnect everything and try again later or call in an expert or a tow truck who knows what they’re doing!

Why is this method of negative battery cable unsafe?

The tester’s alternator can easily be tested with the removal of the car battery cord. But it’s risky because what’s going on? Experts are worried about its effectiveness. These methods work for generators, but alternators have no generator.

If the alternator is used with the method described above to get rid of the charger negative cable, there is a considerable chance of causing it to die.

Common symptoms of a bad alternator

Check several confirming signs of alternators and test them. We believe that this issue can arise at different points in history.

You shouldn’t waste time on the basis of not gaining early warnings that the condition will definitely deteriorate. The following list of some common indicators for alternator failure.

Here are some common symptoms that indicate your alternator might be acting up:

  • Gas mileage drops significantly
  • Noisy electrical noises
  • Flickering dashboard lights or headlights
  • Vibration in the steering wheel when the car stalls or accelerates
  • Shifting gears harder than normal

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

Luckily, there are some easy ways to test to see if your alternator is bad:

1. Check for voltage at the negative battery terminal.

You can check the alternator by disconnecting battery cable from its battery terminal and connecting them again.

This will give you an idea of your battery charged or if there’s a voltage signal being sent from one terminal to another (which means your alternator is working).

2. Turn on your headlights.

If they don’t turn on when you turn the key in your ignition (or if they flicker), that means there’s no voltage being sent from the negative battery terminal to where it needs to go—which means your alternator might be bad!

Which car battery terminals should I disconnect to test the alternator?

Typically car batteries have two terminals: positive and a negative battery terminal. Batteries terminals typically sit on top of batteries and can often be found on the sides.

The battery test can be performed using the negative battery cable when starting the car’s engine. If you disconnect the battery it could be causing problems.


Can you disconnect battery to check alternator?

Yes, you can disconnect the battery to check the alternator.

What happens if you disconnect the battery while the car is still running?

The first thing to remember checking the alternator by disconnecting your car’s battery will cause the car’s electrical system to shut down.

So it’s important to remember that disconnecting the car battery does not mean simply unplugging it; instead, you must also unplug its connection from the alternator. If you don’t do this correctly, there’s a risk of fire or explosion.

How can I tell if my alternator is bad without a multimeter?

To test a car’s alternator, start the car engine and let it idle for a minute or so. Then move forward and remove the negative battery cable terminal from your car battery. If the car turns off, it means that you have a defective alternator.

Video: How to Test Alternator by Disconnecting Battery

In this video, you can watch a simple and easiest way (although not recommended by professional mechanics or technicians) to test to see if your alternator is bad.


So there you have it, folks! You now know everything you ever wanted to know about alternators! The causes, symptoms, and testing procedure for a failing alternator. You’ll be able to tell if your vehicle’s alternator is failing by using these steps.

Please comment below if you have any further questions or concerns—we would be happy to help! Here is how to reset outside temperature gauge in car.

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