How to Test an Alternator?

Are you a DIY enthusiast who would like to handle some things yourself? Maybe your car has been giving you alternator problem and you will like to be able to test it yourself.

You are in luck, as this article gives you practical tips and acts as a guide for you on how to test an alternator.

What Is an Alternator?

In case you didn’t know, an alternator is a car component that generates electric energy. This is then used to recharge your car battery and to run your car’s electrical components. You see your car’s windshield, your car’s headlights (as well as the interior lights), the heater and all other electrical components work thanks to the alternator in conjunction with the battery.

What is an car alternator
The alternator is a very vital component of the car. Credit:

Most people confuse a car alternator with the car battery but the truth is that they are different. While the battery does provide electricity for the car’s electrical system, without the alternator to replenish its electric charge, it will run out and die in no time at all.

The alternator is, thus, a very vital component of the car as it converts the engine’s mechanical energy into electrical energy. An alternator is usually located near the front of a vehicle. A lot of vehicles have their alternator mounted to a specific region of the engine via a bracket.

This component generates electricity by means of electromagnetism and is so named because it generates alternating current.

Why Test an Alternator

You should test your alternator so as to know whether it is in perfect working condition or not. This helps you avoid unpleasant circumstances. For instance, imagine driving along a lonely road on a snowy winter night and your car battery fails. Probably due to the alternator having failed and not being able to recharge the battery.

Such inconveniences are definitely things you would wish to avoid which is the reason why you should test your alternator when you feel like it has a problem or when you see a sign of a developing fault.

The following are the reasons why an alternator might fail:

  • The carbon brushes might fail
  • The contact rings might wear out due to heat and use
  • The voltage regulator might be faulty
  • The alternator bearings (both interior and exterior) might be bad

If any of these becomes faulty, your alternator will not be able to work efficiently again which will lead to it failing and you being at risk of a major inconvenience.

Tools for Testing an Alternator

There are two tools that can be used to test an alternator. One is the battery and charging system tester while the other is the voltmeter.

Tools for Testing an Alternator: multimeter, voltage regulator

The battery and charging system charger can detect whether there is a problem with the charging system of a vehicle. Using this tool shows you whether the charging system is weak or has completely failed. It can also determine whether any of the diodes that drive the alternator has failed or if all is fine.

The voltmeter, on the other hand, tests the voltage of the car’s battery when the engine is on and running and when it is of. This allows you to know whether or not there is a problem with the car’s charging system. The voltmeter test is also known as the load test.

How to Test an Alternator (Step by Step Guide On How to Check Your Alternator with A Voltmeter)

The following are concrete tips and steps on how to test a car alternator using a voltmeter:

  1. Buy or get a voltmeter at the local electronics store;
  2. Choose the DC option on the voltmeter as opposed to the AC. This is because the battery uses DC and that is what you will be testing.
  3. Connect the red wire of the voltmeter to the positive terminal of the battery;
  4. Next, connect the black wire to the other terminal which is the negative terminal;
  5. Ignite the vehicle while ensuring that all of the accessories are off;
  6. Raise the RPM to about 1000;
  7. Observe the voltage registered on the voltmeter;

If the voltmeter registers a voltage of about 14 volts, your vehicle’s alternative is fine. If on the other hand, the voltage registered is less than 12, you may have a problem on your hands.

How to test an alternator with a voltmeter infographic

After reading the registered voltage:

  1. Turn on your car accessories such as the headlights, the air conditioner, the heater and others that rely on electric power to work.
  2. Rev the car engine and just like before, observe what the voltmeter reads.

If again the voltmeter shows a reading of about 14 volts, then, your car is safe. Anything below 12 volts and you should start considering getting a new alternator or rebuilding the one you have.

Warning signs of bad alternator or bad battery

Every vehicle has a battery warning light on its information panel which usually comes on when the engine is ignited. This battery warning light automatically goes off when the engine starts. In the event that it persists even after the engine has started, then, there is a problem with the vehicle’s charging system.

This translates to a fault with the alternator or battery.

How to Know If It’s Battery or Alternator (Bad Alternator Vs Bad Battery)

Having a bad battery or bad alternator can be really vexing especially if you can’t tell which one it is. The following will help you figure out whether your car has a battery problem or an alternator problem.

When the following happens, you definitely have a bad battery:

Bad Battery symptoms:

  • The car engine is not very responsive and/or starts inconsistently.
  • No accompanying light and/or sound when you attempt to start your vehicle.
  • The vehicle does not seem to have any problem when you jumpstart.

The following point to a bad alternator: 

Bad alternator symptoms:

  • The car engine does not start upon ignition.
  • Vehicle stalls while driving.
  • The car’s headlights as well as its dash lights dim when the vehicle is idle but bright when the engine is revved.

Bonus: How to change spark plugs? Easy Steps and Tips

4.8/5 (19 Reviews)