How to Test an Alternator

Need to test your alternator? We are a team of ASE certified mechanics that have created this guide for you so you can save money or at least see what you are paying for when taking your car in for repairs. This simple guide shows you how easy testing an alternator is when using a voltmeter. Even if you don't have a meter you can still see if the alternator is charging by observing the headlights which is described at the bottom of the guide.

What Goes Wrong?

Your alternator is responsible for charging your vehicles battery and keeping the electrical system energized when the vehicle is in operation. When it fails you will slowly lose electrical power until the engine quits running due to insufficient voltage that operates the ignition system which can leave you stranded.

Anytime you have a battery warning light on or you have just replaced your alternator or battery you should perform a voltage output test. Additional reasons for testing include a buzzing radio noise through the speakers signaling the alternator diodes have gone bad and are leaking AC voltage into the system. We will explain how to test for this later on in the guide.

If the battery is being overcharged it will cause a chemical reaction that smells like sulfur letting you know the voltage regulator has failed allowing the system to be overcharged. When an alternator fails not only will the battery light come on but it can cause the check engine or service engine soon warning lights to come on.

Never disconnect the battery while the engine is running to check the charging system. This can cause a major electrical system malfunction such as short circuit to the main computer. Removing the battery cable to test the charging system was used for vehicles made before 1976 and not today's vehicles. The battery is used as an electrical shock absorber which keeps the system stable. If you remove a battery cable it will cause a voltage spike that can damage major electrical components like the computer.

Getting Started

Begin your testing with the vehicle on level ground, engine off and in park with the emergency brake set. Though you can see if the alternator is changing without a voltmeter it will need to be done at night using the headlight. Its best to have a voltmeter which is available from Amazon for about $25.00 bucks if you don't have one already.

A serpentine belt is used to turn the alternator stator which creates the voltage needed. If the belt is worn or loose it will make the voltage supply from the alternator weak and may also make a loud squealing noise when the engine is accelerated. Reach down and check the belt tension which should be taught. If the belt is loose check the serpentine belt tensioner which may have a broken spring that needs to be replaced.

Let's Start Testing!

You will be testing the alternator at the battery while the engine is running so wear gloves and protective eye wear to be safe.

Step 1: Preparing the Voltmeter

Switch your voltmeter onto the V position of DC current testing which is shown by a straight line with a broken line below it. Allow the meter to find it's reading of .000 volts. If the meter has been around for a while it's a good idea to open it up and change the battery. This will help the meter read more accurately and make the meter ready for testing.

Locate the battery which is under the hood in most cases but some car's have them under the back seat or in the trunk. If you have a set of alligator clips that came with your meter you can change them out for the standard point probes which make testing easier but are not necessary. You can simply hold the test probes to each on of the battery terminals during the test.

Identify both the negative (black) and positive (red) battery terminals. If both are either red or black reference the battery itself for markings depicting the polarity + or - . Next, attach the voltmeter leads or simply hold the leads onto the battery terminals.

The voltmeter will then read the battery voltage @ about 12.2 to 12.6 volts. If the battery is down on charge these readings will be lower and can be anywhere from 6.5 to 10.8 volts. If so the battery will need to be load tested, charged or replaced before testing can begin.

Step 2: Testing the Alternator Voltage Output

Have a helper start the engine and hold the RPM's just above idle at about 1500 and observe the meter. It should read between 13.6 and 15.8 volts depending on battery condition and state of charge. If the voltage on the meter stays the same or drops when the engine is started the alternator is not charging. You should then check the fuse panel for blown fuses and inspect the wires in the harness which lead to the alternator. If everything checks out the alternator is bad and needs replacing.

The next step in testing is to load test the alternator to see if it can hold up under heavy usage. While the voltmeter is still hooked up and the engine still at a raised RPM turn the headlights and air conditioner on. This will demand an electrical system load which will force the alternator to perform to its maximum output. If the voltage starts to drop the unit is weak and needs replacement.

Step 3: Diode Testing

When the alternator diode pack fails it can leak AC voltage which can cause disruptions in the electrical system including causing premature battery failure. It can also cause other electrical components such as the radio and gauges to act strangely. To test for this condition switch the voltmeter to AC voltage with the engine at about 1200 RPM, the meter should read 0 volts. If voltage is present one or more diodes has failed and the alternator should be replaced.

Please watch the following video below to watch the test being performed by one of our mechanics. The following information pertains to all internal combustion engines vehicles.

Additional Testing Without a Voltmeter

For this test you will need to wait until evening so you can see the headlight brightness which is best at night. Then turn the vehicle headlights on with the engine off and then observe the headlight brightness, then start the engine. The brightness level should momentarily fade as the engine cranks but then get brighter than when the engine was not running telling you the alternator is charging. If you still cant tell, you will need a voltmeter. This test will not work for LED headlights.

Symptoms of a Charging System Failure

  • Dead battery
  • Battery warning light on
  • Engine cranks over slowly followed by a loud clicking noise
  • Engine stalls while driving
An alternator cannot sustain maximum output for long periods of time, it will overheat and fail. A shorted or old battery can cause premature failure of the charging system so it's a good idea to replace the battery at the same time as the alternator if it is three years or older. Avoid unnecessary sparks near the battery. This is to stop accidental ignition of hydrogen gases present inside the battery causing it to explode. This conditioner is typically present when you can smell sulfur causing hydrogen gases.

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Got Any Questions?

If you need more information please visit our forum where our mechanics have answered thousands of alternator questions or ask one of our mechanics, we are happy to help.

Article first published