Alternator Test Guide

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

I have made this simple guide for you which shows how easy testing is this system using a voltmeter, even if you don't have a meter you can still see if the alternator is charging by observing the headlights. 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 while starting the engine. The brightness level should momentarily fade and then get brighter than before as the engine is running telling you the alternator is charging. If you still cant tell, you will need a voltmeter which and can get from Amazon for about $25.00 bucks.

Please watch the following video and then when done read the guide below to pick up on additional information the video may have missed, the following information pertains to all internal combustion engines vehicles.

Anytime you have a battery warning light or you have just replaced your vehicles charging system or battery you should perform a voltage output test. Additional reasons for testing include radio buzzing noise through the speakers signaling the diodes have gone bad while leaking AC voltage into the system which I will explain testing for later on in this article.

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, service engine soon warning lights to come on as well.

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 the main computer. Removing the battery cable to test the charging system was used for vehicles made before 1976 in today's vehicles the battery is used a kind of electrical shock absorber which keeps the system stable and if you remove a battery cable it will cause a voltage spike and damage major electrical components.

If the alternator has failed you can pick a factory OEM unit such as AC Delco, Motorcraft or Nippon from Amazon for about the same wholesale price shops pay for them which can save you over $100.00 not to mention saving on labor costs by changing it yourself.

Supplies Needed

  • Voltmeter
  • Protective gloves and eye wear

Begin your testing with the vehicle on level ground, engine off and in park, with the emergency brake set.

Step 1

A serpentine belt is used to turn the stator which creates the voltage needed, if this belt is worn or loose this will make the voltage supply 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 belt tensioner which may have a broken spring that needs to be replaced. Learn more

Step 2

Switch your voltmeter on to 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 like mine it's a good idea to open it up and change the battery. This will help the meter read more accurately and make it ready for testing.

Step 3

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.

Step 4

Lift the hood on your vehicle and locate the battery to 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.   

Step 5

Now the voltmeter should read battery voltage @ about 12.4 to 12.6 volts. If the battery is failing or down on charge these readings will be lower anywhere from 6.5 to 10.8 volts, if so the battery will need to be load tested or charged.

Step 6

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 assembly is not charging. Check the fuse panel for blown fuses and inspect the wires in the harness which lead to the rear electrical terminals. If everything checks out the unit is bad and needs replacing.

See Also: Alternator Replacement

Step 7

The next step in testing is to load test the unit 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 make the assembly perform to its maximum output. If the voltage starts to drop the unit is weak and needs replacement.  

Testing for AC Voltage

When diode packs fail it can leak AC voltage which can cause disruptions in the electrical system including battery failure and other electrical components 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 unit should be replaced.

Symptoms of a Charging System Failure

  • Dead battery
  • Battery warning light
  • 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 if it is older than three years. Learn more

Avoid unnecessary sparks near or on the battery if possible to stop accidental ignition of hydrogen gases present inside the battery causing it to explode, especially when you can smell sulfur causing hydrogen gases.

If you have need more information about his subject please visit our forum where thousands of questions have already been answered by our online mechanics. Learn more


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