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
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 in the evening or night
time which I will explain further down in this guide.
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.
Protective gloves and eyewear
Begin your testing with the vehicle on level ground, engine off and in park, with the emergency
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.
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, now the meter is reading for testing.
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
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.
Now the voltmeter should read battery voltage @ about 12.4 volts. If the battery
is failing or down on charge these readings will be off, if so the battery will
need to be load tested.
Have a helper start the engine and hold the RPM's just above idle 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.
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 idle turn the headlights and air conditioner on. This will command a
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.
If a voltmeter is not available, wait until
you can see the headlight brightness which is best in the evening. 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 the engine was running telling you the alternator is charging.
Testing for AC Voltage
When diode packs fail
it can leak AC voltage which can cause disruptions in the electrical system including
battery failure. 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
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
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 experts.