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It's 11:45 p.m. and you just finished working a double shift. It's late, you're tired and you have

lass in the morning. !s you fumble for your ar keys, you an't help but shiver. It's about

to snow.

"ou hop in your #onda $ivi and rank it up. %he engine turns over grudgingly but eventually

athes and fires. !fter warming up the engine, you pull out and begin your half&hour journey

home. ust as the heater begins to remove the frosty onditions inside the ar, the radiobegins

to ut out. (oon after, thedash light starts to dim ever so slightly. "ou keep driving but the

anomalies with your ever&reliable ar pile up.

)ventually, the radio and the heater *uit altogether, and the omfortable warmth is slowly

replaed by the outside hill. "our headlights are the ne+t thing to start going. ow you're getting

worried. -nly 15 more minutes and you're home.

%hose ne+t 15 minutes never ome. %he headlights dim to the point of dangerous, and theunthinkable happens: %he engine starts to miss. ess than five miles /0 kilometers from home,

your engine dies, along with everything else in the ar. "ou oast to a stop on the side of the

road and pull out your  ell phone. It's dead, so you plug it into the igarette lighter so you an

all for help. 2uess what3 o power. It's past midnight, and you're stranded on the side of the

road with snowflakes for ompany.

hat you've just e+periened is an e+ample of an alternator breathing its last breath. "our first

thought might have been the battery is

dying. In a sense, you would be right beause

the battery and the alternator are related, but

the battery tends to get all the press. %his artile

aims to e+plain the mehanis of alternators,

how you an diagnose problems and

what you an do if you have a bad alternator.

ead the ne+t setion to learn some bakground

information about alternators and the war of the

urrents.

This diagram shows the diodes, rotor assembly and stator, all of which you wouldn't normally see withoutopening the alternator. You'll hear more about them in a little bit.

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What is an alternator?

 !n automotive harging system is made up of three major omponents: the battery, thevoltage

regulator  and analternator . %he alternator works with the battery to generate power for the

eletrial omponents of a vehile, like the interior and e+terior lights, and the instrument panel.

 !n alternator gets its name from the term alternating current (A!.

 !lternators are typially found near the front of the engine and are driven by the rankshaft, 

whih onverts the pistons' up&and&down movement into irular movement. /%o learn more

about the basi parts of ar engines, read #ow $ar )ngines ork. (ome early model vehiles

used a separate drive belt from the rankshaft pulley to the alternator pulley, but most ars

today have a serpentine belt, or one belt that drives all omponents that rely on rankshaft

power. 6ost alternators are mounted using brakets that bolt to a speifi point on the engine.

-ne of the brakets is usually a fi+ed point, while the other is adjustable to tighten the drive belt.

 !lternators produe !$ power through electromagnetism formed through the stator and rotor

relationship that we'll touh on later in the artile. %he eletriity is hanneled into the battery,

providing voltage to run the various eletrial systems. 7efore we learn more about the

mehanis of the alternator and how it generates eletriity, let's look at the various parts of an

alternator in the ne+t setion.

 You can spot the alternator (with the tag hanging off it! in the foreground of this shot of a stoc" car engine.

 You can also see the serpentine belt looped around it.

Alternator omponents

8or the most part, alternators are relatively small and lightweight. oughly the si9e of a oonut,

the alternators found in most passenger ars and light truks are onstruted using an

aluminum outer housing, as the lightweight metal does not magneti9e. %his is important sine

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aluminum dissipates the tremendous heat generated by produing the eletrial power and

sine therotor assembly produes a magneti field.

If you losely inspet an alternator, you'll find it has vents on both the front and bak side. !gain,

this aids in heat dissipation. ! drive pulley is attahed to the rotor shaft on the front of the

alternator. hen the engine is running, the rankshaft turns the drive belt, whih in turn spins

the pulley on the rotor shaft. In essene, the alternator transfers the mehanial energy from the

engine into eletrial power for the ar's aessories.

-n the bak side of the alternator you'll find several terminals /or onneting points in

an eletrial iruit. et's take a look at those:

• # terminal & (enses battery voltage

• $% terminal & Ignition swith that turns the voltage regulator on

• & terminal & $loses the iruit to the warning lamp

• terminal & 6ain alternator output terminal /onneted to the battery

• terminal & 8ull&field bypass for regulator 

$ooling is essential to an alternator's effiieny. It's easy to spot an older unit by the e+ternal fan

blades found on the rotor shaft behind the pulley. 6odern alternators have cooling fans inside

the aluminum housing. %hese fans operate the same way, using mehanial power from the

spinning rotor shaft.

 !s we start to disassemble the alternator, we find the diode rectifier  /or rectifier bridge!,the voltage regulator , slip rings and brushes. %he regulator distributes the power the

alternator reates, and it ontrols the output of power to the battery. %he retifier bridge onverts

the power, as we'll learn in the ne+t setion, while the brushes and slip rings help ondut

urrent to the rotor field winding, or wire field. ow let's rak the oonut open.

-pening the alternator reveals a large ylinder with triangular finger poles around the

irumferene. %his is the rotor. ! basi alternator is made up of a series of alternating finger

pole piees plaed around oil wires alled field windings that wrap around an iron ore on the

rotor shaft. (ine we know the pulley attahes to the shaft, we an now visuali9e how the rotor

spins inside the stator. %he rotor assembly fits inside the stator  with enough room or tolerane

between the two, so the rotor an spin at high speeds without striking the stator wall. -n eah

end of the shaft sits a brush and a slip ring.

 !s we touhed on briefly, alternators generate power through magnetism. %he triangular finger

poles fi+ed around the irumferene of the rotor are staggered, so the north and south poles

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alternate as they surround the wire rotor field windings. %his alternating pattern reates the

magneti field that in turn indues voltage into the stator. %hink of the stator as the ather's

glove as it harnesses all the power reated by the spinning rotor.

 !ll these omponents work together to give us the power we need to run our vehiles. %esla

aptured this eletrial energy and used it to light up ities, but we only need enough volts to

power our stereo, lights,windows and loks. et's take a look at how the alternator produes that

power in the ne+t setion.

This is what you might see if you were to open your alternator.

)nderstanding Alternator *ower +utput

In the early days, ars used generators rather than alternators to power the vehile's eletrial

system and harge the battery. %hat's not the ase anymore. !s automotive tehnology evolved,

so did the need for more power. 2enerators produe direct current, whih travels in one

diretion, as opposed to thealternating current for the eletriity in our houses, whihperiodially reverses diretions. !s %esla proved in 100, alternating urrent beame more

attrative as it generates higher voltage more effiiently, something neessary in ontemporary

automobiles. 7ut ar batteries an't use !$ power sine they produe ;$ power. !s a result,

the alternator's power output is fed through diodes, whih onvert the !$ power to ;$ power.

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%he rotor and the stator are the two omponents that generate power. !s the engine rotates the

alternator pulley, the rotor spins past three stationary stator windings, or wire oils,

surrounding a fi+ed iron ore that makes up the stator. %his is referred to as a three

phase urrent. %he oil windings are evenly spaed at intervals of 1<= degrees around the iron

shaft. %he alternating magneti field from the rotor produes a subse*uent alternating urrent inthe stator. %his !$ urrent is fed through stator leads into a onneting set of diodes. %wo

diodes onnet to eah stator lead to regulate the urrent. %he diodes are used to essentially

blok and diret the urrent. (ine batteries need ;$ urrent, the diodes beome a one&way

valve that will only allow urrent to pass in the same diretion.

Threephase alternators have three sets of windings> they're more effiient than a single&

phase alternator, whih produe a single&phase !$ urrent. hen working properly, the three

windings produe three urrents that make up the three phases. !dding all three together

produes the total !$ output of the stator.

%he two basi stator winding designs are delta wound and wye style. ;elta wound are easily

identifiable by their shape, as they're triangular. %hese windings allow for a high urrent flow at

lower ?6. ye windings resemble the flu+ apaitor seen in @7ak to the 8uture.@ %hese

windings are ideal for diesel engines, as they produe higher voltage than delta stators at even

lower ?6.

 !fter the !$A;$ onversion, the resulting voltage is ready to use in the battery. %oo muh or too

little voltage an damage the battery, as well as other eletrial omponents. %o ensure the

orret amount, a voltage regulator determines when and how muh voltage is needed in thebattery. -ne of two types of regulators are found in most alternators: %he grounded

regulator  works by ontrolling the amount of negative or battery ground going into the winding

in the rotor, while a grounded field type works the other way around & by ontrolling the

amount of battery positive. either poses an advantage over the other.

ith so many omponents working to reate the eletriity vital for our vehiles, it's safe to say

the alternator is a ruial omponent under the hood. 7ut like many parts on our ars, they fail.

%he ne+t setion will give you an idea of how to determine if you are about to be stranded and

what you an do if you need to replae your alternator.

The &ife and -eath of an Alternator 

 !s we saw in the beginning of the artile, a failing alternator will kill a battery and ruin your day.

7ut why did the alternator fail in the first plae3 !lternators have moving parts, get dirty and are

subjet to stress from heat and old. !s a result, the internal parts gradually wear out.

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-ne of the most ommon failures is bearing failure. %he needlebearings that allow the rotor to

spin freely inside the stator an break down from dirt and heat. hen the bearings fail, the rotor

will not spin effiiently and an eventually sei9e. Bsually an alternator with bearings failure

makes a loud grinding noise. If you suspet this problem, it's only a matter of time before the

alternator gives up. -lder vehiles with generators tend to re*uire muh moremaintenane thannewer models, but there's no hard and fast rule for how long an alternator will last. It varies from

manufaturer to manufaturer.

"ou an take several easy steps to diagnose whether your alternator is on the frit9. 8irst, most

ars today have a dashboard light that glows when the ignition is swithed on. %his light usually

is represented with a symbol of a battery. #ave you ever heard a bu99ing noise when the key is

on, but the ar isn't running3 %hat's the voltage from the battery running through the harging

system. If this bulb is burned out, hanes are the alternator won't work. %he ar may start, but

as we learned, it's just a matter of time before the battery drains and the eletrial system fails.

 !s a rule, a three&phase alternator an operate with only one of the stator windings operational,

although it's only one&third as effiient. %o test whether your ar had an issue with one of its

stator windings, you'd need to use a voltmeter to hek the voltage. /"ou an buy a basi

voltmeter at an eletronis store. %his is alled a load test. (ine the battery produes ;$

power, set the voltmeter to ;$ rather than !$. $onnet the red lead /or wire to the positive

terminal and the blak to the negative. ith no aessories on, start the ar and raise the ?6

to around 1,===. %he voltage should register around 14 volts. !nything less than 1< may

indiate a problem. e+t, turn on the headlights, air onditioner, radio and anything else that

draws eletrial power. ev the engine and hek the voltmeter. !gain, the voltage shouldregister around 14 volts. If you have a failing alternator, the voltage will be well below 14 volts. If 

so, it's time to replae the alternator.

7efore you deide you need to yank the alternator and replae it, make

sure you hek the serpentine belt. If the belt is worn or loose, the

alternator won't funtion properly. ! bad belt is easy to replae and

won't set you bak muh, usually less than C<=. 7ut if you have to

replae the alternator, you have options. ead on to learn how to go

about replaing an alternator and what it may ost in the ne+tsetion.

  A voltmeter li"e this could help to figure out if your alternator is on the frit.

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 !lternator eplaement

8or the most part, alternators are less e+pensive than say, a power steering pump or air

onditioner ompressor. evertheless, you have alternatives to forking out a lot of ash for a

replaement alternator. 6any automotive stores sell remanufatured or rebuilt alternators at adisounted prie. 8or instane, a 7ekA!rnley alternator for a <==< #onda !ord with a D&liter

EF engine an ost C<<G new, versus C1FD for the same part, only remanufatured

Hsoure: ?arts!meria.

hile alternators are relatively ine+pensive in omparison to some of the other omponents on

your engine, some an be e+pensive depending on the ar make. If you owned a <==5 ?orshe

G11 $arrera 2% for instane and wanted to replae the original 7osh alternator, you ould

e+pet to pay well over CD==, and that's for a remanufatured omponent Hsoure: !utoparts

arehouse.

(ometimes alternators are easily aessible and simple to replae for the amateur mehani.

ith a modest amount of automotive e+periene and the proper tools, replaing an alternator in

your garage an be done. 7ut more and more ars don't have room under the hood, and the

alternators an be diffiult to reah without first removing several other omponents. In this

ase, it's best to take your ar to an e+periened tehniian who an do the job *uikly.

If you happen to own an alternator that has a repair kit available for sale, you an really save

some money. !lternator repair kits run between C1< and CD=, depending on whih omponents

you need to fi+. !gain, you need the proper tools and a little know&how, but if you're able to findthe right kit and know what you're doing, you an rebuild an alternator for a fration of the ost

of even a remanufatured unit.

-ne thing is ertain: ! bad alternator will ruin a good battery if you don't address it *uikly.

7atteries an only be reharged so many times before they'll lose their ability to hold a harge.

8or the most part, if the battery isn't relatively old, it should survive. 7ut an older battery that is

onstantly drained and harged, drained and harged will have a shorter life span than a battery

operated under normal onditions. %he average life span of a battery is usually around 40

months Hsoure: 7attery (tuff .

#igh&output alternators have beome popular with ustom ar shops and in high&performane

ars. %hey produe higher voltage at idle and make more power for vehiles that draw an

e+essive amount of eletriity. 8or e+ample, ustom ars with lavish audio and video systems

draw more eletriity than the stok alternators an handle. !s a result, ompanies suh as #-

 !lternators make alternators for several makes and models. (ome ustom shops are even

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installing dual&alternator systems to handle the additional eletrial loads seen in ustom

vehiles.

8or more information on vehile eletrial systems and related #ow(tufforks artiles, please

hek out the lots more information setion on the ne+t page.

 You may not have to replace your alternator entirely if you're handy in the garage.