What Is An Alternator?
An automotive charging system is made up of three major components: the battery, the voltage regulator and an alternator. The alternator works with the battery to generate power for the electrical components of a vehicle, like the interior and exterior lights, and the instrument panel. An alternator gets its name from the term alternating current (AC).
What does alternator do?
Cars run a number of systems each of which has a specific task, all of these processes function with one commonality. They all in one way or another rely on electricity from the battery to keep them going. While the diesel fuel pumps and diesel injector both run on electrical impulses many other systems rely on electricity as well. Belt driven systems like the power steering pump and water pump may seem to have nothing to do with the electric flow but they rely on the movement of the motor which is caused by combustion started by a spark.
The alternator is the belt driven unit in the motor which generates electricity that can be flowed to the battery in order to keep the unit from discharging all of its power. If the alternator fails the car will not instantly die because the battery retains a charge that can maintain the systems for a short time. Do not expect to make it very far on just the battery since every function in the car is going to be draining the battery of its life.
Electricity or the movement of electrons is created by the alternator by way of a rotating coil driven by the belt of the motor. Around the coil is a larger coil that uses a magnetically opposite metal that forces the electrons to move through the coils and then out of the lines from the unit. This flow of electrons will charge the battery and keep the car going.
For the most part, alternators are relatively small and lightweight. Roughly the size of a coconut, the alternators found in most passenger cars and light trucks are constructed using an aluminum outer housing, as the lightweight metal does not magnetize. This is important since aluminum dissipates the tremendous heat generated by producing the electrical power and since the rotor assembly produces a magnetic field.
If you closely inspect an alternator, you’ll find it has vents on both the front and back side. Again, this aids in heat dissipation. A drive pulley is attached to the rotor shaft on the front of the alternator. When the engine is running, the crankshaft turns the drive belt, which in turn spins the pulley on the rotor shaft. In essence, the alternator transfers the mechanical energy from the engine into electrical power for the car’s accessories.
On the back side of the alternator you’ll find several terminals (or connecting points in an electrical circuit). Let’s take a look at those:
* S terminal – Senses battery voltage
* IG terminal – Ignition switch that turns the voltage regulator on
* L terminal – Closes the circuit to the warning lamp
* B terminal – Main alternator output terminal (connected to the battery)
* F terminal – Full-field bypass for regulator
Cooling is essential to an alternator’s efficiency. It’s easy to spot an older unit by the external fan blades found on the rotor shaft behind the pulley. Modern alternators have cooling fans inside the aluminum housing. These fans operate the same way, using mechanical power from the spinning rotor shaft.
As we start to disassemble the alternator, we find the diode rectifier (or rectifier bridge), the voltage regulator, slip rings and brushes. The regulator distributes the power the alternator creates, and it controls the output of power to the battery. The rectifier bridge converts the power, as we’ll learn in the next section, while the brushes and slip rings help conduct current to the rotor field winding, or wire field. Now let’s crack the coconut open.
Opening the alternator reveals a large cylinder with triangular finger poles around the circumference. This is the rotor. A basic alternator is made up of a series of alternating finger pole pieces placed around coil wires called field windings that wrap around an iron core on the rotor shaft. Since we know the pulley attaches to the shaft, we can now visualize how the rotor spins inside the stator. The rotor assembly fits inside the stator with enough room or tolerance between the two, so the rotor can spin at high speeds without striking the stator wall. On each end of the shaft sits a brush and a slip ring.
As we touched on briefly, alternators generate power through magnetism. The triangular finger poles fixed around the circumference of the rotor are staggered, so the north and south poles alternate as they surround the wire rotor field windings. This alternating pattern creates the magnetic field that in turn induces voltage into the stator. Think of the stator as the catcher’s glove as it harnesses all the power created by the spinning rotor.
All these components work together to give us the power we need to run our vehicles. Tesla captured this electrical energy and used it to light up cities, but we only need enough volts to power our stereo, lights, windows and locks.
The Life and Death of an Alternator 
As we saw in the beginning of the article, a failing alternator will kill a battery and ruin your day. But why did the alternator fail in the first place? Alternators have moving parts, get dirty and are subject to stress from heat and cold. As a result, the internal parts gradually wear out.
One of the most common failures is bearing failure. The needle bearings that allow the rotor to spin freely inside the stator can break down from dirt and heat. When the bearings fail, the rotor will not spin efficiently and can eventually seize. Usually an alternator with bearings failure makes a loud grinding noise. If you suspect this problem, it’s only a matter of time before the alternator gives up. Older vehicles with generators tend to require much more maintenance than newer models, but there’s no hard and fast rule for how long an alternator will last. It varies from manufacturer to manufacturer.
You can take several easy steps to diagnose whether your alternator is on the fritz. First, most cars today have a dashboard light that glows when the ignition is switched on. This light usually is represented with a symbol of a battery. Have you ever heard a buzzing noise when the key is on, but the car isn’t running? That’s the voltage from the battery running through the charging system. If this bulb is burned out, chances are the alternator won’t work. The car may start, but as we learned, it’s just a matter of time before the battery drains and the electrical system fails.
As a rule, a three-phase alternator can operate with only one of the stator windings operational, although it’s only one-third as efficient. To test whether your car had an issue with one of its stator windings, you’d need to use a voltmeter to check the voltage. (You can buy a basic voltmeter at an electronics store.) This is called a load test. Since the battery produces DC power, set the voltmeter to DC rather than AC. Connect the red lead (or wire) to the positive terminal and the black to the negative. With no accessories on, start the car and raise the RPM to around 1,000. The voltage should register around 14 volts. Anything less than 12 may indicate a problem. Next, turn on the headlight
s, air conditioner, radio and anything else that draws electrical power. Rev the engine and check the voltmeter. Again, the voltage should register around 14 volts. If you have a failing alternator, the voltage will be well below 14 volts. If so, it’s time to replace the alternator.
Before you decide you need to yank the alternator and replace it, make sure you check the serpentine belt. If the belt is worn or loose, the alternator won’t function properly. A bad belt is easy to replace and won’t set you back much, usually less than $20. But if you have to replace the alternator, you have options. Read on to learn how to go about replacing an alternator and what it may cost in the next section.
Where is the alternator usually
Alternators are typically found near the front of the engine and are driven by the crankshaft, which converts the pistons’ up-and-down movement into circular movement. (To learn more about the basic parts of car engines, read How Car Engines Work.) Some early model vehicles used a separate drive belt from the crankshaft pulley to the alternator pulley, but most cars today have a serpentine belt, or one belt that drives all components that rely on crankshaft power. Most alternators are mounted using brackets that bolt to a specific point on the engine. One of the brackets is usually a fixed point, while the other is adjustable to tighten the drive belt.
Alternator Parts and Terms
A listing of some common alternator parts and terms BRUSHES: Made of graphite they contact the two slip rings, energizing the field coil of the alternator. They are tensioned against the slip rings by springs. Held by a “brush holder”, they are usually in the rear of the alternator.
SLIP RINGS: These are two circles of copper attached to the shaft of the rotor. Wires connect them to either end of the rotor field coil.
ROTOR: The rotor assembly includes multiple “pole shoes”, copper field windings, and the slip rings, all mounted on the central shaft of the alternator. Multiple “N” and “S” poles on the rotor pass across the multiple sets of stator windings (in groups of 3) to make 3 phase AC current.
STATOR: It fits between the two halves of the alternator with the rotor spinning in the center of it. The stator has multiple poles wound with copper wire. The poles are in groups of three, providing 3 phase current.
DIODE: A “one way valve” for electricity. Diodes have low resistance in one direction, and infinite resistance in the other. RECTIFIER DIODES: 6 are used (3 positive, 3 negative) to convert the 3 phase AC output of the stator to DC
REGULATOR DIODES/ ADDITIONAL DIODES Some alternators have additional diodes duplicating the rectifier diodes, but they have a very small load on them. Some voltage regulators uses this as a reference to regulate the alternator output. Often the brushes (and the rotor field) are powered through a separate set of diodes.
Delco alternators have such a set of diodes, called a TRIODE ASSEMBLY: it’s 3 diodes in one
VOLTAGE REGULATOR: supplies current to the rotor field coil via the brushes and slip rings. It varies this current to control the alternator output. Most auto alternators today have the regulator inside them, called “internally regulated”. Older vehicles had an external regulator, often mounted on the fender or firewall. Some automobiles use the vehicle computer to control the alternator.
Bad Alternator
Symptoms of a bad alternator
Symptoms of a Bad Alternator, alternator problems
Symptoms of a bad alternator include hard or slow cranking when you try to start your vehicle, lights that seem too dim, and of course the red BATT, ALT, or GEN light glowing on the dash.
NOTE: You can have an alternator problem WITHOUT the “alt”, “bat”, or “gen” light coming on. The alternator itself is responsible for turning this light on and off (voltage regulator, actually, inside the alternator on most cars), so it can go bad and not turn the light on. If you think you have a bad alternator, diagnose your problems with the alternator testing procedures below.
repair an alternator
Overcharging
The only exception is if the alternator is OVERCHARGING: in other words putting out too much voltage. This can “DRIVE THE COMPUTER CRAZY” while the engine is running, and cause all sorts of weird problems.!
An overcharging alternator can “boil a battery dry”, so if your battery gets low on elecrolyte, making you add water to it, or if you car “eats batteries”, you may have an overcharging alternator.
You could completely remove the alternator from the car and your engine would still start and run normally UNTIL THE BATTERY WENT DEAD! Some race cars have just a small battery to fire the spark plugs and no alternator or charging system at all.. An alternator takes away power and weighs a bit, and every bit counts when you’re racing! (Top fuel drag racers don’t have alternators, or starters for that matter! Just a small battery to power the electric stuff, and a remote starter spins the engine, usually on the supercharger pulley.)
Are You Sure It’s The Alternator?
BAD BATTERY SYMPTOMS AND BAD STARTER SYMPTOMS can be very much like bad alternator symptoms. Make sure it’s the alternator before you repair or replace it!
A properly operating alternator cannot charge a bad battery! A new fully charged battery can’t spin over a bad starter!
How to test Alternator
ALTERNATOR TESTING
The best method of testing a bad alternator is with a voltmeter.
Testing An Alternator With A Voltmeter
Put a voltmeter across the battery while the engine is running . Voltage should be 14.2 volts. Lower than 14 volts or higher than 14.4 volts could mean an alternator problem. Turn on the lights and accessories: voltage should stay up around 13 volts. Idle engine up to 2000 RPM. Voltage should go up to 14 or so. (Testing numbers are for 12 volt systems)
IF THE VOLTAGE GOES UP TO 14.2 VOLTS YOU NEED NO ALTERNATOR REPAIR!
NOTE FOR NEWER VEHICLES: temperature compensated alternators
The newer vehicles have alternators which are “temperature compensated”. This means they charge at different voltages depending on the temperature. A hot battery requires less voltage to charge than a cold battery, so the temperature compensated alternators charge at a lower voltage when hot than when cold. In any case, note that any voltage reading below 13.5 volts or so is a sure sign of trouble.
A Common But Dangerous Way To Test An Alternator
THIS IS COMMON, BUT DANGEROUS !!! DON’T DO IT!!!
The old “backwoods” way to test an alternator was to disconnect one of the battery terminals and see if the car keeps running. Although this does work, it’s not recommended. First, the battery can blow up from a spark near it. On newer cars it can cause a voltage surge which can fry computer components. So even though this way might work, why not just get a $15 voltmeter and do it right?
JUST BECAUSE THE ALTERNATOR ISN’T CHARGING DOESN’T MEAN IT’S BAD.
If you have an older car you might have an external voltage regulator.” Any good repair manual will have instructions on bypassing the regulator to test the alternator.
Some newer vehicles, and some 80′s Chrysler products, had the voltage regulator as part of their computer. Once again, a good manual will tell you how to do a bypass and test the alternator.
Wiring Tests
Even on an internally regulated alternator there can be faults in the wiring and circuits leading to the alternator which can make it not charge. Test the wiring before you replace the alternator! Some vehicles have a fuse which can blow and keep the alternator from charging. Others won’t charge if the alternator warning light bulb is burned out! On older Chrysler products (80′s), and some newer vehicles of other manufacturers, (2000+) the alternator is controlled by the computer.
The wire from the large terminal (usually labelled “B+”) to the battery can have high resistance, usually caused by a bad connection at either the starter or battery “+” terminal. While the car is running, check the voltage both at the back of the alternator (B+) and the positive battery terminal. They should be within a few tenths of a volt of each other. If not, there may be excessive resistance in the circuit. Many vehicles have a fusible link or a large fuse to protect the alternator. These can be burned up by a short or improper use of jumper cables. If there is no power at the alternator B+ (large output terminal) then a bad fusible link or maxi-fuse is probably the problem.
General Wiring Diagnostic Test Procedures
WARNING!!! THE FOLLOWING DIAGNOSTIC TEST PROCEDURES ARE GENERAL FOR INTERNALLY REGULATED ALTERNATORS.. THEY DON’T APPLY TO EVERY ALTERNATOR EVER MADE. THESE TESTS ARE NOT MEANT TO REPLACE DIAGNOSTIC TEST PROCEDURES FROM A MANUAL OR WIRING DIAGRAM DEALING WITH YOUR SPECIFIC CAR OR ALTERNATOR!!!
(Message to legal types: Shoo! Shoo!)
On an internally regulated alternator there are generally 3 terminals. (not including ground: most alternators ground through their cases where they bolt to the engine). The large terminal goes to the battery: it’s the 14.2 volt output terminal. Another terminal “turns the alternator on”, or provides a sensing voltage for the regulator: it generally gets 12 volts from the “ignition switch on” circuit. The third terminal usually goes to the “alt” or “bat” warning light.
Do these tests before you condemn the alternator and do an unnecessary repair!
On MOST cars, with the ignition switch in the run position, (ENGINE OFF!!! JUST ALL THE DASH LIGHTS LIT UP!!!), good alternator wiring should test like this: The big terminal should have battery voltage (12 volts or so)
One of the small terminals should also have battery voltage (maybe a little less)
The other small terminal should have some voltage (whatever flows through the bulb)
Basically MOST internally regulated alternators should have some positive voltage going to all their terminals with the ignition on. If an internally regulated alternator has those voltages and still doesn’t charge then you have a bad alternator.
How long will can you run on bad
Your car can run for a short time with a bad alternator. How long depends on how much electicity your car uses. A car with a mechanical fuel pump can go pretty far. An electronically fuel injected car with electric pump can’t go very long at all. No car can make it far at night with the lights on. So if the “BATT, ALT,or GEN” light comes on, no need to immediately pull off the road, but get things checked A.S.A.P.! Also watch your temperature gage! You may have broken a belt, and this could cause your engine to overheat. Should the car overheat, you should stop as soon as safely possible. Either check it out yourself or get it to a mechanic for repair. Special note to air cooled V.W. Beetle owners and Corvairs (Yes, there are still some around!!): the alternator or generator belt turns your cooling fan, so if the light comes on, check the belt very soon: it can severely overheat in a very few minutes!
How to repair an alternator
Most of the time the repair for a bad alternator is to remove it from the car and replace it with a new or rebuilt unit from the parts store. If you have pretty good mechanical ability, it is possible to repair some alternators yourself and save some money.
The only alternator I normally will consider rebuilding is the early Delcotron GM alternators (The 10 SI series). They are very modular and no soldering or pressing in of diodes, etc. is required. If you have a good set of sockets you pretty much have what you need to rebuild the early Delcotrons. The diode assembly, brushes and holder, stator, voltage regulator, bearings, and triode are all readily available at parts stores. At one time you could buy a kit with the regulator, triode, brushes, and bearings at Discount Auto Parts for $12. You can still find kits on the web, some in the $20-$30 range. Rectifier diodes are less than $10.
Of course those old Delcotron alternators are available rebuilt with a lifetime warranty for less than $50 at most parts stores.
HOW MUCH DOES INSTALL COST
The alternators on older vehicles, especially the domestic models, are the least expensive. As of 2009, the price for an 80-amp alternator for a domestic vehicle is around $80 with a $10 core. For an alternator with a higher output rating than 80 amps the price would be marginally higher.
Alternators for older foreign vehicles have a separate voltage regulator that must be purchased, bringing the price in line with the domestic vehicles.
For alternators from the mid -90s and later, the price has essentially doubled. A current domestic alternator averages $180 to $240. This is where the domestic and foreign vehicles part ways in most cases. An average alternator for a foreign vehicle is in the neighborhood of $270-$320. Alternators for high-end foreign cars such as Mercedes and BMW are in the neighborhood of $340 and up, even though Jaguar’s alternators cost about the same as a domestic vehicle.
When a new alternator is to be installed, it is usually due to a failure, which would have drained the battery. Caution is in order here because a new alternator will be damaged if it must recharge a depleted battery. Alternators are designed to maintain a charge, not to charge a dead or low battery. Charge and check the battery before the engine is started.
Most alternator failures are due to overtaxing the alternator due to a bad battery or a large stereo amplifier. If the bearings are good and the alternator makes no noise, rebuilding it is a simple matter taking only a few minutes and saving considerable dollars. The only thing it needs is the rectifier bank and diode trio located in the rear inside of the alternator. These can be purchased at any auto parts stores. Take the pulley off, the four bolts securing the two halves of the case, pull the rotor out and the rectifier and diodes are simple to remove.
Another option that can save money is to take the alternator to an automotive alternator and starter rebuilding business in the local area. This is a popular option because not only is it inexpensive, but they install better parts. The average is $40-$60 and a one-day turnaround.
HOW LONG SHOULD IT TAKE
These instructions are VERY general unless otherwise stated!
Most alternators are made of two aluminum shells with the stator assembly “sandwiched” in between. The two halves are held in place with bolts, usually 4 of them, facing the rear of the alternator. (terminology: pully side is the front the other side is the rear) . The rear section of the alternator usually houses the brushes, triode, and rectifier diode assembly.
Next mark, or make a note or drawing of the “clock” position of the two halves of the alternator. I usually just take a screwdriver and make a scratch across the side of the alternator. When I put it back together I just line up these scratch marks. This is important because the alternator will bolt back together 4 different ways. If you bolt it back together wrong it would still work, but the wires might not reach, or be in a bad spot, or maybe it wouldn’t bolt back on the car at all!
