What Is an Alternator?

ALTERNATORAn alternator is a device mounted to a car or truck engine that turns mechanical energy into electricity. With the help of some other components, it manipulates alternating current into direct current to charge the vehicles’ battery and run all of the automotive electronics.


How Does an Alternator Work?

Well, the story really begins with our good friends Michael Faraday, Nikola Tesla, and a few other guys, as they are the glorious men behind the discovery of alternating current (AC). Without their genius, electricity would not travel great distances, or help power up our vehicles, like it does each morning. Sure, direct current (DC) is great and all, but alternators begin by making AC before DC is spit out the other end. Let’s hit the basics really quick before we get too carried away in the history.

Direct current is the kind of electricity found in batteries; it always flows in one direction (as noted by the + and – labels on AAA batteries). Alternating current is the kind of electricity that flows through your house. It flows in two directions, continuously changing from positive to negative many, many times per second. An automotive alternator uses both of these types of electricity to keep your battery charged.

Inside the alternator there is a “rotor” that spins inside the “stator”. In colloquial terms, it’s basically two big magnets spinning next to each other, but never touching (which is really too bad, because they are quite attracted to each other). As the alternator rotor is spun (rotor = field windings) within the confines of the stator (stator = induction windings), the magnetic field between them creates alternating current.

Since cars run on direct current, diodes called a “rectifier bridge” need to enter the equation to turn AC into DC. They are basically devices that straighten out the “alternating” electricity waves. As the alternating current is produced by the spinning magnets, it is pushed through the rectifier bridge, which has direct current smoothly sliding out the opposite side. From there, the newly made DC electricity travels over to the battery through some wires. Since the car battery is a giant ocean of beautiful DC energy, it further cleans up the ever-so-slightly imperfect direct current that it is receiving from the alternator. This makes the electricity perfectly clean for sensitive automotive electronics to enjoy.

Now, along with your battery, there is a voltage regulator that plays a big role in this whole event. Old cars had a voltage regulator that was separate from the alternator, but all newer alternators have voltage regulators built right inside them. The voltage regulator tells the alternator how much electricity to make based on what the vehicle needs. If the voltage regulator notices that the headlights and heat are on, with the radio pumping up the fly beats, it will tell the alternator to work a little harder. If the voltage regulator notices that you are cruising down the sunny highway listening to the sounds of the open road with all of the accessories off, it will let the alternator take a little breather and relax a bit. The voltage regulator is basically the boss of the alternator.

What do the Labeled Terminals on the Back of the Alternator Mean?

  • S terminal – Senses battery voltage so that the alternator knows how much effort to put in. It should be wired to a power source.
  • IG terminal – This is the terminal that turns the voltage regulator on. It is wired to the power source from the ignition switch.
  • L terminal – Closes the circuit to the charging system warning lamp. If the alternator stops doing its job, this terminal becomes the ground for the warning lamp, which turns the lamp on. When working properly, this terminal will show battery voltage.
  • B terminal – Alternator output terminal. This should be connected to the battery, as it is where all the electricity comes from.
  • F terminal – Full-field bypass for regulator. This is for testing purposes only. It’s best not to mess with it.

When is a Replacement Alternator Needed?

The majority of automotive alternators fail in the same exact way. They don’t produce any worthwhile energy to charge the battery with. This is usually prevalent when you stop the car, because it won’t restart again due to a dead battery. You can easily test the health of an alternator with a digital multimeter (or any device that measures DC voltage). Just start the car (jump start if needed), and put the meter across the positive and negative battery terminals. It should read about 13.5 volts or more. If it is at 12 volts or less, your alternator is not doing its job.

Replacing an Alternator

Replacing an alternator can be incredibly easy or extremely difficult, depending on the type of vehicle. If you are in need of an alternator replacement for your car or truck, the best thing to do is get yourself a service manual to see what is involved in replacing one. Some alternator removal procedures require motor mounts or intake manifolds to be removed, and others are merely two easily accessible bolts. The degree of difficulty is heavily based on the type of vehicle that it is attached to.

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