How an Alternator Works

An alternator is essentially an alternating current (hence the term alternator) electrical generator. You may have noticed that everything on a car runs on DC, so why are they using an AC generator as opposed to a DC generator (dynamo)? Cars used to employ dynamos, but there were a few drawbacks: they weren't as reliable as an AC generator and were harder to keep running at the right voltage. The development of high current silicon diodes made the use of alternators possible.

All electrical generators work by moving a coil through a stationary magnetic field. In most generators, the magnetic field is provided by permanent magnets, but in car alternators it's provided by an electromagnet. The electromagnet is made up of coils looped through iron laminations in a circle around the interior of the alternator. This is the stator coil. An electromagnet is used because it makes it easy to regulate the output. The higher the coil voltage and current, the higher the output.  With permanent magnets, the output would be fixed - since the engine speed varies from 1,000 RPM at idle to 4,000 RPM during heavy acceleration, permanent magnets wouldn't be able to produce the proper voltage at all speeds, and the engine would be working to produce the maximum current the alternator is capable of, even when it's not needed. Regulating the stator coil current, which is under a few amps, is a lot easier and simpler than directly regulating the alternator's output, which can be well over 100 amps.

The rotor is the part that turns inside the stator, and is responsible for generating the electricity. The two ends of the coil are connected to a pair of copper slip rings. A pair of graphite brushes are used to pick up the electricity generated by the stator from the slip rings.

The brushes are connected to a bridge rectifier, which is an arrangement of four really hefty high current silicon diodes that converts the AC output of the alternator into DC (sort of). The output of the rectifier is connected directly to the battery. A voltage regulator circuit inside the alternator measures the voltage of the rectifier's output and adjusts the stator coil voltage and current accordingly.

Diagnosing the Problem

Probably the easiest way to see if an alternator is working is to start the car and disconnect the battery. If it keeps running, the alternator is producing enough current to at least keep the engine computer, fuel injectors, and spark plugs running. Start turning on electrical loads one at a time: radio, fan, lights, rear window defroster, etc... The alternator needs to be able to produce enough current to run all of the electrical loads of the car. If not, you have a problem.

Isolating the Problem.

Before rebuilding or replacing the alternator there are a few things you need to check. DC power and moisture can corrode metal really fast, so check the connection between the alternator and the battery. Make sure the connectors are clean and the bolts are tight. Disconnect the cable at one end and measure the resistance with a multimeter, it should be close to 0Ω, replace it if it's higher than about 20Ω.

With the car running, disconnect the power to the alternator's stator coil, a small two or three pin connector usually located at the rear of the alternator. With the black lead of your voltmeter clipped to the negative battery terminal, measure the voltage of all of the pins; at least one of them should be close to battery voltage (usually 11.5-13.5 VDC). Anything lower means that the problem is this connection. Trace it all the way back to the battery and look for corroded connectors or blown fuses.

Disconnect the battery lead from the alternator and measure the voltage. Without the stator coil plugged in it should be a few millivolts, plug the stator back in and measure again - it should be something above battery voltage. I got 18 VDC on a good one, but it could be higher or lower depending on how it was designed. This is where mine failed the test: I got 10 VDC while revving the engine to driving speed.

The way mechanics test alternator current is with a carbon pile, which is basically just a really big resistor with an ammeter attached to it. If you have a clamp type ammeter you can measure your alternator's current output by clamping it to the cable between the alternator and the battery. Turn everything on and measure the current, it should be pretty high - over 30 amps or so. If you're still in doubt you can remove it and take it to a garage or auto parts store where they can test it.