an Alternator Works
An alternator is essentially an alternating current
(hence the term alternator)
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
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
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
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
Disconnect the battery lead from the alternator and
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
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