Alternators and the care and feeding thereof

[Mike Arman <armanmik@n-jcenter.com>]

In an effort to distract our list from flaming 2WD, 4WD, noWD,
redneck-toting shotguns (huh?), etc., I'm going to offer some content . . .


Diagnosing and rebuilding alternators.

How it works: The part that rotates is called the field, or rotor. The
stationary part is called the armature, or stator. (This is the reverse of
the little DC motors we had in electric train sets when we were young, and
is the source of endless confusion.)

The field makes magnetism - it is just a big electromagnet. Since it
rotates (driven by a pulley), we use brushes and slip rings to get power to
the field. This power is supplied by the voltage regulator.

Remember that moving a magnet past a wire induces a current in the wire.
(Works the other way, too: You can move the wire past the magnet and get
the same result.) The faster you move the magnet, the higher the current in
the wire. Also, the stronger the magnet, the more current you get. This is
why we need a regulator: At 5,500 RPM the alternator would be putting out
1,000 volts and promptly fry our expensive eurolights and trick stereos.
(And climate controls and window motors and trip computers . . . )

(And yes, I am using the words "current" and "voltage" interchangeably here
- this is not correct, but will work for the purposes of this discussion.)

The regulator functions by sensing the output voltage of the alternator -
now listen carefully - and *reducing* the power to the field (less power
equals less magnetism equals less alternator output) as required to keep
the alternator output DOWN to the required 13.2 to 14 (approximately) volts.

The armature is the part that makes the electricity. Armatures are nothing
more than three coils of wire. They are connected together in two ways: in
a "wye" or in a "delta". The wye looks like a letter "Y", and the delta
looks like a triangle. This is only important when we start troubleshooting
- stay tuned.

The output of the armature is alternating current, but we require direct
current to charge the battery and run all the goodies. Alternating current
is converted to direct current by a rectifier, which is simply an array of
six diodes.

There are no more electrical parts (except sometimes a noise supressor
which is nothing more than a large capacitor). The only mechanical
wear-item parts besides the brushes are a pair of ball bearings. These are
usually sealed and require no lubrication. They are cheap, and should be
replaced whenever you have the alternator open. (There are some mechanical
differences - some alternators have an unsealed needle bearing at the brush
end - same difference.) A bad ball bearing in the alternator will whine -
or grind, if it is very bad.

Electrical diagnosis: Ohmmeter or continuity tester required, alternator
disassembled.

Field: continuity between both slip rings and no continuity to the shaft.

Armature: continuity between all three "ends" or output wires, no
continuity to the frame or laminations. Now here's a gotcha - on the wye
wound armature, an open coil will be obvious - one of the tests reads open
- infinity - test light doesn't light. On the delta wound armature, an open
coil is harder to detect because instead of going through the open coil, it
goes around it though the OTHER two coils and will check OK with a test
light even though it isn't OK. You need an ohmmeter - if the delta wound
armature is OK, you'll get readings of (say) two ohms, two ohms, two ohms
between the wire pairs. If one coil IS open, your readings will be two
ohms, two ohms, four ohms. Bingo.

Rectifier: You'll find five connections: three for the armature output, a
ground and a hot. There are six diodes in the rectifier array. Each one
must measure continuity one way, and when you reverse the wires to your
tester, no continuity. Start anywhere you want - I suggest with the ground
wire only because you have to start somewhere - connect your meter to the
ground wire and any one of the three armature connections on the rectifier
array. You will read either continuity or no continuity. Now reverse the
wires to your meter - you MUST read the OPPOSITE of what you just read. If
you read continuity before, now you must see no continuity.

Any one of the six diodes that reads the same both ways is bad. Continuity
- continuity means the diode is shorted, no continuity - no continuity
means it is open. Replace as needed.

Check the brushes for free movement in the carrier and make sure they are
long enough (they wear down).

85% of all alternator problems are brushes and/or diodes. Most of the rest
are bad regulators. The windings, field and armature, give very little
trouble.

Before you take an alternator apart, mark both case halves and stator
laminations for alignment. These can be assembled 120 or 240 degrees off,
and while it will go back together, none of the brackets will line up and
you won't be able to bolt it back to your car. Also, keep track of all the
little bits and pieces of insulating washers and spacers - you NEED to put
them back where they came from!

Happy alternators, and that's really about all there is to them!

Best Regards,

Mike Arman