In a message dated 97-05-26 07:18:27 EDT, sean_johnson@milacron.com (Sean
Johnson) writes:
> Also, could someone who understands electrons/smoke a little better than I,
> explain how/when the warning light works?
>
Sean:
I can try. Very simply, in order to produce electricity, the stator windings
in an alternator have to be in a rotating magnetic field. Since there is no
permanent magnet in an alternator, the magnetic field is produced by current
flow through the field winding, which is rotating when the engine is running.
One side of the warning light is connected to the battery, via the "on"
position of the ignition switch, and the other side is connected to the field
winding of the alternator. When you turn the key on, current flows through
the warning light and to the field winding. Since the alternator is not
spinning, There is no voltage being produced by the alternator, so the light
is on. When the engine is started, and the alternator is spinning, the
alternator will now produce it's own voltage. There will now be 12 volts on
both sides of the warning light, and it will be off. The alternator is now
producing it's own field current, and current from the battery, through the
light, is no longer needed.
If the alternator should fail to produce sufficient voltage, current will
again flow from the battery to the field winding, and the light will come on.
If the alternator voltage drops a little, the light will be dim; if it drops
a lot, the light will be bright. Similary, if the battery voltage should
fall, current from the alternator will flow through the light to the battery,
and the light will come on. The later rarely happens, because the voltage on
even a very bad battery rarely falls low enough to turn on the light. The
usual failure mode for a battery is a severely reduced current capacity.
The second small wire to the alternator, the small brown wire, is used by the
regulator in the alternator to measure the system voltage. If the voltage
drops, the regulator steps up the current through the field winding, and the
output of the alternator is increased. Conversely, if the system voltage is
too high, the regulator reduces the field current.
The light itself is a fairly good diagnostic tool. However, I strongly
suggest you take the car to an automotive repair shop that specializes on
battery/alternator work and have them do the tests. They have all the
required equipment, and usually, the expertise. Normally, there is no charge
for this, as they hope to sell you a battery or an alternator. Unless you are
really into originality, the Lucas unit can be replaced with either a GM
unit, or, as someone posted on this list earlier today, a Ford Fiesta unit.
Hope this helps!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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