On Thu, 22 Jul 1999, Ian Frearson wrote:
> I fear that my Spitfire's alternator is dead...my progressively more dim
> headlights and dashlights were the giveaway. But the ignition/alternator
> warning light never came on... Can someone explain the process by which the
> warning light works?
When the ignition is on and the engine is off, the light receives 12 volts
from the ignition switch and no volts (0V) from the alternator. A
difference of 12 volts. Light is out. Plus, there's current entering the
alternator and activating the stator windings (or the commutator windings,
I can't remember which. Maybe both)
You can make electrons move around in wire by waving magnets over it. You
can make a magnet out of wire by running electrons through it. The stator
is a stationary magnet, the commutator (commute = move...) is a moving
magnet. That's what makes the alternator do its stuff. B fields an E
fields and intercepting flux lines and other great mathematical
representations; I won't go into those (mostly as I risk doing it wrong!)
Back to the light: without some startup voltage (and current), your
alternator is just a bunch of copper being whizzed around in another bunch
of copper. Once you put electricity into it, you have magnets, which can
generate more electricity!
When the red light comes on, it's due to a 'problem' similar to when you
first turn the key- 12V coming from the ignition switch (from the battery)
and 0V coming from the alternator. If the engine is running, then you've
got a problem. If the light goes out (and stays out) when you rev the
engine, then you don't have a serious problem, just a tired / mismatched /
under-driven alternator. If the light dims at high RPMs and then stays on
at low RPMs, you have brush problems.
On my car (55A Chevy alternator) the red light will glow if I have ALL the
accesories on (hi-beams, wipers, signals, radio, heater, brake lights).
It's not losing the charge (the ammeter reads no discharge); I beleive the
'fault' is that I'm drawing lots of voltage from the battery, pulling it
down to 10V (maybe! I've never measured it) and the alternator is pumping
out 15V, as Chevy alternators are prone to do. So the red light is
glowing due to a 5V difference.
If the red light glows (bright!) and never dims or goes out, you've either
got a bad break internally in the alternator, or a bad wiring break
externally; check the surrounding wires first (I had this happen a few
days ago, the field windings lead had snapped); if all wires are sound,
replace the alternator.
-Malcolm
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