OK, I'll try to trim it down a bit once...
A bad diode can. The circuit has already been adequately described,
perhaps a bad ASCII schematic?
D1 D7 Winding A
D2 D8 Winding B
Terminal D D3 D9 Winding C
/--------------------------+------|<|---------+-()()()()--\
| S1 | /------|<|----/ |
/----+--o-->o---+-----+ +------|<|---------+-()()()()--+
| | | | |------|<|----/ |
=== | | ==== \------|<|---------+-()()()()--/
= ===== +-|Lamp|------+-----|<|----/
=== Bat. |Misc.| ==== | Terminal B
= |Loads| =====
=== ===== |Volt.| Field
= | | Reg.|----BR------()()()()--\
| | ===== |
| | | |
\---------------+--------------------+-------BR----------------/
|
Rest of Car <-- | --> Inside alternator
|
Not shown are D4, D5, and D6, the other three heavy diodes that
complete the three-phase rectifier system. (They don't matter for
the purposes of this discussion, and it was hard enough to draw
the above and try to make it clear.) Notice particularly that
the connection between the battery and alternator is unswitched.
(It is also unfused, but this simplified schematic doesn't show
fuses anyway.)
Normally, when everything is working right, battery current runs
through S1, the ignition switch, and powers various loads (ABS,
radio, SRS, ACC...), as well as running through the lamp to the
voltage regulator, thence through the brushes to the field winding
and to ground. The lamp is lit by this field current. No current
goes through any of the diodes.
Once the alternator is spinning, its generated current runs
through D1-D3 (and virtual ground is supplied through the not
shown D4-D6) to charge the battery. The voltage regulator
monitors this voltage, and is powered by it too, through D7-D9,
resulting in a net zero voltage across the lamp which remains dark.
Now, if any of D1-D3 are shorted, even when the key is off battery
current will flow through the offender, through the windings and
through D7-D9, to the voltage regulator which will power the field
winding, _and_ through the lamp to the other normally-switched loads.
There is obviously, in your case, enough current to light the lamp.
It only takes something like 100m-200mA to light the thing. It's
the alternator field winding that sucked the battery dry so quickly.
All clear?
-- Jim
