Battery light alternator

The Charge lamp comes on with the Alternator fails to keep battery charge going. (lots of way to fail, and ALT Is 100% good) it's a system. each part of it must be good, end to end.
so...
i put my voltmeter across battery, i start car, i see 13.3 to 15vdc, alt is good.
no? , i see now, 12.2v. ALt is dead, (i didnt say bad) and battery is sightly discharged, 12.6v is charged. (key off, rested voltage)
This 12v tells me the ALT is shut down. only why is the question.

how can any charge system fail,
bad battery tops the list here, they LOVE to fail.
1: belt slips.
2: belt missing
3: bad alt.
4: bad wires to ALT . (corrosion on any wire or lug)
5: fuses blown , any listed below.:
your alt is not an ECU controlled, alt ,its 100% self contained

did you ever remove the ALT and blow the 100 amp fuseable link
the first step of any alt pul, is the pull the NEG, battery lug or BOOM.

im logged in to alldata.com so can you.

and what do i see.
a 5 wire ALT. one is the frame ground, so it must not be rusty or..... it fails. frame to engine block. no rust here.

wire L , yellow.goes to dash LEDs (alt failure idiot lamps) (this works)
wire. S white goes to ALT 5a fuse, is it blown?
wire :B B/G color, goes to the 100 amp link fuse. (blown>?)
wire IG Red-white, goes to a fuse marked 10A gauge.

i think the IG wire is the voltage sense wire.
no sense, no charge. I think the whole I/P dash cluster goes dead when this fuse blows.

The S wire is the boot strap voltage sense, it wakes up the regulator .
if open , the reg stays asleep, as does ALT.
Alternators can not self start, the regulator has boot strap device
that cures that. called self excitation. so this wire is critical.
All wires except the L wire is critical. in this case.

the B wire is the big wire, it charges the battery . 50amps or more..

any of the 5 paths open , kills the ALT dead.
The REG inside the ALT can also shut it down.
eg: battery bad, with huge cell resistances. total.
eg: overvoltge for any reasons. including attempts top run car with no battery, (dont ask)
eg: over current (battery cells 1 to 6 shorted) or 25 KC lamps added.
eg: over temperature shut down.

that is about it.