'87 4kq: temporary no-juice condition

[Lines Peter]

Based on the events you explained, I'm not so sure the alternator is the
culprit.  Even with a complete alternator failure, the voltage shouldn't
drop much below 12 volts.  I would be searching for a really big current
drain to pull the voltage down that low, that quickly.  Some sort of short
circuit possibly?  Or maybe a problem with the battery?  In the original
post, you said it read 8V, then you (correctly) switched off all the
accesories.  It stayed at 8V but the engine was running.  This means the
only things drawing current from the battery at that time are the fuel pump,
CIS computer, ignition system and a few gauges.  The fuel pump is a moderate
draw, but certainly not enough to pull a good battery down to 8V.
Remember normal resting battery voltage is 12V.  Alternator output voltage
should peak at 14.4V.  If the alternator output drops below 12V, the
dashboard light will come on.  If the alternator was failing, you'd see the
light.  Do you see the light come on with the key before you start the
engine?
I'd be inspecting the battery cables for chafing and shorts.  Look around
the starter for obvious wiring shorts.  Also, take a look at the wiper motor
for shorts and possibly the radiator fan wiring.  How old is the battery?  I
don't know much about batteries but maybe something is wrong with it.  Good
luck.

Peter Lines