'86 5KTQ: Bad alternator or voltage regulator?

[L DC]

I drove my '86 5KTQ last week for about 1 hour,
traveling some 35 miles including stop-and-go traffic
as well as some highway driving.

I pulled over to a gas station to fill up some gas and
when I tried to start the car, the battery did not
have enough juice to crank engine over. I jumped
started the car but it die after driving some 90 feet
away. My brother brought over a spare battery and made
it home, which was some 5 miles away.

I took the battery to Advanced Discount Auto Parts was
told it was good but needed charging.

With the battery fully charged once again, I tested
the battery before I started the car and it showed
12.6 volts. It also showed the same voltage at the
starter and big threaded rod on the alternator.
However, power would declined as time went on and
under load when the radiator fan came on and some 20
minutes later, while idling while I tested things out,
the car die.

Battery once agai was drained to 11.25 volts and,
thus, car would not start.

I took the alternator off the car and had it tested at
Advance Discount and was told that it is good.
However, I don't know if the test was properly done.
For instance, when the guy at the store hooked up the
testing wire to the bigger threaded rod to which the
big wires in the car attached to, no power was
evident--dead. When he hooked it up to the small
threaded rod to which the little wire attaches to,
power was evident in the area of 15-16 volts, so he
said alternator was good.

I had one of the alternators they stock tested and it
yielded the exact same results.

Not confident on their results, I had the alternator
tested at an independent shop and the guy there told
me it was good too, but I did not see the test itself.


The wiring from the alternator to the starter looks
fine.