Verify that the FIELD fuse for the alternator is good. On my '92, this
fuse is at the rear of the main fuse box under the hood on the right
(pass side on US cars) side fender support. The alternator may be is
self-exciting once the revs are high enough for residual megnetism to
make the initial excitation current. The excitation may also go
through a dashboard idiot light too. I'm not at the car or the
manuals, or I'd give a better description.
ALSO: Please state the year and model of the car you are working on.
My great aunt Claire Voyant died a few years ago, so I can only rely on
what you share in your posts concerning the year and model. ;)