"Gary Ober"  wrote:           1999 F250   97000 miles

The alternator light comes on during hard acceleration.
I have verified that the voltage does drop below 12V.
It starts OK, and the voltage is fine at idle.    Any ideas
besides replacing the alternator?
___________________________________________

The cause could be a loose belt or a bad brush/slip ring contact.

If the brushes are worn out, open the alternator and install a
replacement brush kit.      Clean the slip ring face with alcohol
and fine emery cloth.

If the bad contact is due to contamination from oil or dirt,
You may be able to clean the slip ring without disassembly
by shoving an alcohol-soaked rag through the vents while
turning it by hand.   Follow with emery or a pencil eraser.

Good luck.

Rodan.