Measure the voltage at the battery AND also across the clamp-post connection (particularly while cranking).

I had a similar failure recently with my '90 -- rolled up to a stop light, stalled, and would not crank. Idiot lights, headlights, and accessories appeared to work fine, however, a bit dim.

Turned out that worn brushes allowed the battery to drain, and after falling below about 8-9 volts, accessories worked but wouldn't crank. New brushes solved the problem (and now a voltmeter in the dash gives me a warm, fuzzy feeling....)

Your battery should read 12.5-13 volts fully charged (engine off). It should read 13.5-14.5 volts while charging (enigine idling), even with headlights, blower, and 4-way flashers going.

You should measure essentially 0 volts between battery post and battery clamp, even while cranking (maybe 0.1 volt while cranking). If you see any voltage drop at the clamp, then you have corrosion and resistance between the clamp and post. You'll need to clean and tighten the posts 'n clamps.
--
Don Foster (near Cape Cod, MA)