Voodoo in the fuse box!
Well, the 350 got sorted out today. Its electrical problems, the result of a required Maryland State inspection, are solved, and she works!!
To digress, when the 350 was driven back from the shop that did the inspection, she failed in the middle of the road--completely dark. A battery charge or two later, at home, still alternator not charging, tach not working, voltmeter on dash not working, psi gage for air suspension not working. Headlights (one of which had been replaced during the inspection), when turned on, cause a warning buzzer to sound whether the engine is running or not. Engine will, however, start and run.
After tracing down every wire from the alternator, checking all the fuses, cleaning up all connections at the battery box, frame, and battery isolator, I had the alternator tested and determining that it had a blown diode package.
Replaced alternator. Still doesn't charge.
I eventually gave up and turned the project over to my mechanic. He began looking for fusable links--turns out there aren't any in a 91 350. He duplicates the work that I did, no doubt, with a critical difference--he found the problem.
A defective fuse. Not a blown fuse. A defective one. Apparently, the fuse (which resides in the GM fuse box in the glove box) only fails when warmed by current trying to flow across it. It looks OK. Cold, it tests OK with a voltmeter. But--warmed by current trying to cross from one spade to another.........it fails.
Word to the wise. Check fuse continuity with another fuse!! And, you may want to replace fuses which are getting old.......
Everything works!!!!!!!!!!!!! (but I still need to clean up the ground strap from the engine to the frame)