Hello all. I have a '92 GMC G2500 Vandura (5.7, 4 spd aod, 180K +) which I use as a work van. I've been having an electrical issue as follows:

Van was starting and running fine until about 2 weeks ago. I'd started it 4 or 5 times that day with no issues. Got in, turned the key, and got absolutely nothing. No accessories, no start, no warning lights, voltmeter pegged at low end.

So:

Checked voltage at battery and terminal block on firewall: 12.6

Headlights, horn, non-ignition switched accessories all worked.

Cleaned bat. terminals. Still nothing.

As I was semi-stranded, I tried a jumper between the horn fuse and the turn signal fuse. This fired up the check engine and seatbelt warning lights and buzzer. Turned the key, and had all electrical accessories again.

Turned key to start: van started and ran like normal. I was able to drive home with no problems. Had to disconnect the jumper when I got there as the warning lights and buzzer stayed on after turning the key off.

I was able to use the van by hooking and unhooking the jumper. Alternator charges normally, everything works, just like nothing ever happened. After using it this way for a few days (fortunately I've been very busy and haven't had time to pursue a proper repair), the horn fuse (which I jumpered from) blew while driving. Van died in traffic. Pulled over, jumpered from battery side of fuse, and drove home.

Any insight as to what's going on here?