electrical headscratcher

i have a 94 safari awd vin w that recently developed an intermittent charging problem.dash meter drops from 14 to 12 volts, then slowly drops as battery drains, until van sputters and stalls as battery drains. then, alt will work again but intermittently.
checked battery cables, pos and neg,all are tight and clean. battery is 5 years old, but takes a full charge, starts van without hesitation. even so, i swapped out battery for my wife's brand new one, no difference. positive lead behind batt on firewall checked, no corrosion there, all fusible links good. thinking it could only be alternator i tried a new one, still intermittent charging. old alt back in, new one back to farm&barn. serpentine belt good tension, don't know what else to try. read through every forum re similar problem and none seem to correct my problem. help please

I had the exact same problem with an 1996 Express Van It was sent to 3 different dealers, they changed the battery, alternators (3), etc. They kept telling me that it was a bad batch of alternators. Still had the problem but was intermittent. After dealing with this problem for a few months, I finally discovered a group of wires (near the alternator) wrapped in that plastic shielding (about an inch thick) was very corroded. I repaired the wires and problem was solved :) This happened more often when it rained. Don't know if this will help, but I thought that I would mention it.

When the problem is ongoing, engine running, use a digital multimeter at the generator bat terminal. What Is the reading?

14.16 -14.19 volts thanks for you response

update: tested second time after running for a few minutes. watched voltmeter applied to battery drop continuously like a countdown, faster when headlights and a/c were on . tested at bat terminal and reading was 12.2 volts, mirrors the battery output and continues to drop as battery discharges.

You need to check for excite voltage to alternator, L terminal. Without excite voltage, the alternator won't work.

update - on fri found that a test light jumped between the batt and the disconnected pos cable would light, indicating a short. when disconnecting the plug with the 4 fusible links behind the battery the problem corrected, further isolated terminal c on fusible plug as the culprit but that red wire disappears through the firewall and then up under the dashboard. i have no idea where it goes to. haynes nor chilton wiring diagram don't show enough detail
possibly the ignition switch? L terminal is hot so that is not the problem, thanks for the suggestion

That doesn't necessarily indicate a short, only a load of some kind. It could be a short, but it could just as easily be the backup systems for the SIR, radio, BCM, interior lights, or numerous other devices placing a load on the system. The typical parasitic load is around 35mA, but that wavers a bit as systems charge, change mode, etc. The size of the lamp will matter as well. Most test lamps are only 100mA or so. Connecting an ammeter in place of the lamp will provide a more useful indication of what is happening.

no longer scratching, good thing because my head was beginning to bleed. after checking and rechecking EVERY battery cable, ground connection, even running a neg. jumper from the battery directly to the alternator body, i put the new alternator back in before i returned it thinking it HAD to be the reason for the intermittent charging. i then realized that i may have run the serpentine belt around the pulley below the alternator on the wrong side, leaving a little play in the belt when i first tried the new alternator. anyway with the belt routed correctly on the new alternator, bam 14 volts on the dash meter, battery dash light off, charging as specified. i then checked my records and found that i had last replaced the alternator over 10 years and 100k ago. i guess it was time. at least now i have become intimately familiar with the van's battery, starting and charging circuit. just in time because my wife's montana just started leaking antifreeze underneath. oh well the tools and the ramps are still out, on to the next project. thanks to all who offered help.

It's the little things that will get you. Thanks for reporting back with the results.