I think that a 2002 Chevy Express will have a voltage sense wire from the alternator. That smaller gauge wire connects to one of the outer posts on the isolator, not the center post. With the OEM alternator, voltage at the outer posts should be 14.4V or even higher if the alternator is cold. When hot, it might drop to 13.8V or so. The isolator center post will be about 0.7V higher than the outer posts. The OEM alternator tries to maintain 14.4V based on the input from the voltage sense wire. It compensates for the 0.7V drop caused by the diode type isolator because the voltage sense wire shows the 0.7V drop.
That's how it is on my '97. That series of van spans 1997 to 2002.
The alternator is likely a CS144. The isolator should be a 4 post type and have 5 wires connected to it.
There are a couple of files attached to this post -> https://www.classbforum.com/forums/f...tml#post129804
<- that should help with isolator troubleshooting.