|"Is your power supply harness missing the wire to pin 18? That supplies a -5 volts required by ISA slots. "|
The ISA slot will still work if the (usually) white wire is not there at position 18 of a 20 pin standard ATX connector, or position 20 of a standard ATX 24 pin connector (if any mboards with a 24 pin connector socket have ISA slots). It carries only a small number of millamps of current.
It's for a rarely used feature of ISA slots / cards, that of course won't work if that wire is missing. Someone else who answered posts who had more knowledge than myself informed me of what the wire is for a long time ago on this site, but I can't find his info now, or even remember who it was, and I can't find any note about that on my computer I've used the whole time I've answered here.
"ISA cards were not made Plug and Play."
Some of the later ones were Plug n Play. e.g. I have an ISA PnP Soundblaster SB16 card, and some cards could be set to either PnP use, or jumpered or manually configured like older ISA cards.