This may tend towards over-simplification, but my understanding of active power control (soft power) is that it depends on 5V TRKL being present in the PSU at the time that the start up signal is given (with a key or switch), and on its falling to <1V in response to the Shut Down command.

Tomlee59 is probably able to extend/correct this from his far more intimate circuit knowledge about Macs, but the upshot of those requirements may very well be that a little component massage may rescue your PSU, and save you the expense of a new(er) unit.

The auto-recover-from-power-failure switch on the IIci (and a few other Macs) is a mechanical means of doing what many more-recent Macs can do through a control panel.

de