> That circuit in the FI ECU that shuts down the Alternator to save fuel?
> How can I find and test the sensors that trigger that circuit *without*
> replacing the ECU with a "known-good" unit?

The ECU does not actually shut down the alternator, it just lowers the
output voltage. This is not based on any single sensor but a host of
conditions including engine RPM, vehicle speed, coolant temperature etc.
On my '94, stepping on the brakes will defeat this power saving mode and
bump the voltage up to normal.

Does you service manual outline what conditions need to be met for the
lower voltage to be engaged on your particular car? The one from Helm
does for my '94, but I don't know if Haynes and others go into that much
detail.