I think the older supras just run a weird volt range. Cause my 91 used to run around 12.8 volts idle and 14 volts under a load before the alt took a crap. After putting a new alternator and battery in it I'm now running 12 idle and 13.5 under a load.

All those numbers are from my afc.

 

^ Same here basically....

I was reading the alternator is supposed to put 13.6 max, after it distributes itself it can only go 12 max throught out a 12 volt system. Anything more it will overcharge leading to Diodes burning up and the alternator and possibly battery taking a crap.