Not a single peson has mentioned voltage regulator.

Engine part - moving to engine forum.

 

its in the alternator

 

But it is NOT the alternator. Bad winds in the alt, dead spots, worn bearings, etc...... lot's of things can go wrong with the alt, the voltage regulator is NOT the alt and is easily replaceable without replacing the alt. (and about 1/8th the price)

 

Well i had my 1JZGTE soarer alternator laying around, i put that in, BATT light is gone..

So far that engine swap has come in handy, so far i have the soarer fuel pump and alternator in my 2jzGE sc300, its like having a parts shop in your garage.