help with wiring problem

I'm trying to wire a sears suburban and i'm running into problems.I'm trying to use a car voltage regulator and I can get the tractor to crank and run but it wont charge.I had the starter/generator checked and it is good.My problem is on the diagrams it shows an f,gen,bat terminals on the reg. but the one i'm using has an f,i,s,a terminals.Can anyone help?