The idle on the 5.0 is non-adjustable. The computer controls it. If you try to adjust it down, the computer will try to raise it up by opening the blade on the IAC.
The only reason you should touch the idle screw is to set minimum idle. WHat you do is disconnect the IAC and set the idle with the screw so it idles as low as it possible can without stalling out.
Then you reset the computer and plug the IAC in and reset TPS to 0.95 volts.
The IAC will function only very slightly open at normal idle and can open further since it has most of it's range available.
Everything vehicle related is a 12volt DC system. The alt may produce AC internally, but all output from the power output should be about 14VDC. Also, when it comes to working on vehicle electrical systems, Black wires are grounds and red wires are usually 12V power source wires