Powerstroke SOS .....Alternator.....correct testing procedures
 

97 psd,90k original,had a headlight switch burn up as well as fuse.The battery light is on,new cables /batteres that are charged.The Alt 3 pin plug has a light green /red,white, and yellow.With key on and engine off theres 12v to the lt green/red when not plugged into the alt.When plugged into alt only reads 4v.This is a brand new Alt from DB electrical in TN.Unplugged black/orange heavy gauge in backside to test circuit.This is the only hope i have to fix so all help is appreciated!!!!!!!!!!

Wiggle the wires to the 3 pin plug around and see if the light flashes on and off. The light might just be from a shorted wire near the plug and not have anything to do with the alternator.

How many volts does the alternator put out at idle? That's the important question.