Volts lower with car running .

So i gave the scanmaster its own ground to the battery. got it up to 12.8 orso gonna keep going to see if i can get it up morem no pun intended
The scanmaster reports the voltage the ECM tells it. Has nothing to do with the voltage going into the scanmaster.
So replaced the ignition switch the one down on the steering column did not make a difference . but when i replaced the fuse and it went up to 13.7. 13.8 and thats the best i got it up to read .one of you guys mentioned it so i did it and that made the biggest difference.thanks all you guys.