when running ac turn lights on volts drop could it be alt not big enough?

What is your question?

replaced comp. for no charge problem charges fine until ac on, turn on lights and charging drops to 12.37 volts and gauge drops down. turn everything off and the van charges properly,14.2 volts. i just got van and found it has junk yard alt on it is it possible the wrong alt was put on? it is a conversion van.