Check for a malf battery cable or ground connection at the frame. If these are good check out http://www.f150forum.com/f4/alternat...-problem-12668
you will need a VOM to properly test these fusible links. You can pick one up at Lowes or Home Depot. You want to follow Bucko's # 7 thread reply closely. You are dealing with high amp carrying wires containing fuseable links inside them.