John Deere LT 160 not charging?

I have a JD LT160 that will not charge. The John Deere dealer said to replace the rectifier to the tune of $50some dollars. No joy still does not charge. I am getting 20 to 40VAC from the stator (alternator) and 14 to 16VAC from the center post of the rectifier that connects into the wire harness and directly to the + side of the battery. I read 12.4.VDC at the battery (should be reading around 14V). The dealer is telling me there is a diode in the harness between the rectifier and the battery that may be bad but I cannot find any diode any where. Any one have any ideas?