The 14v post does not get grounded on that setup. I had the same question with the N3135 and N1225-1 setup - here is the installation diagram: http://www.ceniehoff.com/Documents/Ctrl_Hyperlink/II0028D_uid121020091201492.pdf33v - 38v is too high. Bad things can happen if you continue to over charge batteries.
It may have no effext on the 28v output, but the 14v tap should be grounded if not being used. Run a wire from it to one of the four regulator mounting bolts and check the volts at battery again.
pay attention to the notes - when used with the N1225-1, the alternator phase cable is attached as shown in Manvstaco's pictures. Don't ground the 14v post.
you won't see 14v on that post unless there's a load on it.