charging system diagnosis?

I am working on a John Deere 4100 narrow. I have replaced the voltage regulator with a new one from John Deere. I have 40 volts ac at the terminal connecters at the alternator wires at the regulator ; disconnected. Ihave battery voltage on the battery voltage sensing wire at the regulator. I only have 17 volts at the alternator wires connected to the regulator. Battery will hold a charge until it is used up. Can anyone help me on where to go from here?