It sounds like you've miswired something. "F" is the Field terminal and "A" is the Armature terminal of the Generator. The wire from the regulator should connect to the - or discharge side of the ammeter. The wire from the F terminal of the generator should go to the switch resistance terminal. The switch has a resistor built in to control the low charge rate. The switch shorts out this resistor to obtain the high charge rate. The high charge rate can be adjusted with the third brush on the generator. I would get a Service Manual for the WD. It has the wiring diagram in it along with a full explanation of how to wire the system and how it works. Check with your local AGCO dealer first on the manual.