The original early model kubota b7100 had a 3 wire dynamo. One wire went straight to the brown wire (AC lead) of the regulator and the other 2 went to the light switch (yellow and white) then from the light switch via red wire to the other AC lead of the regulator. The light switch is a double switch acting as a high/low charging switch. It produced more charge when lights were on high.
Later they changed it to a 2 wire design with one charge rate and currently you can only buy the 2 wire design dynamo.
To convert your 3 wire to a 2 wire is simple. Connect one of the 2 wires from the dynamo to the brown wire on the harness side. Then choose either the white or yellow on the harness side and connect it to the second wire on the dynamo. Now at the light switch cut the red wire (G2 on attached diagram) and cut the white or yellow that you have connected to the dynamo on the other end (G1 or S on attached diagram) and splice them together. And your done. Of course you could run new wires from dynamo to the 2 AC leads on the regulator if you choose. Either way your bypassing the light switch. Any questions feel free to contact us. Thank you
