The Author of the article gave his explination.
"The fix
The regulator senses the voltage between D+ and ground and adjusts the current to the rotor to obtain the nominal voltage. If one could raise the regulator ground slightly above the alternator ground, this would be seen as a higher output voltage."

What I understand from the article and other things I have read is:
The Bosch Alternator uses the Ground to alter the Voltage output to the Fields. The Diode increased the resistance to the Ground so the Voltage Regulator has to up the Voltage to overcome the Resistance.
The Author was using a Diode as if it was a Resistor not to rectify any wave form.

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84 300D, 82 Volvo 244Gl Diesel