I was referring to the replacement of the coil with the bulb.
It was done for testing purposes. The bulb was connected to the brushes of
the regulator, and it gave direct indication of the supply of the excitation.
As the voltage raised (by using a variable PSU) there was a narrow voltage
range when the bulb light became dimmed, then went off as set the voltage
above 14.4Volts. Which is OK, because during normal conditions the exciter
coil supply must be shut off above 14,4V in order to prevent overvoltage on
the alternator output.
But further reading about the warning light and bias, couldn't the warning light be paralleled with a resistor instead of a bulb?
Of course the original warning light can be paralleled with a resistor too, just
keep in mind if the resistor has lower resistance than warning lamp, the lamp
light will dim/fade. For testing i'd recommend a 120 Ohm, 2W resistor.