Alternator Warning Lamp Circuit Question

[dom14]

Hi Guys,
I'm trying to expand my understanding of the alternator lamp circuit, so I won't end up "cooking"
anything during my repairs, tests or mods in the electrical system.

I'm referring to my GQ RB30 here, but I'm guessing the question(s) apply to any charging system.

1)The lamp circuit of the GQ alternator/charging circuit reads +1.5v when the ignition is on and the engine is NOT running. The wire colour in GQ Patrol is white with a red stripe or yellow with a red stripe(depending on the location).

2)When the engine's NOT running and ignition's on, this white wire with red stripe acts as an earth.

3)When the engine's running this wire get energized by roughly +12V

Now, referring to below standard circuit of of the alternator/charging system, I can understand the number 3 above.

But, I'm still lost with 1 & 2, namely how to explain the +1.5V reading on the wire & how it manage to earth.(I'm using typical alternator circuit diagram below to understand it)

I'm guessing regulator circuitry has something to do with that?

I would appreciate if you guys can explain this to me, perhaps with a sketch or a regulator circuit diagram?!

Thanx