Perhaps you will understand this better. . .take both paragraphs in their entirety as the second states the same operation that you concluded.

The way you have it drawn is it a ground path for the alternator "not charging light." "not charging light" = the light on the dash that turns on when the alternator is "not charging".

The current would normally flow through the field but if the brushes have failed there isn't a path. In normal operation, once the engine is running, voltage at the diode trio ( where the light is attached ) matches the light and the light goes out. ( + 12 V on both sides of the light )



I never stated that the circuit as drawn functioned as a min charging level.

I left it open that the resistor function on my car might set a min charging level but needs to be verified. The resistor was 220 Ohm from what I recall. Cars with EFI and other electronics have a minimum amp draw that never goes away so it would be possible to set a min charging level with a resistor from the regulated ground side of the field to battery ground.

My explanation in post 12 was incorrect: "Vreg has its own mini-alternator coils".

Vreg has its own "half-rectifier circuit" (3 small diodes going to D+), but it uses the same field and stator coils as the main part of the alternator (6 big diodes). The schematic in post 4 details it. Sorry, I recalled a different schematic.

In simpler alternators, the field current is supplied externally via 2 brush terminals, from an external Vreg. The rest of the alternator is just the 6 diodes on the stator windings. GM has used the internal Vreg since the 1970's, which M-B followed. Hot rodders term that "1-wire alternator" and many like it since simpler connections (not really 1-wire, even in GM world). Trouble-shooting is not as easy, but "replace it all" becomes easier.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I have a guess: the resistor, or it's absence, is used to vary the charging voltage.

My basis: the 55 - 65 amp Bosch alternator (same as the one on 300D -300SD W123-W126) is used on various skid steers, loaders, pumps & other construction equipment (some ag stuff also). In the alternator re-builders parts manual several brush/regulator assemblies are listed, with differing charging voltages.

My experience has been that you can move the charging voltage up by adding a slight amount of resistance (50-100 ohms) to the sensing circuit. A 194 bulb in series usually moves the charging voltage up by a half volt (194 bulb is about 47-50 ohms). This is a good trick for a slow turning engine (think diesel) that idles most of the day and has a hard time keeping the battery charged.

There is no separate sense on these alternators. Sense is completely internal...D+ direct to regulator.

The resistor is there to provide an alternate path to ground for the indicator light, so that there's an indication of a regulator, brush, or rotor failure.

Yes, the sense is routed internally, but it can be intercepted & modified. Here is an article (BOSCH Voltage regulator mod) that better demonstrates the concept I refer to, that is, adding a slight resistance in the voltage sense circuit will push up the voltage set point in the regulator. This can be accomplished on the positive side (D+) or the negative/ground/earth side (D-). The technique used in the article uses a diode, I was taught to use resistance. It works - I've modified a 55a Bosch (in the field) using 18ga wire, solder, and a 194 bulb & socket. It kicked up the voltage output enough to keep the AGM batteries charged until we could get back to civilization.

Remember, I am guessing as to the reason for the resistor referred to in the original post. It would be interesting to bench test an alternator using each of the three regulator/brush assemblies. (just my $0.02)

Yes, series resistance would raise voltage on B+, because it would result in a voltage drop in the D+ circuit. But that's not what the resistor in question is doing. It's located between D+ and ground, in parallel with the regulator, so that there is an alternate path to ground. It simply allows the indicator light to detect an open circuit in the regulator or field. I posted a link to the Bosch documentation a few posts back, read through the thread and you'll find it. Also review the OP's circuit diagram, which accurately shows the location of the resistor.