Sorry if this info is redundant. I found more questions being asked than definite answers so thought it would be helpful to post what I figured out about the factory Bosch 55A alternator on my 85 300D.

Initially, I was totally confused by both the connectors on the alternator and the factory wiring diagram. Probably because my prior experience is old Mopar alternators which are the simple - 2 field wires and a big output stud. An external voltage regulator controls the field, so one can control the field directly, such as apply 12 V across the field w/ jumpers to get full output, ex. get home if key switch bad (been there). M-B alternators have an integral Vreg on the brush assembly. I found an excellent description at a Danish ship website:
www.kb-kbh.dk/shipslib/el_ombord/alternatorhandbook_ocr.pdf
This text might have been written by a retired auto-electrical engineer years ago (hand drawings). While we don't often need to take an alternator apart to fix like these sailors must, the info can avoid thinking a good alternator is bad (as I did). Read the full text, and you won't need to read my rant below. The author refers to our type as Motorola rather than Bosch.
While there, also download
www.kb-kbh.dk/shipslib/el_ombord/DIN72552_electrical_terms.pdf
which details the cryptic "DIN numbers" you see in the M-B diagrams, ex. the "30", "85", "86", "87", "87a" you already know from standard automotive relays.

As a quick summary, the "B+" output (2 terminals & wires that run in parallel to the battery) is simple, but the "D+" output is misunderstood by many. It is more than just a dash monitor. One must apply a small current there to initially excite the field and "boot up" the alternator. The D+ circuit is a parallel "mini-alternator" that generates the field current (controlled by the internal Vreg). Once flowing, the D+ could be disconnected from the dash. However, to boot up, current comes from the battery thru the "charge indicator" bulb on the dash. M-B engineers were concerned that if that bulb burns out, owner's cars would not charge and they would get mad, and more importantly return under warranty. Therefore, they provided an alternate path thru the brake pad warning bulb, via a diode and resistor to isolate the two. That path only works if your brake pad wires aren't grounding, but even when worn that shouldn't trigger unless you apply the brakes. This is per my Haynes manual for 82-84 cars. I don't have an 85 diagram, but suspect mine has other paths since with key on, my "brake level/parking", "pads worn", and "low fuel" bulbs all light, and go out once the alternator is outputting, plus my circuit board has more diodes & resistors than in the Haynes diagram. This system is not M-B specific, and other cars with the Motorola system rely on a bulb to boot up. For sure, replace your bulbs and don't drive long with the cluster unplugged, unless you boot-up the alternator yourself (below).

I read other posts here that claim the alternator will not work if the "charge indicator" bulb is bad. Not quite true. In my car, that bulb (maybe all 3 above) is sufficient to start it outputting at idle. If I remove the charge bulb, I must rev the engine a bit to get it to kick in. Where the threshold lies depends on your bulb resistance. My confusion started when I replaced all cluster bulbs w/ LED's. They didn't flow enough current to boot the alternator at idle, though it still kicked in ~1500 rpm. I could live with that, but am preparing this car for my clueless son who will have it out of town, so I put a regular incandescent bulb back for the charge indicator. If that burns out, the alternator stills boots ~2000 rpm, but warning bulbs being lit until then will surely be disconcerting. I guess years ago when I used to drive this car, I noticed these bulbs initially on, but figured that was some type of computer bulb test. No computer, the "feature" is all in the quirky alternator regulator.

My confusion was initially worse since I had just fixed my dash circuit board and tested every pin on the connector, then installed it in the car and found several warning bulbs lit, even after I started the engine (WTF). I didn't think to rev the engine. The individual sensors (brake pads, fluid level) showed no connection to ground. I suspected I had messed up the circuit board even worse than when I smoked a trace (LED bulb issue), but just artifacts of the Motorola regulator system. For more detail, see
www.peachparts.com/shopforum/diesel-discussion/346954-instrument-cluster-repair.html.

The photo shows how I measured DC current output with a clamp-on probe ($400 list, but cheap on ebay), using a temp extension for the "B+" wires to connector block. In my 85, the "D+" wire doesn't go thru the block, but is a separate single round connector (like temp sensors) strapped to the side of the coolant reservoir (yellow woven w/ blu stripe). In an emergency, one could disconnect that and feed +12 V for a few seconds, preferably thru a ~100 ohm resistor (or test light), to boot-up the alternator. I'll carry one in the car, now that I understand the system. Direct +12 V briefly might work, but risks damaging the regulator. BTW, the grey box is a "Battery Brain" I have in most cars to avoid getting stranded.

Attached Thumbnails

 

Good writeup. Had to noodle this out a couple of days ago when I put the saab AL129X 115amp in my 240D to replace the 55A that couldn't keep up with my 'off road' H4 bulbs and other add-ons.

I use a similar battery monitor called a 'battery bug'. Connected directly to battery and mounted in the cabin.

__________________
Eric, CPO, Submarines, retired.
Here's a sig line...
Mine: '68 Corvette LS1/4L65E, 83 240D, 2000 GMC 4x4, 08 FLSTC Anniv
Hers: '72 Corvette 454/4spd, '99 MB SLK, '93 Metro vert, 78 240D, '92 Silverado, '65 Fjord Rustang, '59 Fjord Fairlane, '17 Slingshot.

Smoker,
Your "Battery Bug" in the cabin sounds more like a voltage monitor. I use a monitor in all my newer cars that don't have a dash ammeter. They plug into a cigarette lighter. Simplest and cheapest is the Harbor Freight one, which has LED's to monitor the battery off (~12.6 V is good) and the alternator when running (~14.3 V). Another I have is an Eqquis (Amazon) that gives the exact voltage. My "Battery Brain" is different. It isolates the battery if it senses the voltage too low, so you will always have enough left to start the car. Even though its relay is rated ~200A, I don't run the starter current thru it.

Battery Bug is a bit different. It works best when synched with a new fully charged battery, and then does a starting amp draw test every time you start the car. Using that it calculates the percentage of original battery health as time progresses, and predicts when battery should be replaced.

It also has a nasty audible alarm if the resting battery voltage dips below a healthy amount.

I'd like it if it had a cutout as you describe, that would be quite handy!!

http://www.oreillyauto.com/site/c/detail/BBG0/BBSBM12/N0366.oap?ck=Brand+Search_BBG_-1_-1&mn=Battery+Bug&mc=BBG

__________________
Eric, CPO, Submarines, retired.
Here's a sig line...
Mine: '68 Corvette LS1/4L65E, 83 240D, 2000 GMC 4x4, 08 FLSTC Anniv
Hers: '72 Corvette 454/4spd, '99 MB SLK, '93 Metro vert, 78 240D, '92 Silverado, '65 Fjord Rustang, '59 Fjord Fairlane, '17 Slingshot.

What you are seeing is the result of residual magnetism in the rotor generating enough current to self-excite the alternator. Many people will install Delco alternators, which have similar external connections to the Bosch / MB alternator, with only one wire and will wonder why they need to increase engine rpm for charging to begin.

Ensuring sufficient charge at low rpm could be as simple as installing a resistor across the warning bulb circuit in parallel with the LED. Select a resistor with slightly less resistance than the bulb you replace and you should have enough voltage drop to operate the LED.

__________________
When life gives you lemmings... make lemmingade.

Could be that at high enough rpm it would self-generate enough to boot-up without any starting current from the dash, particularly an older alternator w/ residual magnetism. I didn't try it without any bulbs.

My owner's manual says that if the "charge indicator" bulb doesn't light when you first turn on the key, to immediately take the car to the dealer for a critical repair. I am guessing the dealers appreciated that - change a light bulb, charge $100. I like the idea of permanently installing a resistor to bypass the bulb. However, it would probably have to be a 5 W resistor. The factory probably didn't do that because such resistor is fairly large and expensive, plus why starve the dealers of income. Rich Mercedes customers normally have regular contact with their dealer to at least change the wiper blades and re-fill the wiper fluid, while they sip complimentary cappucinno.