Never though much about Bosch alternators, corrosion was always a major problem with these things. Circuit shown in Nick's drawing is not quite complete, these units are internally sensed. A wire from the positive battery output to the VR is missing. A spade lug terminal kind of thing that corrodes.
Field voltage is supplied by a diode trio that works in conjunction with the three main negative diodes to produce a 14.5 V 5% ripple voltage to the voltage regulator and is also, "inputed" via the D+ input.
What the diode trio does, is to isolate the field circuit from the main battery output, so when the alternator is stopped, no field current loading is presented to the battery. That would result in a dead battery if connected directly to the main diode output.
But when the alternator first starts spinning no output voltage is generated because the diode trio output is zero volts. It does need help so a very small voltage is required at the D+ terminal to get the ball rolling. At start up, the battery voltage is greater than the output diode voltage, these diodes are reversed bias, so there is no load on the stator so its voltage builds up. As you can see in the circuit typically a 194 bulb that limits the D+ input current is all that is needed.
First sign is when you turn on the ignition switch, that charging lamp should come on bright. The voltage at the D+ terminal will only be a couple of volts. As the stator voltage builds up, the diode trio outputs the same voltage as the main diodes, and since the other end of this bulb is connected to that main diode output, it should go out. If the lamp glows dim, a sign of either an open main or trio diode.
I worked on one these recently, the real bitch was separating the case halves, the stator is in the front half, its wires go to the rear half, and that stator was corroded in tight. Don't dare pull on the halves, that will break the stator wires. I tossed it in my chamber and heated it to 400*F, and by tapping on the front case halve, finally broke it free. If only Bosch used anti-seize. Major problem with this one was the slip rings were coated with copper oxide, an excellent insulator, so just wouldn't start up. Brushes were hardly worn, VR terminals were all corroded. Just gave it a good clean up, used anti-seize on the stator core to make it easier for the next guy, but don't know why.
Front and rear bearing had the usual dried up very little grease in them, after a good cleaning, checking the balls and races for pits, they were fine, and got filled with great Wolf's high temperature red bearing grease. Just saying, didn't spend a dime for parts, was all janitor type work. But something about German aluminum and copper, not as good as ours.
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