Quote:
Originally Posted by ah-kay
The resister is used to provide 'excitation' current to the alternator winding during initial startup. Alternator ... needs an initial dose of current to 'excite' the winding to give sustaining voltage.
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Theory above isn't exactly correct. There is an excitation current required, but it is to "boot-up" the internal voltage regulator. The Vreg has its own mini-alternator coils (shown in M-B schematic). This is particular to the "Motorola-style" design used in Bosch alternators. Several web-sites detail all you might care to know about various alternator designs.
Quote:
Originally Posted by ah-kay
Normally, the battery light provide a path for the excitation current so the resister is not needed. However, if the light bulb burnt out and there is no resister then the alternator will not generate any voltage.
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Correct. I'll add that my 1984 & 85 300D, the "worn brake pads" and "low fuel" bulb currents add in for alternator boot-up. That does double-duty as a "bulb check" function. One correction is that in old alternators the armature has often become magnetized, which can allow them to "boot-up" at high rpm even if the dash bulbs are blown. If you need to apply a resistor in an emergency, my 300D's have a 1-wire connector strapped to the aft side of the coolant reservoir that goes to D+ on the alternator. Disconnect that and apply ~100 ohm between ground and the side going to the alt to force a boot-up. If no resistor, you could just ground the wire, but better to limit the current using a small 12 V bulb or such. If the Vreg ever fails, you can change it on the car. It unbolts from the alternator as a regulator/brushes assembly.
Most older GM cars used the same type alternator. They well know the "bad dash bulb, no charging" issue and long ago started the load resistor addition.
For humor, read your M-B Owner's Manual. It says if your BATT bulb doesn't light w/ the key before starting, drive immediately to a dealer for an emergency bulb replacement. I'll bet the dealers loved that business.