An automobile alternator does not have any magnets and in order to generate electricity the field coils must be charged. When the alternator starts running and generating current it keeps the field coil charged. All cars with alternators (except maybe new cars that may have some useless computer controlled replacement) have a bulb that supplied power to the field that also serves as a warning that the alternator has stopped working correctly and is no longer keeping the field charged which allows power to flow through the bulb and light it up.

In short this is not a uniquely Mercedes design. Any car with a charge or alternator bulb on the dash will not work correctly if that bulb is burnt out. That's why if you have a voltage or amperage gauge you still have the light.

Yes, but this light does not go on when the alternator is not working, it stays off. The only telltale sign is the battery light not lighting up briefly at startup.
My F150 works as you describe. When that battery light comes on I better be on my way to buy an alternator. I've gone through 2 in 209K miles....{voltage reg/brushes not replaceable}

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

Alternators do not have field coils. The rotor contains a coil of wire that creates the magnetic "field" which when it rotates around in close proximity to the 3 phase stator windings produces alternating current, which is then rectified to DC. The rotor magnetism is controlled by the amount of current supplied by the voltage regulator. Many alternators (marine units in particular) are self energizing, using the residual magnetism in the rotor to excite the regulator rather than relying on an external source of current.

Many cars do not use an idiot light. The regulator is directly energized with current when the key is in the run position without incorporating an idiot light in series. Most early GM vehicles used a shunt resistor on the PC board which allowed current to bypass the idiot light & energize the voltage reg if the bulb was burned out or missing. This can be added to our MB's to solve the idiot light failure problem. It's a fairly simple mod if you can trace out a circuit on the PC board.

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82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K

Brought up in reference to the pulsing alternator light thread.

How to test alternator, cheap wagon style.....

Thank You Johnt49. This is closer to my impression of the philosophy of the situation than having a car which is totally dependent on a little bulb filament being intact to activate the charging system. This needs to be in the bulb pulsing thread also...
In the electrical charging diagram the function of wires 2 and 3 entering the alternator was not discussed. The arrangement of the diodes ( or the way they were drawn ) seems to preclude positive voltage being applied by either of them... even though the electrical manual says the diagrams are arranged so that it flows from the top of the page down.... and it really does not show how the transistor in the regulator is hooked up...

when you checked the voltage after installing the reman alternator did you give the engine a rev before checking the voltage? Im not to familiar with the electric system on these cars (they are usually pretty solid) but I do know that the alternator or a relay or something somewhere has to be activated before the cars electrical systems come alive.

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1981 300SD 512k OM603