Date registered: Dec 2005
Vehicle: W211 E320 Bluetec R129 SL500 - $1300 car ........ Ford F350 6l
Location: CA Bay Area
Mentioned: 9 Post(s)
Quoted: 1168 Post(s)
Alternators are not fuel-dependent.
Big advantage of alternators v/s generators is that they can produce good amps at low speeds. I can observe voltage on car display on my W210 and glow plugs on or off, high beams on or off, the voltage stays the same at idle or not.
Might test the W124 for that out of curiosity.
Bottom line, tests machines for alternators do have whole program that last some time and it did happened to me that malfunctioning alternator from my Ford passed the test with flying colors, before it definitely died 2 months later.
From your description your alternator might have dead diode, or 2 and it does charge, but at some loads just not enough.