One real problem in analysing charging problems on early 300SDs is that they are way undercapacity on the size of the alternator. The first models had an AL 80x (Bosch rebuilt #) which only puts out 55amps. Later models, around 83 and up, had AL 117x models with 65amps. The problem with these alternators is that they don't produce that number till about 2000rpms. At idle they make about 20-25amps. The newer cars are above 120amps and they get 90% capacity at idle (good stuff).
Unfortunately the A/C with aux fan running takes about 25amps. So if you are idling with your A/C on and the wipers and headlights going you are about 25amps in the hole. Combine this with the high losses during glowing and starting and short, stop and go, trips and you have a problem. We see 10-20 cars a summer that the driving conditions cause the problems not a lack of original performance.
We no longer ever use the AL 80x. We replace them all with AL 117x, but it only helps marginally. I don't do the exchange on working systems only when the alternator is bad.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician