I had a very similar problem recently on my euro 84 300TD turbo. System not charging, swapped in known good alternator and battery, system still not charging.

Aside from the not charging, I was most bothered by the fact that the idiot light on the instument cluster (charge indicator lamp) did not light up when the ignition was keyed prior to starting. I pulled the instrument cluster and determined that the bulb was functional. I then tested the continuity of the thin blue wire that runs from the plug on the alternator (the other two big red ones run to the starter) to the big round plug on the fuel/temp/oil pressure gauge on the back of the instrument panel (pin #6 on my car), and sure enough there was an internal break. So, I spliced in a new wire from the alternator to the idiot light and viola, light works = charging system works.

The reasoning behind all this (as I understand it from reading the forum archives) is that in order for the alternator to generate an output charge it must first have a "priming current" to generate a magnetic field between the rotor and the windings. That low voltage current comes from your battery and the circuit passes through the idiot light. No light = no priming current = no magnetic field in the alternator = no electrical output from the alternator.

Once the alternator is spinning (in the presence of the priming current) it generates its own self sustaining magnetic field and the higher voltage current it produces somehow shuts off the idiot light. Hence the idiot light staying on when your alternator is not charging and the weak glow from the light (especially at idle) when the alternator output is sub par. That idiot light is not so dumb after all. It can tell you alot about your charging system, if you understand how it works.

BTW, I learned all of this by searching the archives of the forum. If I mispoke, please correct me (but blame the forum).