I believe that this is a smart alternator, so the small wire is NOT an exciter terminal, it is a LIN bus digital control line.
Try running the engine with the alternator control line disconnected. If it is a smart alternator, the charging voltage should jump up to 14.5V and the engine management light will come on. If it is not a smart alternator, then the alternator probably wont charge at all, but there will be no engine management light.
If it is a smart alternator, then I would replace the battery without going any further, as a battery with reduced capacity (even though able to start the car after charging) will cause the alternator to under-charge the battery.
Component codes can be found in the pages linked from
viewtopic.php?t=80489
The
diagram that I believe to be applicable is:
Oops this diagram is for a C4 Picasso, my error, see next post by wheeler.
