Hello,Some of the C-classes had alternators with smart interface to engine control unit. Their voltage regulators are able to send a signal out to engine control unit about the output state of alternator. When this output is not ok- engine control unit stores a fault code and sends a message to activate the warning.
At the same time there same generation C-classes with simple excitation wire. If the alternators are mixed up- warning message will be triggered.It seems that you got an alternator with unsuitable voltage regulator for your C-class.
This will not make the alternator to communicate. Check with the supplier if they can offer the alternator with a smart communicating voltage regulator.