To test the alternator only disconnect the green wire & place a temporary ground to that terminal, your voltage should go to maximum output, if this works it would indicate a problem with the regulator although wiring cannot be ruled out.
The weak spot seems to be the voltage regulator, from what I've read this is the problem the majority of the time.
Other places to check are the regulator to body ground, connection at the bulkhead connector (if your still using the stock wiring) & connections at the ammeter. If the connection at the bulkhead or ammeter is bad consider adding a wire directly from the alternator output to the battery with a fusible link for protection, this bypasses the ammeter so it will no longer work therefore adding a voltmeter would be a good idea.
