nominal output of the stock alternator should be 12v @ 50A
the output current should run around 13.5v and ammeter should read 10A or more at 1500 rpm, 35A or more at 2500rpm, or 49A or more at 5000rpm.
These units have an internal IC regulator.
Unloaded voltage: 14.7v +/- 0.3v

When checking your alternator output do not check it at negative, you check it at pos (B+) terminal.
Generally you want to make sure your voltage output is between 13.2v to 14.8v. (this may vary depending on how discharged the battery is).
There is a test to check the rectifier on the alternator but you will need the service manual to show you the table on how to test the rectifier array (best scenario is to remove alternator and take it to a repair shop for testing).

FYI: the ecm does not control the charging

Make sure ALL your connections are good, check the harness and make sure there are no faulty fuse links inline in the harness going to the alternator, and most of all make sure the battery is in good condition, load test it with a load tester.