Freightliner with MB engine low voltage at alternator

We have a 2005 freight liner with a MB engine, the alternator output voltage is 1-2volts at idle,the batteries are at 12v. Traced lead from alt to starter still 1-2 v. Should I have battery voltage 12v at alt.It started and ran before we checked voltages and fuses, now it will turn over,but not start. Thanks in advance

Freightliner with MB engine low voltage at alternator