Alternator won't charge the battery

There may be and issue with the ECM,it controls charging through it,

How would I fix it?

You will need to check for code,s usually when the ecm is bad it will throw numrous codes or none at all,

Go have it scanned if possible,

ALL so have the new alternator checked,it is possible it is a bad one.

It is throwing codes I'll post when I can find the paper where i wrote it on.

It threw cat o2 sencer hrt egr evap

The alternator is putting out 12 volts. When you turn on the air it drops down to 11.25 volts.