I have a 2002 mazda protege5 put new alernater on and battery tested good still not charging.

I did and I could not find any bad . I also checked for voltage coming from the output post on the back of the alternator there is none. Is it possible the new alternator is bad?

Good morning Brian, it is possible the new alternator is not working, but you would want to eliminate some other possibilities. With the engine running at idle, check at the battery with your digital multi meter to see what the voltage is. At the battery it should read around 14.5-DCV. If your not getting that, check continuity of the large output wire from the alternator to the B+, a lot of times there is a fuse-able link in that line that could be burnt out.

when unhook the large wire from output post on alternator and check voltage at that post I have nothing.

You would have nothing at the alternator post. It is in the wire you should have showed battery voltage.

I think I have a computer issue the computer tells the alternator to charge. alternator tested good as well as battery. I don't think I have a short because I drove car about 6 different places in about 5 miles and still had about 12.15 volts in it when we got home. starting car each time. They don't make a repair manual for this car. all fuses are good. That i could find.

12.15 DCV is showing the battery is discharging. The battery should have a minimum of 12.5 DCV if the alternator, charging system was working. Did you check for voltage at the main wire that runs to the alternator? There should be battery voltage there. As a matter of fact you did check..." I also checked for voltage coming from the output post on the back of the alternator there is none" ...There should be battery voltage on that wire.