1990 Oldsmobile Cutlass charging system

I have a 1990 Oldsmobile cutlass ciera that will not charge the battery. I removed the alernator and had it tested, and it came back ok. I checked the voltage of the battery with the car not running and it was 12 volts I also checked the votage with the car running and it was also 12v. I checked the voltage coming from the main alternator wire with the car running and it was about 15v. I checked the excitor wires to make sure they had power and they checked out, I also checked all of the fuses and they checked out as well. I found that as long as Im not running a high electrical load that my car would run for quite a while. I had a guy tell me to check the voltage regulator to see if the charge being created was making it back to the battery. One thing I dont understand is why the charge "dummy light" would not come on untill the service engine soon light came on when the battery was already dead. I think running a resistance check through the main alternator wire may help me find if the vote regulator is good or not. I also made sure I have a good battery, I put it on a load tester and it came back 12v. I hope you could help me with this problem I really need my car to run and hope you know more than I do about this problem.
Thanks much,
caleb mccary