Ford Repair: Battery Light, ford taurus, proper diagnosis

Question
Our 2002 Ford Taurus recently had the dashboard battery light come on intermittently.  The mechanic checked the battery; it's fine.  He replaced the alternator twice; both times the same problem occurs.  He reinstalled the original alternator and only charged for diagnostics.  He said the dealership may be able to reset the car's computer, where he thinks the problem lies.  Is this possible?  How can I prevent getting ripped off for a alternator I might not need, in view of the fact this problem has occurred with three different alternators?  Thanks for your advice!

Mark  

Answer
if you had an intermittent battery light coming on and everything checked fine a logical conclusion would be alternator but as you found out that is not always the case-the only way you can prevent being ripped off as you say is to leave car long enough and be prepared to pay some diagnosis time hunting the problem-could be loose connection, actual bad alternator and aftermarket alternators have a lot of problems
i don't care how many you put on-a bad computer can turn light on-it would have to be on when checking for a proper diagnosis unless the tech finds a bad connection (corroded,loose,burnt etc.) tps sensors have been known to cause charging system problems, a sulfated battery can also. good luck