What year is your bronco? Battery fully charged? 11 volt’s at idle? What about revving the engine at about 2,000 rpm’s, how many volts? Why I asked about when revving at 2,000 rpm’s, diode’s could be bad. Went through 5 on a Nissan pickup back in the day, before I got a good one! Which it’s odd to get that many bad! Is your voltage regulator on the firewall or built internal within the alternator? Have you checked out wiring for resistance to insure all is good! A lot of variables when checking electrical systems.
Battery must be fully charged or you will get incorrect reading’s for starter’s! Check wiring for resistance if all good! Battery is in good working order (meaning fully charged and no bad cell’s)! Part’s are made cheap and seems to be almost as many bad as they are good! If everything else check’s out good with the wiring and battery, no question about it, you have a bad alternator!