15.2 volts is too high. That is typical of a bad diode. If the voltage regulator is shorted, the battery voltage will be much higher, especially when you increase engine speed.
AC generators all put out three-phase current. When one diode is defective, you'll be missing one of those phases. It also shorts out half of each of the other phases. That's where the loss of two thirds of the output current capacity comes from. On some designs, the regulator responds to those momentary dips in voltage during the missing phase, and it tries to bump up the output current to try to get battery voltage back up. In effect, it over-reacts and can't respond fast enough to bring the voltage back down to below 14.7 volts.
The generator needs to be tested for maximum output current. Most auto parts store testers don't test for that, and if they do, the motors in them aren't strong enough to run a generator wide-open. That test needs to be done with the generator on the car. The test only takes five seconds once the cables are connected, but it should be done while the warning light is on.
Friday, December 27th, 2013 AT 5:20 PM