Your regulator contact could have welded together too or something is applying power to the field terminal. Pull the plug off the alternator and check to see if there is power on the F terminal. If you see power there, you need a new regulator or need to find the source for the voltage.
To test for a faulty diode, set your multimeter to ac volts. Test black lead to ground and red lead to charge post of the alt. Replace or rebuild the alt if more than 50mV is detected. In order to get a reliable reading, your battery should be fully charged before the test.
To test for a faulty diode, set your multimeter to ac volts. Test black lead to ground and red lead to charge post of the alt. Replace or rebuild the alt if more than 50mV is detected. In order to get a reliable reading, your battery should be fully charged before the test.