It is important whenever a car is serviced to check the alternator ac diode pattern. This will help that the customer doesn’t have problems later, because as a mechanic we know drivers are not checking the little battery light in the speed clock or some will call it the cluster. When the key ignition is switch on this battery light illuminates and when the engine starts it should go off. It should go off completely. If it just glows a little bit, then the alternator is not charging properly and of cause, if it does not go off at all then the alternator is not charging at all.
The following car came in with a flat battery. The alternator test reveals that the alternator was not charging. I decided to replace the diodes and the regulator. After that was done the alternator charge properly but after the engine was idling for a while I could hear there is suddenly a whining sound coming from the alternator. I thought let me test the alternator with a scope at its output wire to the battery to see what the diode pattern is and the next image reveal the problem.
Now this pattern reveals that the diodes are faulty or something is serious wrong with the alternator. I have removed the alternator and strip it. What I found was that one wire was broken off from the stator. There are 3 x sets of 2 x wire each and one wire was broken off. This caused the alternator to whine and not to charge properly. The alternator was not charging when the engine is idling, but if you increase the rpm then the charging voltage goes up to 14v. The battery light also did not go off completely.
I have decided to replace that alternator with a new alternator, see the next image of a good alternator diode pattern. The ac voltage should be averaging .5volt.