How do I check for open diodes in the rectifier? I'm sure I need to take the alternator apart but will it show when tested? I'm having the same issue. I have a new Ron Francis Express fuse box/wiring harness, 1 gauge battery cables, 6 gauge ground from battery to body in 2 places. The fuse box, Ron Francis voltage regulator, are all grounded. Alternator reads 14V and above at idle. At higher rpm it reads 15V. I have checked voltage with a fluke multimeter and have very minuscule voltage drop. For example, at 2500 rpm, I get 15V at the battery terminals and 14.91V using the master cylinder, alternator body or intake manifold as a ground. I'm thinking alternator even though it's new.
Diodes are easily checkex with a ohmmeter.
There should be a large difference in ohms between the test leads placed one way vs the other. An open diode reads more or less infinite resistance.
For me, the easiest way is just listening to the alternator. If all the diodes are good, there's no humming-whirring sound, because there's very little AC ripple in the three phase after summing into DC.
If one diode is out, you'll start to hear the whirring sound, if two diodes are out, the whirring is unmistakable, and you'll notice the ammeter never goes way over into charge like it should, say like if you left the headlights on for 10 minutes with the engine off, and then went for a drive. Three diodes out, and you'll notice the ammeter hardly goes over into charge at all, and can't even accomodate the headlights at idle in neutral. It will go into the discharge immediately. This is why I love ammeters, for the additional information they provide.
Now, this whirring is not high pitched at idle speeds, it's more like like a hum, about the pitch of the first note in "Happy Birthday to you". It can be heard just as easily in the car as standing in front of the alternator, maybe better. If you have a factory radio, the hum will be quite clear as soon as the radio is turned on with the volume down all the way, and it doesn't get louder with turning the volume up. The hum is AC ripple superimposed on top of the DC level.
If you have an oscilliscope, it sticks out like a sore thumb, you'll see the ripple where the dead diodes didn't fill in the output, and that's the ripple that makes it audible both physically and in the radio's output stage.