My Test Mule

Unless you can load the output to full rated current, you are not fully testing them. For example, a "let's say" larger maybe 50A alternator with just one open diode will still output enough to run an older car.

A bad diode CAN cause big problems especially with solid state devices because it causes a lot of AC ripple on the output, as well as a nasty whine in any audio systems.

"In a previous life" in the Navy, my 69RR failed I think it was 2 diodes. This was in the later part of "before payday" so I just pulled it apart and cut loose the two bad ones, and went ahead and ran it. This turned a ?37? A alternator into I don't know, a ?15-20A? alternator. This was in San Diego, so I avoided the heater, did not drive much at night, and a 69 383 RR does not usually have much in accessories. You would not get away with that trick with today's EFI systems............

On the other hand, your test method is likely as good (bad) as these parts stores. Most these parts store testers do NOT fully load an alternator