short circuit ? electrical prolem? help!

Diodes can fail 2 ways. Opened or shorted. We don't see the shorted example nearly as often as the open. When all 6 work, the alternator is strong, but if one or more open up the alternator works but is weak, and a
unique whine emanates from the alternator ( can be heard in the car).
This is due to uneven AC rectifying out of the 3-phases.
The "missing" pole's output then can be heard.
You can go years like this, if only one goes out.
Often the wires break on the old press-in diodes, right at the base of the wire.
This is due to side tension being left on the lead in soldering.
The wiring from the stator should be arranged to have no tension or stress
on the diode lead before soldering.