Who is right regarding a noisy alternator: Technician A claims it can be caused by a bad diode, while Technician B claims it can be caused by a worn bearing?

The correct answer is that both Technician A and Technician B are right because a noisy alternator can indeed be attributed to issues related to both bad diodes and worn bearings.

A bad diode can cause a variety of problems in an alternator, including electrical noise. Diodes are responsible for rectifying the alternating current (AC) generated by the alternator to direct current (DC) suitable for charging the battery. When a diode fails, it can create an uneven output, leading to whining or buzzing noises as the electrical current fluctuates. This noise is often a result of the rectification process being compromised, generating harmonics or interference.

On the other hand, worn bearings are a mechanical issue that can also lead to alternator noise. The bearings support the rotor and allow it to spin freely. If the bearings are worn out, they can produce grinding or rumbling sounds as the rotor moves within the stator. This mechanical noise is different from the electrical noise caused by a bad diode, but it is still a significant symptom to notice when diagnosing alternator problems.

Thus, both technicians are indeed correct in stating that their respective issues can cause noise in an alternator, making the answer that both A and B are right the most appropriate.