This video covers the essential steps to properly test a car's alternator and diagnose common alternator issues. The guide ...

A failed diode often whines accompanied by reduced charging output. Have your technician test your alternator with an oscilloscope. It should show a straight, scalloped waveform with no weird spikes.

Contenders can be as simple as an always-on interior light bulb such as the one in the trunk, to a shorted alternator diode, or more complex items dealing with failing onboard electrical modules.