Voltage readings only tell part of the story about the charging system. If you find the battery voltage is between 13.75 and 14.75 volts with the engine running, you know the generator is doing something, but that's all. It also has to be professionally load-tested for maximum output current and "ripple" voltage.
There's six diodes in the generator. If one fails, you will lose exactly two thirds of the generator's maximum output current. That leaves you with not enough current to run the entire electrical system under all conditions. The battery will have to make up the difference.
With a bad diode, you lose one phase of the three-phase output. During the periods of the missing phase, voltage drops significantly. That variation in output voltage is the ripple voltage. That goes very high. That can affect computers and confuse them so weird things can happen. Most often people notice that as dash warning lights turning on for no apparent reason.
A totally unrelated cause of intermittent dash lights has to do with something rubbing and shorting inside the generator and that can occur at higher engine speeds when there's more centrifugal force acting on the rotor. That is harder to detect and it won't show up with a load-test.
A point to keep in mind is that due to how the voltage regulators work, AC generators, ("alternator" is copyrighted by Chrysler), can produce voltage spikes. GM has a huge problem with this. I haven't really heard of it being a problem on any other brands, but the battery is the key component in absorbing and dampening those harmful spikes. To prevent repeat generator failures, if you have them, replace the battery at the same time unless it is less than about two years old.
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Monday, October 14th, 2013 AT 1:55 PM