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Originally Posted by kwren
...None of the people here care about how a diode works...Keith
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You are clearly not speaking for yourself (or me) and there is no need to speak for others. Especially, if you are going to disparage their intelligence and/or curiosity.
A generator produces direct current (DC) which has a fixed polarity, + and - are always on the same side (wire). An alternator is more efficient than a generator, but produces alternating current where the polarity (+/-) switches as the armature is turned. A diode is like a one way street sign, or a turnstile. It allows current to flow in one direction only. By placing a diode on both sides of the output an alternator can be polarized to supply a DC system.
A diode can fail in 2 ways.
It can stop passing current in either direction. If only one fails, this will result in only half of the output being supplied to the system. Because there will still be voltage of the correct polarity, this condition is difficult to test and is the reason why so many bad alternators are tested and proclaimed good.
A diode can also fail by allowing current to pass in both directions. Diodes are designed to resist a certain amount of reverse current flow. This is called the reverse breakdown voltage. If it's capasity to resist this flow is diminished, things will not be well. The symtoms of this are difficult to predict, except that the greater the load, the greater the symtoms will become. This condition will diminish a battery's ability to accept and deliver a charge and eventually render it useless. Note that such a battery might provide acceptable voltage readings if not under load.
What Trevor is suggesting is that this condition can be discovered by using a voltmeter set to A/C to find what he refers to as 'alternating current component'. Prior to reading his repeated suggestion in this forum, I was unaware of any test for this condition. I have not had the opportunity to perform this test, but it does make sense. Historically, a repair would envolve replacing both the battery and the alternator. Usually one, then when that didn't work, the other.
Charl
Edit:corrected spellings.