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Originally Posted by Trevor
The suggestions made here is that a faulty alternator can be detected using a DC voltmeter, because the output measured across the battery, will not be close to 14 volts, if the machine is defective.
It is fact that members here have had an alternator checked by professionals after experiencing the flashing trouble lights problem, then have been advised that the alternator is OK. One would expect that the output voltage has been measured under load by way of a test, but one must presume that the procedure has not detected an AC component in the output, due to only DC test gear being employed.
My contention is that the alternator could produce adequate voltage, albeit at reduced current, even though a faulty diode may exist, but at the same time produce an alternating current component within the output. N.B. --- Hence my reason to dispute the validity of a simple DC voltage test.
The alternator is in effect a three phase device, incorporating nine diodes in rectifier bridge networks, so that the failure of a single diode may only somewhat reduce output current, and the voltage regulator could compensate in respect of voltage.
We need someone who has an alternator with the fault in question, able to carry out proper tests and experiments, in order to be able to exactly confirm the several factors which apply. The understanding required, will likely be above the head of the average auto electrician.
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One of us is able to use a simple test to check an alternator with a DC voltmeter.
None of the people here care about how a diode works. If an alternator doesn’t charge the battery, the alternator has to be replaced. . I have a lot of education and years of experience including one electronics school that began with basics including how every part of an alternator is made, the composition of batteries, and eventually getting to electrical schematics that you couldn’t put on a a sheet of paper as big as a double page of a newspaper. Just one school was 8 hours a day for a full year. I would be void of logic if I couldn’t tell that the alternator was bad if all my red lights came on and I put a voltmeter on it and only got the 12 volts from the battery. When I drove it again and the red lights went off, checked it with the lights off and the output was 14 volts. When 14 volts are put into a 12-volt battery, it charges the battery, period. More logic! I do not need all my education and experience to find out why my alternator doesn’t work.I only need to know to set the meter for dc volts, put the red lead on the positive battery terminal and the black lead on the negative terminal. I don’t even care why the alternator doesn’t work. Why, because to replace the parts in our old alternator would cost at the least $2,000.00 at the Subaru dealer. Several of the parts cost more each than replacing the whole alternator! More logic, just pay the $200 and get a whole new alternator at the same Subaru dealer!
You might be amazed at how little electrical expertise these auto “mechanics” actually have!
KISS Keep It Simple, St----
Keith