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Old 26-09-2011, 14:43   #1
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Alternator Story (Continued)

Having been told how to determine if my battery was being drained back thru my alternator by kind folks on this forum, ie. unhook the wire from the back of the alternator that goes to the battery and measure voltage or amps with the engine not running and switched off. Anything above a few milliamps shows a leaky diode. I measured 3.5 amps. Took alternator to a very well respected shop and he found it to be perfect. Said to check if I am getting power at the R terminal when switched off. Shouldn't be. He said the L terminal might or might not show power depending on how it is wired. When I look at the wiring diagram in the Yanmar manual I can't see how either L or R should have power when ignition is off. I'll put the alternator back on and do as he said but I'm doubtful because even the Fluke handbook instructions on checking alternators says that any power at alternator battery terminal when engine if off means a leaky diode.
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Old 27-09-2011, 14:06   #2
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Re: Alternator Story (Continued)

Not sure what more evidence do you want? Your battery is pushing 3.5amps through the alternator, right? As long as you are sure that this is going through the alternator not some other hidden leak, that is all you need to know that alt is toast. Sorry.
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Old 27-09-2011, 15:08   #3
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Re: Alternator Story (Continued)

Wish it was so simple. Here is what happened today when I reinstalled it. With no wire attached to alternator I hooked up the meter in series between the B terminal and the red wire and got 2.5 amps. Sometimes I got nothing. Sometimes it varies up to 3 amps. When I try continuity between the B terminal and ground terminal I get it one way, switch leads and get none. Must be a diode between them but surprised that I should have any continuity there at all. This is an alternator that puts out 14.4 volts and was pronounced perfect by a well respected repair shop.
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Old 27-09-2011, 17:10   #4
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Re: Alternator Story (Continued)

Make sure there is no wire attached to the alternator main terminal. Then measure the current between battery positive and alternator terminal. (is set your multimeter for DCA, clamp positive to battery, negative to alternator). If this is what you are doing and getting 2-3amps - that is the whole story. It may be possible that the alternator will put out 14V even if some of the diodes are shorted (although it will not be as nice a waveform if you were to look at it on the oscilloscope).

I mean, if your alternator is pulling serious amps out of your battery, how do you want to use it? Disconnect it each time you shut down the engine?
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