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Old 02-09-2011, 12:35   #31
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Re: Alternator Question

Excellent diagram, Eric.
Where did you find it?
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Old 02-09-2011, 12:53   #32
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Re: Alternator Question

Quote:
Originally Posted by chala View Post
any positive diode leaking will feed the regulator, brushes and may produce a drain depending how the regulator behave.
Yes, I see that a shorted positive diode would bias the diode trio and power the regulator. Definitely could cause a drain depending on the regulator. Good call chala. I'm up for some more bench testing.

Here ya go GordMay, there's a whole pile of pics here:

http://www.google.com/search?q=alter...AQ&safe=active

Eric
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Old 04-09-2011, 07:12   #33
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Re: Alternator Question

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Originally Posted by fairbank56 View Post
Yes, I see that a shorted positive diode would bias the diode trio and power the regulator. Definitely could cause a drain depending on the regulator. Eric
In regard to this drawing not really but again the drawing is incomplete. For this schematic to work the ignition switch must be wired to the negative. When the switch is open a faulty diode will cause no drain. The Alt light to dash if wired to a positive cannot be a drain. So Eric you are right: no drain.

Now your drawing is a little more complex that a simple drawing that contain only 3 terminals: POS, NEG, D+. D+ goes to the Alt light and then to ignition switch and then to the battery positive. In that type of schematic a faulty diode can cause a drain.
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Old 04-09-2011, 14:41   #34
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Re: Alternator Question

Well, I finally got back to the boat and determined that in fact the alternator is the cause of the problem. It's a Hitachi fitted as standard to the Yanmar GM1. It's the 30 Amp model and seems to work perfectly, charging the battery at 14.3 V. With the engine stopped and everything turned off except the battery selector switch it draws 3.5 Amps thru the alternator. Disconnect one of the wires to the alternator--- zero draw. The battery selector switch had also been defective, never really switched off, but I fixed that. Now my problem will be to figure out which diode is the bad one.
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Old 05-09-2011, 23:19   #35
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Re: Alternator Question

And to truly stop the pi__ing match how many bad diodes and which ones..
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Old 06-09-2011, 15:37   #36
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Re: Alternator Question

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And to truly stop the pi__ing match how many bad diodes and which ones..
Sorry to upset you.
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Old 07-09-2011, 22:00   #37
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Re: Alternator Question

Eric, according to Charlie Wing's Boatowner's Electrical handbook (p.57):

"
Figure 4.20 shows three small diodes (the diode
trio) connected to the three coil terminals in a type-N
alternator. These diodes feed a terminal labeled “Aux.”
When the alternator is not turning, there is no current
to power the regulator, so there is no drain on the battery.
Reverse current flow from the battery to the auxiliary
terminal is prevented by the charge diodes.
"
and
"Figure 4.21 shows a second method of supplying
regulator current. A large isolation diode is placed
between the three charge diodes and the alternator +
output terminal. This isolation diode prevents reverse
current flow from the battery to the regulator."
[emphasis mine]

Not in the rectifier section, but in the diode trio. I wasn't clear in the differences between the two, physically, because I've seen the "diodes" referred to as the "diode trio" "diode frame" and generally just "diodes" in various manuals and as replacement parts.

Thanks for making me dig enough to get clear on that. Whether, in practical terms, there's a lot of confusion about which diodes can be bad, or whether it is actually a dual failure that really makes alternators eat batteries...the bottom line is still that somewhere in an alternator, it can go bad and cause battery drain. And some of them do exactly that.
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