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Old 27-08-2011, 07:12   #1
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Alternator Question

Can the alternator on a small Yanmar diesel which is putting out 14.4 volts and charging the batteries fine still have a bad diode in the voltage regulator that will drain the battery power when the engine is not running. If so, how do I detect it? Thanks,
nial
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Old 27-08-2011, 07:15   #2
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Re: Alternator question

How about before you leave the boat you disconnect the alt and see what happens?
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Old 27-08-2011, 07:52   #3
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Re: Alternator question

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Can the alternator on a small Yanmar diesel which is putting out 14.4 volts and charging the batteries fine still have a bad diode in the voltage regulator that will drain the battery power when the engine is not running. If so, how do I detect it? Thanks,
nial

process of elimination Niall as was siad before is the only way to sort this, disconnecting wires and a voltmeter should track all this down in a hour or two.

Dave

PS: why start a new thread
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Old 27-08-2011, 14:48   #4
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Re: Alternator Question

I agree with the previous. But the alternator on old Yanmar don’t give much current. I hade a Yanmar YSB8 and the alternator rotated at the same speed as the engine. The engine work best at 2500 rpm and at that speed the alternator gives between 25 - 30 A. I made my own pulley and that way doubled the speed on the alternator. On the picture I have also change the alternator.
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Old 28-08-2011, 10:02   #5
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Re: Alternator Question

I would disconnect the positive at the alternator, if it spark then you have a current flow. Then I will use a test lamp between the + terminal and the + wire to see how much current flow by judging by the brightness of the lamp, then I may replace the test lamp with an ampmeter to see how much current flow.
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Old 28-08-2011, 10:05   #6
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Re: Alternator Question

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I would disconnect the positive at the alternator, if it spark then you have a current flow. Then I will use a test lamp between the + terminal and the + wire to see how much current flow by judging by the brightness of the lamp, then I may replace the test lamp with an ampmeter to see how much current flow.
With the engine off of course. With the engine on, disconnecting the output will blow the diodes.
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Old 28-08-2011, 10:41   #7
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Re: Alternator Question

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With the engine off of course. With the engine on, disconnecting the output will blow the diodes.
Courageous is the one who work on an alternator with the engine running.
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Old 28-08-2011, 10:43   #8
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Re: Alternator Question

Get a clamp ammeter or a regular ammeter/multitester to see if you have current flowing when and where there is not supposed to be any current flow.
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Old 28-08-2011, 10:46   #9
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Re: Alternator Question

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With the engine off of course. With the engine on, disconnecting the output will blow the diodes.
Courageous is the one who work on an alternator running. May be not.
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Old 28-08-2011, 11:02   #10
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Re: Alternator Question

If a diode is blown OPEN the output voltage will be low. If a diode is shorted then you would have no DC output and will not charge the batteries. It is possible that the stator is being energized by the regulator but that is turned off with the key switch. Measure the voltage of this small wore running to the alternator with the key off. Should be 0 volts. You are probably looking in the wrong place.

If your batteries are going flat you may have bad batteries or another source of current flow.
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Old 28-08-2011, 23:14   #11
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Re: Alternator Question

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Courageous is the one who work on an alternator running. May be not.
I just finished installing a new sea water pump. The instructions tell you to tighten the bolts while the engine is running, being careful of the fan belt!
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Old 29-08-2011, 14:26   #12
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Re: Alternator Question

Nial, with the engine OFF, take a dc multimeter, set to the 10A or 20A range. Disconnect the alternator output wire from the battery, connect the ammeter between the two. If there's any current flowing in that line when the engine is off, it means there's a bad (leaky) diode.

You can use a multimeter with a 2A range, but the odds are the leak would be higher than that and blow the fuse in the meter or harm the meter right away. With the 10A or 20A range...you should at least be able to see it showing a load before the fuse blows, IF the load is higher.

Also bear in mind, when you're working on the battery positive directly, at the battery post, any "oops" short to ground can carry 3000 amps or more, and do real damage to fingers and equipment. Do take precautions against that.
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Old 30-08-2011, 02:22   #13
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Re: Alternator Question

All this testing would be unnecessary if you just had a monitor and could see whether or not there is current flowing out with everything else off.
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Old 30-08-2011, 02:36   #14
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Re: Alternator Question

There are a couple of battery monitors that will show current and even AHrs for a second bank but most battery monitors will only show current and AHrs on 1 battery bank. Its generaly volts only for the start battery.
A clamp on DC multimeter gets my vote for the best equipment investment. In a large portion of the electrical questions asked on this forum it would make solving the problem much easier.

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Old 30-08-2011, 03:57   #15
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Re: Alternator Question

Amen.

And, you can buy a true RMS AC/DC clamp-on ammeter with multi-functions, including in-rush, frequency, voltage, etc. for less than $70 these days.

Bill
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