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Old 18-07-2007, 17:47   #1
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Question How to turn off an alternator?

We have a Universal M3-20 diesel engine with the stock Prestolite internally regulated alternator which seems to put out 14 to 14.2 volts constantly no matter what the batteries state of charge is. It never drops below 13.8 and usually is around 14. This alternator was just totally rebuilt this spring, new everything except the case and windings. I want to protect the diodes from accidentally shutting off the batteries (this necessitated the spring rebuild) and also to be able to turn off the alternator so it doesn't fry our little house bank which currently is two group 24 dual purpose 12 volt but will shortly be two Group 27 Trojan 12 volt deep cycles. We have minimal electrical requirements as we have no icebox, use LED lights, have a 4 stage charger, etc., etc.

We have a separate starter battery wired direct to the starter with its own disconnect switch. The alternator voltage sense wire is on the alternator output so it sees the house bank, not the starter battery. The starter battery is charged by a Xantrex Echo charger off the house bank. We really don't need the Perko switch, but it is central in the panel, would leave a big ugly hole if not there and it does give us the ability to run the house off one battery if we have a dead cell or something in the other one. We carry an AWG1 jumper cable if we ever need to boost the starter from the house.

I replaced our Perko battery switch (8501) with an identical Perko switch that has the field disconnect circuit in it (8503). The field disconnect circuit takes the feed from the purple wire feeding the EXC (Excite) terminal on the alternator to the F1 terminal and then from the F2 terminal returns to the EXC terminal on the alternator. Then I put a manual switch in the line from the Perko field disconnect F2 to the alternator EXC terminal.

I thought that by breaking the power on the EXC terminal with the manual switch I could turn off the alterator output when the batteries are known to be topped up and not needing charge thus avoiding boiling them to an early death. But when I installed all this stuff and tested it, nothing seems to happen, the alternator keeps on putting out 14 to 14.2 volts even after I turn off the EXC circuit with the manual switch I added.

So I am guessing that this alternator only needs voltage on the EXC terminal until it starts making power, then somehow it is getting voltage on the field internally and no longer needs anything from the EXC terminal.

This raises two questions:
1./This indicates that the diode protection on the Perko switch is useless since breaking the field feed from the purple wire seems to do nothing, is that correct?
2./How do I turn this alternator off?

I know the real solution is a P type alternator with an external regulator, but we are 5 years and 10 feet of boat away from being able to justify this equipment. I just don't want to be buying new house batteries every year.

Thanks in advance for any assistance.
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Old 18-07-2007, 20:35   #2
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Prestolite / Leece-Neville Technical Bulletins

The above link may help. I'm not sure what alternator you have but usually it's the wire going to the "Field" terminal where you want to insert the switch.

I would strongly recommend you convert your alternator to externally regulated. Rick posted a writeup here on how to do that. You should be able to search for that.

Actually it's here

Smart Regulator for Yanmar Alternator
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Old 19-07-2007, 10:07   #3
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If still having trouble, post a photo of the back of the alternator.
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Old 19-07-2007, 13:46   #4
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Thanks for the responses, appreciate the help. I had seen the other thread about converting to external but I've also read that doing this with these low output alternators often just kills them quickly. Prestolite has on their forums that they supply Balmar with certain alternators, though I doubt this is one of them.
I'll try to get a shot of the back of the alternator this weekend but it was painted black by the rebuilder so I'm not sure what's going to show up. I can't read the model number anymore without taking it off and scraping off the paint.
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Old 19-07-2007, 14:44   #5
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I looked at some of the wiring diagrams at the Leece-Neville site and they do show a switch on the lead going to the Exitation terminal. However, some of the units seem to have a wire at the alternator to this terminal. If there is a wire coming from inside the alternator to the Exc terminal you could try disconnecting that to see if the switch can then control the output.
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Old 23-07-2007, 12:37   #6
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Switch actually works

I checked with a voltmeter and found that the switch and the field disconnect on the Perko switch actually are working, they both put the voltage at the excite terminal to zero with the ignition on and either of them opened. With the switches closed and ignition on, the voltage is 12.72.
Have done some more reading on this and found some info that indicates that an alternator has enough residual magnetism in the rotor once it is running that they can continue producing output with nothing on the excite field. Not sure if that is what is happening with mine but it seems likely. Have posted a question to Prestolite about this issue.
These alternators are simple to convert to external reg, Prestolite has a plate that they sell to do this, so I may end up doing that.
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Old 23-07-2007, 14:39   #7
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By all means, wait for the reply from Prestolite. With modern alternator designs, disconnecting the wrong wire can make them burn up in ten seconds. That varies with the design, and there are many different designs.
Some put out exactly and only 14.3-14.4, that tightly regulated, and they put it out "all the time" but in short pulses, ranging from 10x to 10,000x per second, so they always are "on" but they may really be "off" when they are not putting out the other 9,990 pulses every second.< g >

That's why it pays to ask the maker.

Diode protection at the battery switch may or may not also be unnecessary, some alternators have more brains than others, and protect themselves internally.

"I'll try to get a shot of the back of the alternator this weekend but it was painted black by the rebuilder" Beware of pretty paint jobs. Some years ago a very senior man at Delco was kind enough to teach me a bit about alternators. He said, all they have to do is spray ONE COAT OF PAINT over the bare metal parts (diodes especially) that are supposed to be AIR COOLED, and a rebuilder can guarantee that your newly rebuilt alternator will die in a matter of months. Some parts are simply never, ever, to be painted for any reason--unless something else is done to cool them. Typically the installation of dual fans (one on each end) for marine use.

"These alternators are simple to convert to external reg, Prestolite has a plate that they sell to do this, so I may end up doing that." Now, that's NICE. Your batteries will love you for switching to a real marine regulator, and you will save money in the long run by not replacing batteries as often.
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Old 24-07-2007, 18:04   #8
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Answer from Prestolite

On the Prestolite forum their support folks advise me that the alternator field is getting power through the diode trio. Solution is that there are two green wires under the regulator, disconnect one of them to be able to turn off the field circuit. Will be trying this out as soon as I can.
They seem to have an excellent website and very good support.
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