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Old 29-01-2017, 19:33   #1
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Why do I have an on/off switch coming off my alternator?

Hello all,

Decade long reader, first time poster.

I am cleaning up the wiring on my Perkins 4.108 and today noticed that the hot charging wire coming off the alternator goes into a Blue Sea Systems ON-OFF battery switch and then straight the battery.

What is this switch for?

I've read that disconnecting the batteries while the engine is running can destroy the diodes in the alternator - but wouldn't turning the ON-OFF switch off do the same?

Why even have the switch?

Thanks for your help.
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Old 29-01-2017, 19:38   #2
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Re: Why do I have an on/off switch coming off my alternator?

Turning off the switch is just an easy way to disconnect something so yes, the effect would be the same.

No idea why it's there. I suspect a PO did it for some reason. I would pull it from the system or put a BIG sign over it saying "Do Not Switch Off".

Related, is there a fuse on that wire? If not I would add one.
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Old 29-01-2017, 19:43   #3
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Re: Why do I have an on/off switch coming off my alternator?

No fuse. What's weird is the boat is a 1972. Most of the electrical system looks dated to that era EXCEPT this Blue Sea Systems switch, which is much newer. I have no idea when it was added but it WAS added. There is no fuse. It's just in line.
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Old 29-01-2017, 19:47   #4
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Re: Why do I have an on/off switch coming off my alternator?

Some alternators have more leakage current than they should, so that the batteries can be drained in a matter of days if the boat is sitting with no charging source, as might happen if it were moored for an extended time or in a slip without shore power. Before solar panels were as inexpensive as they are now, the quick fix was to put in a switch. Cheaper and easier than replacing the alternator, and sometimes a newly rebuild alternator will still have high leakage current...

The other alternative is that the disconnect would facilitate safer engine maintenance (or alternator replacement) without disconnecting the battery.
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Old 30-01-2017, 03:26   #5
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Re: Why do I have an on/off switch coming off my alternator?

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..... high leakage current...

The other alternative is that the disconnect would facilitate safer engine maintenance (or alternator replacement) without disconnecting the battery.

Yes, the switch is likely there so that maintenance on the engine can be safely done, otherwise the big lug on the alternator is always hot. Have to be sure not to switch off while the alternator is running or you could blow some diodes.

Never heard of "alternator leakage current" being a problem. Easy to check though with a multimeter. There should be NO current in the B+ cable with the engine shut down.

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Old 30-01-2017, 03:51   #6
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Re: Why do I have an on/off switch coming off my alternator?

That sounds sort of like my setup, but I am not sure what you mean by hot charging wire.

In my setup I can switch the 12 volt house bank alternator on and off independently of the engine ignition. This is partly because the engine has its own separate 24 volt alternator and ignition circuit , but more importantly it gives you independent control of the house charging alternator if needed. Mine is switched using the standard "ignition" terminal of the alternator, which I would not call a hot charging terminal so may be a different setup.

I have heard of alternator leakage, not surprisingly observed the same on my old Range Rover though the alternator in question was Italian not English. Had to put a relay on the ignition feed to the alternator too, otherwise the engine could not be switched off once it was running, the alternator would feed power back out the ignition feed terminal which kept the engine management computer live.


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Old 30-01-2017, 06:34   #7
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Re: Why do I have an on/off switch coming off my alternator?

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Originally Posted by btrayfors View Post
Never heard of "alternator leakage current" being a problem. Easy to check though with a multimeter. There should be NO current in the B+ cable with the engine shut down.
I have found that it is the one-wire alternators that are the most frequent offenders.
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Old 30-01-2017, 08:41   #8
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Re: Why do I have an on/off switch coming off my alternator?

Quote:
Originally Posted by theglo View Post
Hello all,

Decade long reader, first time poster.

I am cleaning up the wiring on my Perkins 4.108 and today noticed that the hot charging wire coming off the alternator goes into a Blue Sea Systems ON-OFF battery switch and then straight the battery.

What is this switch for?

I've read that disconnecting the batteries while the engine is running can destroy the diodes in the alternator - but wouldn't turning the ON-OFF switch off do the same?

Why even have the switch?

Thanks for your help.
It is referred to as a "service disconnect." Leave it on, until you need to remove the alternator for some reason. Turn it off before disconnecting the alternator, this allows the rest of the electrical system to remain on while the alternator is being repaired or replaced.

Without the service disconnect, it is necessary to either completely disconnect the batteries by either tracing the alternator wire to where it connects to the batteries or disconnect the negative wires from the batteries, thereby shutting down the system.

If the alternator wire (B+) touches the engine block while the ground wires are connected, there will be an immediate massive short with ruined batteries and perhaps a fire.
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Old 30-01-2017, 08:47   #9
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Re: Why do I have an on/off switch coming off my alternator?

Quote:
Originally Posted by Jammer View Post
Some alternators have more leakage current than they should, so that the batteries can be drained in a matter of days if the boat is sitting with no charging source, as might happen if it were moored for an extended time or in a slip without shore power. Before solar panels were as inexpensive as they are now, the quick fix was to put in a switch. Cheaper and easier than replacing the alternator, and sometimes a newly rebuild alternator will still have high leakage current...

The other alternative is that the disconnect would facilitate safer engine maintenance (or alternator replacement) without disconnecting the battery.
I rebuild alternators and never heard of leakage current. Also that big terminal is hot all the time as some are connected to the big lug on the starter. The diodes prevent the current from coming back to the alternator.
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Old 30-01-2017, 09:11   #10
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Re: Why do I have an on/off switch coming off my alternator?

Exactly!
I wonder if this was a PO attempt to get around a set of failed diodes?
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Old 30-01-2017, 09:31   #11
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Re: Why do I have an on/off switch coming off my alternator?

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Exactly!
I wonder if this was a PO attempt to get around a set of failed diodes?
No, if there are failed diodes the alternator will not put out any amperage. You need all six diodes to put out alternating current to direct current and amperage. If one fails, no output.
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Old 30-01-2017, 13:19   #12
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Re: Why do I have an on/off switch coming off my alternator?

Does your alternator only have two wires, or is it tree? If it is more than two, it could be one of those old alternators that need magnetization current for the rotor. If you switch of the magnetization current you would effectively turn off the charging without damaging any diodes. If the alternator has this solution, it would also need to have brushes for the magnetization circuit. The brushes would need to be replaced from time to time. Permanent magnets in the rotor eliminates this maintenance, and thus is the modern solution but then it is no way to turn of the charging while the alternator spins.
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Old 30-01-2017, 13:36   #13
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Re: Why do I have an on/off switch coming off my alternator?

Hi I have a Guest 2 bank selector switch on my boat,twice in the last year I left the switch on the starting battery positon,when I returned two days later the battery was totaly drained. I was told that was caused by current drain through the alternater or the conection to the starter solinoid.As a result starting on my house battery allowed me motor out into open water which allowed me set sail and while out of gear with the engine running switched to the starting battery and recharged it.Rushing away and forgetting the first time,dumb second time dumb dumb!!
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Old 30-01-2017, 13:39   #14
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Re: Why do I have an on/off switch coming off my alternator?

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Originally Posted by seabreez View Post
No, if there are failed diodes the alternator will not put out any amperage. You need all six diodes to put out alternating current to direct current and amperage. If one fails, no output.

OR 4 Diodes


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Old 30-01-2017, 14:13   #15
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Re: Why do I have an on/off switch coming off my alternator?

Every alternator I've seen for a 12v or 24v DC system is 3 phase. They typically have a six-diode full wave bridge.

== servicing ==

There are procedures that can be followed for safe servicing of the DC system while it's energized. You have to tape your tools, remove jewelry and any conductive items that may be a part of your clothing etc (eyeglasses, metal button on jeans), and cover conductive surfaces, and be careful and deliberate. There has to be enough clearance to be able to work safely.

There should be a fuse or a fusible link protecting the alternator circuit, because shorts can happen for reasons other than mistakes during service.
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