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Farmall alternator #5161922 07/15/19 07:19 PM
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chilson450 Offline OP
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I'm working on an old Farmall bn. I took the old non working generator and put a 10si gm 3 wire alternator in it's place. I'm running new wires to connect it up. The main charging lug I ran to the pos. Wire on the starter. The sense wire I just looked back to the charging stud. The Exciter wire is what my question is. This tractor has a magneto so no key switch or positive wires on the tractor. Just a wire from magneto to a switch that runs to ground to shut the tractor off. Can I just run the sense wire right to the battery or do I need a switch inline that I would have to switch every time I start the tractor to turn on the alternator.


2008 Chevy 2500hd 6.0
2011 Ford Explorer 3.5
Re: Farmall alternator [Re: chilson450] #5161953 07/15/19 07:55 PM
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Kira Offline
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There was something about exciting the field coil for just a moment....

...both magneto and alternator?

Just now lookin' over people's shoulders

Re: Farmall alternator [Re: chilson450] #5161965 07/15/19 08:10 PM
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Fsharp Offline
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Hook it to the starter solenoid so it will excite the alternator while starting and then won’t be powered once the engines running.

Re: Farmall alternator [Re: Fsharp] #5161980 07/15/19 08:30 PM
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chilson450 Offline OP
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There is not a typical solenoid on this tractor. It just has a manually operated lever that engages the starter.


2008 Chevy 2500hd 6.0
2011 Ford Explorer 3.5
Re: Farmall alternator [Re: chilson450] #5162002 07/15/19 08:51 PM
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Fsharp Offline
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Okay I have an idea, get a single throw switch to replace current kill switch (if it is momentary), have switch set so that when it turns on it sends power to the exciter wire. Have the switch also open a relay that is connected to the kill switch wire. That way you wont have to have two switches.

It is also possible to leave the exciter wire disconnected and the alternator should self excite at a certain rpm like a one wire alternator, but on a low rpm tractor motor that might not be a good idea.

Re: Farmall alternator [Re: Fsharp] #5162151 07/16/19 04:16 AM
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Linctex Offline
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Originally Posted by Fsharp
Okay I have an idea, get a single throw switch to replace current kill switch (if it is momentary), have switch set so that when it turns on it sends power to the exciter wire. Have the switch also open a relay that is connected to the kill switch wire. That way you wont have to have two switches.


I sat down and sketched out how this would be possible without having a relay energized when it is shut down.
Though it IS possible to do this... in the end there are only 2 "simple" options:

1) Just add another switch and wire to energize and be done with it already
2) Sell the good working magneto on Ebay and convert to battery ignition

You really can't "tie in" to the magneto P-lead since IT IS NOT JUST A GROUND wire! It is actually part of the primary ignition circuit.


"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."
Re: Farmall alternator [Re: chilson450] #5162252 07/16/19 06:58 AM
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Donald Offline
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The sense wire should be connected to +12V connection and can get power all the time. No need to turn on an off but you could. The excite wire needs to be switched on and off with ignition switch. I know magneto but I am just telling you the basics. If you leave excite wire connected all the time the alternator will get warm and drain battery.

A simpler approach is to get what is called a single wire alternator. Only one wire leaving the alternator. Common in hot rods and boats. The alternator does not put out a charge until its RPM is rev'ed up (kick starts it) and from then on it puts out charging voltage.

A rebuild should should be able to convert to a single wire setup.


2015 Ford F-250 w/Powerstroke
2016 Subaru Crosstrek CVT (wife's)
Re: Farmall alternator [Re: chilson450] #5162278 07/16/19 07:17 AM
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thastinger Offline
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My 49 C has a single wire GM on it and a "slow speed exciter" in it. I've had the tractor for many years but it was already 12V converted when I bought it. I keep it at my vacation place and won't be back there for a couple of weeks but I can take a look when I go back but I'm pretty sure it is wired as below, there is only 1 single wire leaving my alternator.

alternator to amp meter, amp meter to pull type kill switch, kill switch to battery, then the large cable from battery to starter

Re: Farmall alternator [Re: Linctex] #5162501 07/16/19 11:50 AM
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Fsharp Offline
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Originally Posted by Linctex
Originally Posted by Fsharp
Okay I have an idea, get a single throw switch to replace current kill switch (if it is momentary), have switch set so that when it turns on it sends power to the exciter wire. Have the switch also open a relay that is connected to the kill switch wire. That way you wont have to have two switches.


I sat down and sketched out how this would be possible without having a relay energized when it is shut down.
Though it IS possible to do this... in the end there are only 2 "simple" options:

1) Just add another switch and wire to energize and be done with it already
2) Sell the good working magneto on Ebay and convert to battery ignition

You really can't "tie in" to the magneto P-lead since IT IS NOT JUST A GROUND wire! It is actually part of the primary ignition circuit.


I you cant figure it out then you may need some more imagination smile

The exciter lead is not a ground wire, it needs +12V

Re: Farmall alternator [Re: chilson450] #5162947 07/16/19 08:15 PM
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chilson450 Offline OP
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Thanks everyone. I think I will just throw a switch in line to the exciter wire. I'll just have to remember the switches. I could also put in a light so I can see when it's on.


2008 Chevy 2500hd 6.0
2011 Ford Explorer 3.5
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