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Allis Chalmers Discussion Forum
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FM-X4B3 Magneto Question

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Davr101

02-24-2007 12:30:51




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I have a 1950 Allis-Chalmers Model B with an FM-X4B3 mag. The kill post coming out of the mag is always grounded, yet I have good spark. When I bought it, I tried to kill the engine by pushing the tab against the mag case. It kept right on running. I disconnected the coil wire and the ground issue went away. I'm a little confused by all of this. Any answers?




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chalmersbob

02-25-2007 18:31:49




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 Re: FM-X4B3 Magneto Question in reply to Davr101, 02-24-2007 12:30:51  
i think one side of the coil attaches to ground. if you have spark, definitely not shorted. Bob



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Davr101

02-25-2007 20:13:22




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 Re: FM-X4B3 Magneto Question in reply to chalmersbob, 02-25-2007 18:31:49  
Thanks for the info, Bob. I have good blue spark.

Dave



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Glen in TX

02-24-2007 14:02:31




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 Re: FM-X4B3 Magneto Question in reply to Davr101, 02-24-2007 12:30:51  
third party image

This a FM RV 4 but shows you same principal how a mag is build inside and works. You just need a good wire from points screw to the kill terminal and the tab on outside should kill it. Keep all other wiring OFF the mag kill terminal unless you are just wiring a seperate kill switch to it. The insulator tube and washers on kill terminal screw have to be good and present or it will short the coil out. If you need a parts breakdown of the FM-X4 mag email me later.

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Glen in TX

02-24-2007 12:48:57




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 Re: FM-X4B3 Magneto Question in reply to Davr101, 02-24-2007 12:30:51  
Sounds like wire or insulators on post are bad? New insulator tube and washers are available as are the wires and kill tabs in kit.



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old

02-24-2007 12:39:33




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 Re: FM-X4B3 Magneto Question in reply to Davr101, 02-24-2007 12:30:51  
If that post is always ground it will not run, unless some one disconnected the wire inside of the mag that went to it and if thats the case the tab will also not work. The post sould only be grounded when you push the kill tab, what that does is grounds out the points so they stop working and that causes the engine to stop running

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Davr101

02-24-2007 12:48:51




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 Re: FM-X4B3 Magneto Question in reply to old, 02-24-2007 12:39:33  
That"s exactly what i thought too, but all the wires are attached. I even opened the points and the post was still grounded.



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Glen in TX

02-24-2007 12:52:49




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 Re: FM-X4B3 Magneto Question in reply to Davr101, 02-24-2007 12:48:51  
If you're checking with a ohm meter yes it will be grounded to mag frame. The primary side of coil is connected inside to the lamintated coil bar. If it fires a good hot blue spark don't worry about it! If it doesn't fire you have short someplace, bad condenser, corroded points or bad coil. It's a magneto and produces it own power and does not need any external voltage from a battery.



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Davr101

02-24-2007 13:19:23




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 Re: FM-X4B3 Magneto Question in reply to Glen in TX, 02-24-2007 12:52:49  
I was checking it with a test light. I put the test light ground clip on the post and the other end to a power source. The test light always comes on, but dims considerably when I crank the engine.

I have the carb off right now for a rebuild, and some other parts off as well, so I can't try to start it. It does have good spark, though.

I wouldn't even know it had this issue, but I rewired the tractor for a new delco alternator (had a Ford alternator that didn't work)and while I was at it, I rewired the lights and the kill switch. I was checking all the wiring when I found this issue and remembered unsuccessfully trying to kill the engine by pusing the tab on the mag against the mag case when I bought the tractor. I thought that was odd.

Anyway, I'm havig trouble understanding why I have spark when I shouldn't.

Dave

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chalmersbob

02-24-2007 13:36:15




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 Re: FM-X4B3 Magneto Question in reply to Davr101, 02-24-2007 13:19:23  
disconnect the wire inside and check between the screw and the case with a ohm meter. should show open. Bob



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Davr101

02-24-2007 13:49:15




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 Re: FM-X4B3 Magneto Question in reply to chalmersbob, 02-24-2007 13:36:15  
I disconnected the coil wire and there was no ground. I hooked it back up and it was grounded again. This led me to believe the coil was bad, but when I checked the spark, I had nice blue spark.



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