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Allis Chalmers Discussion Forum

A.C. B magneto woes

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Pete B.

09-23-2003 09:29:35

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When I went to restart my 1941 A.C. "B" w/FM-J4B3 magneto & hand-crank starter, after a 2 yr. storage- No Luck. I pulled the plugs(non-resistor), replaced the plug wires(solid-core), grounded a plug attached to #1 plug wire and cranked away-No Spark. When I attached a continuacy meter from ground to the grounding switch on the mag housing, I found the switch was grounded. suspecting a short in the wire between the points & the grounding switch, I replaced everything,-Points Still Grounded. Disconnected the coil lead from the points,-Grounding Switch now Not Grounded. Backtracked the coil lead thru the 2 metal mounting ears on sides of the molded coil & found a complete circut. I would think that this would ground the points & be the source of my no-spark problems. Is this just a bad coil or have I overlooked something? Where can I get Info & Pix/Diagrams of this magneto system. This is my first exposure to magnetos & any help would be greatly appreciated. THANX

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Steve in N.J.

09-24-2003 04:37:37

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 Re: A.C. B magneto woes in reply to Pete B., 09-23-2003 09:29:35  
The info Rod gave you is accurate. Rod, you do a great job with information to folks no matter what the subject! If your machine sat for a couple of years, moisture got into the mag and I'll bet you have a corrosion problem at a connection or at the contacts set itself. I doubt if the coil is the problem unless your F/M mag has the older tar coil installed which caused multiple spark problems from age because of leakage. That would be one thing to check. Another thing you could try is disconnecting the ground circuit to the grounding lever and see if it starts. If it does, the grounding circuit inside may be pinched or grounding to the housing where it's attached to the lever due to corrosion. Another thing, always clean the contacts with a tuner cleaner or Brakekleen to remove corrosion. Do not use a file because it will ruin the contact set down the road. If she still doesn't start, drop us a line and we'll work on a couple other spots. She'll fire up once we nail the problem....
Steve/B&B Custom Circuits

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Rod (NH)

09-23-2003 20:09:25

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 Re: A.C. B magneto woes in reply to Pete B., 09-23-2003 09:29:35  
Hi Pete,

I think you are always going to show continuity between that grounding lever (switch) and ground. I suspect you are just going back to ground through the primary side of the coil. I just checked my good mag that way and I show continuity there. Check out the neat animated gif at this site for a circuit diagram. Scroll down a ways to see it. The mag is of different design but I think the theory is the same.

Have you made sure that the points are clean and that the insulating tube and washers surrounding the grounding lever mounting stud are in good shape? Make sure there is no corrosion in the sockets of the cap and that the rotor and adjacent cap electrical surfaces are clean and not rusty or corroded.

Also check the continuity of the secondary side of your coil. Go between the sping clip on the side of the coil and ground. You should show continuity between those points.

Maybe Steve at B&B custom circuits will stop by and shed a more experienced opinion on your situation. If he misses your post here, try posting a question with "Steve at B&B?" as the subject. He drops by here routinely and is more likely to notice it that way.

third party image Rod

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Pete B

09-26-2003 08:13:03

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 Re: Re: A.C. B magneto woes in reply to Rod (NH), 09-23-2003 20:09:25  
Rod & Steve- you the men!! I may be the Prince of Rust but you guys are the Counts of Corrosion!You were correct that it was a corrosion problem. After disassembling the mag innards & checking for continuity, I discovered a fault in the hinged section of the points where the contacts were pinned. Once cleaned, got a nice spark & the machine started on the second crank!Thankx again. One more Question- is there a method (aside from checking for continuity from lead to case) to check a condenser?

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