Magneto Question

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aball36
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Tractors Owned: 1949 Farmall Cub
1951 Farmall Cub

Magneto Question

Postby aball36 » Thu May 14, 2020 8:17 pm

49 Farmall Cub - Magneto Ignition Question

This Cub has worked fairly well for the last couple of years, but it recently has started giving me trouble running. It has started sputtering after a while and gives me trouble occasionally, especially after warming up. Finally this past weekend it wouldn't start at all and would only puff here and there when cranking it time and again. After reading many of the posts, it acts like a spark/ignition issue; like a potential points/condenser and/or Magneto problem so I decided to tear into it tonight. It looks like the points are worn, so that will be on the to-do list shortly, but while I was in there I ran into a question about the wire coming from the Magneto's coil, etc. (See attached picture)

The wire that runs from the coil to the threaded post near the points, appears to be a dead ground when I test it with the ohmmeter. Because this threaded post is insulated from the main housing with that plastic insulator, it certainly leads me to believe that the wire from that coil shouldn't be a dead short to the ground because it kind of would defeat the purpose of the insulation. I even removed the coil from the case and tested between the end of that same wire and the metallic mounting posts that the coil rests on and the same result occurred, dead short with no resistance between them. I am struggling to see how this can be correct. It doesn't seem like that should be the case, but I have almost zero experience with magnetos, points, condensers, distributors and all of the other goodies in that area of the cub tractor. The 51 cub tractor that I own is not a magneto so I can't compare it with that one.

Any thoughts would be appreciated.

Thanks,

Adam
Attachments
magneto4.JPG
magneto3.JPG

tst
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Re: Magneto Question

Postby tst » Thu May 14, 2020 8:26 pm

if you are testing the with the points closed like they are in the picture it will read shorted

aball36
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Tractors Owned: 1949 Farmall Cub
1951 Farmall Cub

Re: Magneto Question

Postby aball36 » Thu May 14, 2020 8:34 pm

Thanks for the reply but my original test with the ohmmeter was done prior to this picture with the points still open. I even took the points out completely in one test in an attempt to find some other short that I was missing.

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Glen
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Re: Magneto Question

Postby Glen » Thu May 14, 2020 8:45 pm

Hi,
The ignition points and condenser have to be good for the engine to start and run good.
The points get old and worn from use. They should have flat, shiny surfaces to work best.

Below is a listing at TM Tractor for new points and condenser, you can look at the pics. :)

http://www.tmtractor.com/new/el/367fp.htm

If you have the points out of the magneto, file them flat with a large sharp file, unless they are completely worn down. Wipe off all filings with a clean cloth.

If you are testing through a wire, and have both ends of the wire disconnected, it shouldn't read shorted.

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Re: Magneto Question

Postby MiCarl » Thu May 14, 2020 8:52 pm

If I understand you correctly you're reading 0 resistance from the points wire to the bar that runs through the coil. Correct?

The coil has two connections. One goes to the points (wire) and the other (tab) grounds with the bar at the mount point. If that ground tab is clear of the bar and you read continuity from either the wire or tab to the bar the coil is shorted to the bar and needs to be replaced. There should be resistance between the tab and points wire. I don't know the number off the top of my head but it's not 0. If you're reading 0 points wire to tab the coil is shorted internally.

That bar is a separate part and should push out of the coil without a lot of force.
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k hutchins
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Re: Magneto Question

Postby k hutchins » Thu May 14, 2020 9:38 pm

What your original descriptuion told me that your coil is bad and needs to be replaced. Starting to cut out when hot, then not restarting right away.
By all means if the points appear bad file and reset them first, but my money is on a bad coil as the culprit.
Why is there never enough time to do the job right, but always enough time to do it over. :?:

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Re: Magneto Question

Postby tst » Thu May 14, 2020 9:42 pm

he is probably correct that your coil is going bad

aball36
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Re: Magneto Question

Postby aball36 » Fri May 15, 2020 5:55 am

The summary that MiCarl provided above are the correct symptoms I was trying to describe. The terminal end of the points wire shows continuity to just about every darn piece of metal on the magneto housing, metal bar, mounting post, etc. when the coil is mounted in place. I did not realize that the metal bar will come out of the coil, but I will slide that bugger out tonight and see what it looks like in there as well. I am just curious why all of a sudden the darn thing would be shorted out like that unless those other symptoms of hard starting, etc. were signs that it was starting to develop the short.

In the one picture above, where I removed the coil and the metal bar, there was some brown residue, almost like a hard epoxy that I tried to capture in the picture as well. There was also some oily residue coming from the coil. Are any of these normal/abnormal aspects of the magneto coil as well?

Thank you for continued feedback.

Adam

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MiCarl
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Re: Magneto Question

Postby MiCarl » Fri May 15, 2020 6:31 am

aball36 wrote:In the one picture above, where I removed the coil and the metal bar, there was some brown residue, almost like a hard epoxy that I tried to capture in the picture as well. There was also some oily residue coming from the coil. Are any of these normal/abnormal aspects of the magneto coil as well?


I didn't notice the residue first go around. Those are also indications that the coil is toast.

Id check the resistance from the coil tab to the black wire. I don't know the spec. but I'd expect 1.5 - 10 ohms.

Points and condenser are cheap. If you're replacing the coil you might as well do them too. Be sure the crud in the housing doesn't interfere with the new coil.

**EDITED** I gave resistance value for secondary coil initially. :(
Last edited by MiCarl on Fri May 15, 2020 7:10 pm, edited 1 time in total.
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Bill V in Md
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Re: Magneto Question

Postby Bill V in Md » Fri May 15, 2020 6:49 am

Inspect the area between the magneto and governor for presence of engine oil. If there is oil there, the governor ignition oil seal is the likely the cause and may need to be replaced.
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Re: Magneto Question

Postby Rick Spivey » Fri May 15, 2020 6:57 am

When the coil is installed in the magneto housing, the tab is connected to the bar, and therefore housing, by way of the points. So you will have continuity with the housing when the points are closed. The tab is simply the other end of the primary winding in the coil, the first end being the connector to the stud, and therefore points. For testing the coil, disconnect from the housing. Between the tab and the "nub" on the coil (secondary winding, you should read around 10,000 ohms resistance (9,5 k up to 11 k). Then, between the nub and the points connector (other end of primary winding), it should read essentially the same. Between the two ends of the primary winding (tab and connector), you will have continuity and virtually no resistance. Hope this helps determine if your coil is bad; one that is "going bad" is much more difficult to detect, other than by the symptoms when running (no power, dies when warmed up, etc.).
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Re: Magneto Question

Postby jckengr » Fri May 15, 2020 7:52 am

I have found that a good rule to remember is whenever the mag functions well cold but deteriorates with engine use and heat, the first item to suspect is the coil. In those instances the engine will ultimately stop yet restart after adequate cool down.

John K

aball36
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Re: Magneto Question

Postby aball36 » Fri May 15, 2020 9:17 pm

I've attached a few pictures taken tonight of the coil along with the ohmmeter readings. You can see no continuity between the nub and the points connector wire. Also no no continuity between the nub and the tab. Finally no resistance between the points connector wire and the tab. Does this change anything or prove that the coil is bad?

Thanks,

Adam
Attachments
magneto8.JPG
magneto7.JPG
magneto6.JPG

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Re: Magneto Question

Postby Eugene » Fri May 15, 2020 9:47 pm

aball36 wrote:Does this change anything or prove that the coil is bad?
No and no. Doesn't prove the coil is good either.

Coils can test good with an ohm meter but be breaking down in operation. Frequently happens.

Your continuity tests just how how an ignition coil is wired.
I have an excuse. CRS.

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Re: Magneto Question

Postby MiCarl » Sat May 16, 2020 7:04 am

Re-do the test in your first picture with the meter on the 200 ohm scale. You should be getting a non-zero reading, but small enough it'll read as 0 on the 20K setting.

I think the readings in pictures 2 and 3 should be about 10K ohms, not open.
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