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Magneto on 1949 MD all of a sudden won't even spark on the tractor but works great on the bench


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Hello All.  My Dad has an 1949 MD Farmall that he has driven since it was brand new.  A few weeks ago he drove up to the shop and turned the tractor off.  When he went to start it again it would not start even though the battery and the starter turned the engine over just like normal.

Dad could not get a spark to form when he touched a spark plug wire to the metal on the tractor.  

He pulled the magneto and placed it on the bench.  When on the bench it sparks fine on both slow hand turning and on a fast drill spin.  When he puts it back on the tractor he can't get it to fire at all.  He has three other magnetos in the shop that fit the tractor.  All three of them work fine on the bench and all three were put on the MD but still no spark or starting.

The wire from the manifold to the magneto is not the problem.  Turns out he never had a manual or parts catalog for the tractor for the 71 years he has had it.  It wasn't until this week when I downloaded a manual that he even knew there was suppose to be a wire to shut off the magneto from the manifold when placed in diesel mode from gasoline.  The spark plugs have been firing for all of these decades on diesel when they did not have to.

We are stumped at why the magnetos work great on the bench, but once mounted won't make any spark.  We can't find any wires on the MD that are showing a short.  I haven't been able to find anything on the internet that describes this problem.  

The tractor must be the problem as 4 magnetos can't all be bad at the same time.

Any help would be most appreciated!  Thank you!  

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4 hours ago, R190 said:

Is the mag turning on the tractor can you hear the impulse snap? did you leave the kill wire off and try it? 

Hi R190.  Thank you for your reply.  I apologize in advance if I use some wrong terms as I am not a mechanic.  When I am at the seat of the tractor and the magneto is not installed, I can hit the starter switch and see the magneto drive turn, so I think the magneto is turning when it is installed.  I can not hear the magneto impulse snap happen from the seat of the tractor.  I don't think my Dad who is testing for spark could hear it if it was, so when I see him next time I will switch places with him.  The magneto has never had a kill wire on it.  The engine has always been killed while in diesel mode by pushing down on the throttle level until the engine died.  Dad has had this tractor since 1949 and until just this last week when I downloaded for him a parts catalog, he did not know that a wire could be run from the manifold to the magneto which would shut off the spark while the tractor was in diesel mode, so for about 70 years the spark plugs have been firing when in diesel mode.    Thank you again for you assistance!  Sincerely, Tom. 

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The 1949 Farmall MD - diesel, starts on gas and switched to diesel is all mechanical.? ?

mags spark on the bench, mags don't spark on the tractor, mags are getting grounded out on tractor ? ??

check the mechanical rotating switch in the manifold, opening & closing 

 

butt butt boss, no butt's just do what I tell you!!??##%%$$ ?

 

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You pose a seemingly impossible situation. Just for clarification. Just about any H4 mag will fit on the MD. Only a mag built for diesel will spark on tractor. They turn backwards from the rest. Second were any components such as cap or rotor used on all 4 mags?

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3 hours ago, snoshoe said:

You pose a seemingly impossible situation. Just for clarification. Just about any H4 mag will fit on the MD. Only a mag built for diesel will spark on tractor. They turn backwards from the rest. Second were any components such as cap or rotor used on all 4 mags?

I was going to get there also are the other three mags of the proper rotation if you not hearing the impulse snap.

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On 10/14/2020 at 4:13 AM, superman said:

The 1949 Farmall MD - diesel, starts on gas and switched to diesel is all mechanical.? ?

mags spark on the bench, mags don't spark on the tractor, mags are getting grounded out on tractor ? ??

check the mechanical rotating switch in the manifold, opening & closing 

 

butt butt boss, no butt's just do what I tell you!!??##%%$$ ?

 

he said he has no wire hooked to the manifold, as in no wire to the mag. so that means mag has to spark. he has given this info on another forum and was given pretty much the same info. ... as is it correct rotation mags? have you removed plugs and actually checked for spark by grounding them? is the mag turning ? nothing has been verified so how can a person keep guessing as yes it sure is a seemingly impossible situation. some are saying timing ... well thats totally off the wall. you can just pick up the mag and bolt it to tractor and it should be sparking.  timing is another issue, and procedure. if it sparks on bench but not on tractor it it is either not turning or wrong mag rotation! that is presuming the impulse is working. yes, 4 mags ... seamingly impossible situation. find someone with mag  experience to look this over. we can only give so much correct info till it gets checked as to what is happening here.

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and how do we know that it is not a gas problem . does the little tank have gas in it? makes more sense to me as yes you can shut it off then it wont start if no gas.

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we must be left hanging in the dark! dont know why people even ask for help then just disappear. be interesting to find out what is going on .

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  • 3 weeks later...

I recently had a problem with a new rotor purchased for my UD-9 - turns out the metal contact on the rotor was just a tiny bit too short to allow a strong spark to reach the plugs.

I switched back to the original IH rotor (which wasn't really in too bad of condition) and it runs fine on the gas cycle.

The magneto seemed to work fine on the bench as well, but wouldn't start or would run really poorly with the new rotor.

 

            - Matt

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On 11/5/2020 at 3:10 AM, mrsell said:

I recently had a problem with a new rotor purchased for my UD-9 - turns out the metal contact on the rotor was just a tiny bit too short to allow a strong spark to reach the plugs.

I switched back to the original IH rotor (which wasn't really in too bad of condition) and it runs fine on the gas cycle.

The magneto seemed to work fine on the bench as well, but wouldn't start or would run really poorly with the new rotor.

 

            - Matt

i can see that, but he is saying that he put 3 magnetos on it and they all do the same thing. hummm. plus he is not doin the work and is speaking for his father so who knows if we even have all the info here. 

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  • 4 months later...

Hi, I am having exactly the same problem with a Wico type A mag. This magneto has new coil, new condenser and good clean points. The mag gives a really good spark on the bench. I attached a piece of wire from the HT connection leaving it just short of the cap screw to keep consistency throughout the test. I can 'click' over the impulse or spin it with a drill and have a brilliant spark jumping almost 1/4". When I put the mag on the engine and spin it over, there is no spark. The rotor turns and holds up for a while until the impulse releases it, but no spark. I have tried everything I know , but I am now lost.

Is there anyone out there who can help me as I am now tearing my hair out. There must be a simple explanation I have not found yet surely!

Terry

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can you post photos ...vid???

has to be ground/short, bad electrics wires plugs , fuel /air issue  +- 10% of the mags do need a recharge

just had an issue on cub this weekend with pump gas and sluggish valves everything pointed to spark until 12 v on the spinning

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On 3/12/2021 at 5:31 AM, Terry Gibbons said:

Hi, I am having exactly the same problem with a Wico type A mag. This magneto has new coil, new condenser and good clean points. The mag gives a really good spark on the bench. I attached a piece of wire from the HT connection leaving it just short of the cap screw to keep consistency throughout the test. I can 'click' over the impulse or spin it with a drill and have a brilliant spark jumping almost 1/4". When I put the mag on the engine and spin it over, there is no spark. The rotor turns and holds up for a while until the impulse releases it, but no spark. I have tried everything I know , but I am now lost.

Is there anyone out there who can help me as I am now tearing my hair out. There must be a simple explanation I have not found yet surely!

Terry

If it sparks on the bench and not on the tractor you are grounding it out.  Are you hooking up the kill wire on the tractor every time? 

Is your "test bench" made of wood?

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