JD Model 40T no spark
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  1. #1
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    JD Model 40T no spark

    Greetings! First post and its going to be a long one, so I appreciate everyone's patience with this no0b. I need some help bringing "Pappy" back to life. He belonged to my grandad for many years. My dad inherited him when gramps passed. Dad gave Pappy a cosmetic makeover about 7 years ago. Pappy last ran probably 4 years ago. Dad passed away just shy of 2 years ago, so now Pappy has found himself in my care. I'm a pretty fair hand when it comes to working on older vehicles, so I definitely have a solid grasp of the basics. Pappy's fuel system was in bad shape and his electrical wiring is a bit sketchy, but gradually improving on my watch. I've had the fuel tank boiled and lined. I also rebuilt the carb, so the fuel system is much improved. Pappy still has his original 6 volt, positive ground configuration. I bought a new 6V battery which I've made sure is fully charged and displaying a surface reading of 6.4V on my dvom. It took a little adjusting to the starter contact switch, but now Pappy's starter cranks the ol twin popper over at a very strong clip every time I engage it.
    In addition to the coil that was mounted on Pappy, I found two other 6V coils of unknown condition in the parts inventory. I figured for sure at least one of those three would get Pappy poppin', but I was wrong. I've performed primary and secondary continuity tests, and all three coils failed to register continuity across their secondary winding. Naturally, none of the three would produce any facsimile of a spark with the HT wire loose from the dizz cap and positioned with an air gap to ground. Today, I purchased a new, universal 6V coil. I did the continuity check on it and it showed continuity on both windings. Yahtzee! I figured it would make Pappy happy too, but it didn't. There's still no spark coming from what appears to be a perfectly good coil. I even swapped out the HT wire with another known good one and still no joy. I have the power feed from the Ign switch connected to coil Neg terminal and I have the feed to the distributor connected to coil Pos, which as near as I can tell from the manual is the correct orientation. Both of these wires are brand new from end to end with clean, tight terminations. I know the pitfalls of corroded and sloppy or loose terminals and connectors, so they are top shelf. I have 6V at both coil terminals when the Ign is switched on. I have the same reading from the loose end of the HT cable where it would plug into the dizz cap. When I crank the starter, this voltage reading drops slightly until I stop cranking. I would expect the voltage to step up significantly during engine crank, so that has me puzzled. I really don't know what I'm missing here. This thing should be making sparks like mad. I'm at a loss as to what to try next. Thanks in advance for any light you can shed.

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  3. #2
    Senior Member Wi11y's Avatar
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    You have done everything but replace the points and condenser. The condenser is actually a capacitor, yet some one in the automotive industry just had to be confusing, yes?

    The condenser is the usual culprit for lack of fire. You can with the points exposed and the switch on, drag a screwdriver across a open set of points (presuming the condenser is any good) to get a spark.

    Please post pictures so we can admire your treasure.

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  5. #3
    Moderator Jim in NC's Avatar
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    Your ohm meter will be a valuable tool as you chase down the problem. Procede through the wiring one step at a time. Check the insulator on the distributor where the points connect to the coil wire. It is common after 50 or 60 years for those to be cracked and damaged and the points will be shorted to the distributor.
    "Farming looks mighty easy when your plow is a pencil and you're a thousand miles from a cornfield." ~ Dwight D. Eisenhower

    "Heaven is under our feet as well as over our heads." Henry David Thoreau

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  7. #4
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    Thank you both for your input. Now I must admit that I am acquainted with 12V Neg ground systems, but this 6V Pos ground is a concept that I'm still wrapping my feeble brain around. I've probably built it up to being a bigger deal in my mind than it actually is. I think I have been mistakenly thinking that I would see sparks out of a good coil with the HT cable loose from the cap despite what might be happening or not happening inside the distributor. I have a NIB set of points and condenser in parts inventory for this tractor that my dad must have purchased several years back. Might as well toss 'em in and have a good look-see at that distributor cap from all angles while I'm at it. I know this has to come down to something simple like that. There's only so much involved to make a spark, and I know I'm most of the way there. As for pictures, Pappy is super filthy right now, and he's quite ashamed of his present condition. He's admonished me not to post any pics until he's had an opportunity to properly address his personal hygiene. I'll post an update after I've had a chance to get elbows deep into the bowels of that distributor.

  8. #5
    Senior Member Wi11y's Avatar
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    There is nothing to positive or negative ground. It is the charging system that sets the polarity. 95% of all generators used are positive ground. Unless you are dealing with some chryslers and the briggs and stratton engines from the 1960's into the 70's. Alternators and modern small engines all use positive ground.
    Please post pictures so we can admire your treasure.

  9. #6
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    Pappy is poppin'! The replacement tune up parts that I referenced before turned out to be totally incorrect for this distributor, even though my dad had clearly labeled the boxes as being for the JD 40. I'm going to blame it on parts monkeys. I don't know why he didn't take that crap back to the store for a refund, though. Oh well. Anyway, that discovery left me with no choice but to proceed with existing parts. I shined up all associated contact surfaces, reset the point gap and turned on the juice. The ol' boy fired right up, I'm happy to say! Its difficult to keep it running since I broke the governor control rod during a prior attempted length adjustment. I've sourced an experienced, but functional replacement that is making its way to me as we speak. I had to hold pressure against the carb throttle linkage to keep it running. I ran it for several minutes altogether. The muffler is broken and leaking badly just above the mounting flange. It is super loud and stinky, especially when you have to be standing that close to it to keep the engine running...not to mention the tiny sparks that flew out occasionally. I have a new muffler on its way to me also. I think I will refrain from running Pappy any further until those two critical items have found my doorstep and I get them installed. That will make the experience more bearable for sure. The main point has been proved anyway. Pappy is in hibernation no more. A tuneup kit will also be on the short list of items to acquire and install. Thanks again!

  10. #7
    Senior Member Wi11y's Avatar
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    LOL video or it didnt happen. Grin just messing with ya. Glad you could get it going.
    Please post pictures so we can admire your treasure.

  11. #8
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    Yeah, I wish. Didn't have a device with me and even if I had one, my hands were pretty busy between working the starter and keeping the throttle applied to keep it running. I'm glad its running too. Even though my dad had it running it recent years, I wasn't around to witness it in person any of the times that he was operating it. So the last time I actually heard/saw that tractor running was in the early 90's when gramps was still using it. That's been awhile.

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