720 pony ignition and such

michael59

New User
Hi: My name is michael and I am going insane due to working on this pony motor. insanity equals doing the same thing over and expecting different results. That being said my main problem with this 720 is I have no spark. Haven't had any spark and don't see a future of having any spark and this has to change.

Things I have done:

Changed coils, spark plugs, checked spark wires for continuity, checked capacitors and set points. All this over and over and...

I have been looking for the firing order and this morning I found it due to this forum, what i have not found is specifics for resistance on things. But as I have been reading through all the JD720 posts it has occurred to me that maybe my ohm meter is not working properly.

So is the first set of points and coil that attaches there to in the rotation of the cam lobe the coil that plugs into the first pistons as viewed from the gas tank? If so then the other coil is for the back plugs, right? If this is so then that I can correct but that does not help me with the spark.

K, I checked the capacitors and one will not hold a charge while the other one bleeds off very slowly. I always thought that a capacitor was supposed to dump its voltage. Anywhoo, I went and talked to an ag-west mechanic and he told me that the capacitors were actually a shock absorber for the voltage spike...K, that i can deal with but I still have no spark....?

I checked the spark and found that the voltage coming out was at 14V. I know this because I hooked up my Vmeter/ohm to one on the highvoltage setting at 500 and the numbers were 1428 and 13xx. The fact is the voltage stayed above 12xx. Also hooked up a light, Umm?, I don't know what it is called but it is one of those little lights that is in the middle of spark wires and attaches to the plug and the spark wire. The color of the light showed red, so I know the spark is going to ground. So I closed the point gaps and the spark gaps and achieved a little brighter red spark.

Also tried, courtesy of ag-west mechanic, energizing a coil and taking the point/capacitor feed wire and grounding to achieve a spark at the plug. Was not successful in this endeavor, what I mean is I did not get a spark at the plug.

K, I know the points are not grounding out or the coils along with the capacitors. The resistor carries continuity with a volt drop of .02 volts.

As you can see I have checked, rechecked and over checked the system and most likely am correct in the assumption that the multimeter has went gunny sack.

Any help? Or do I just get a big hammer and pound everything into submission?

Oh and another thing I have the chance to work on a lot of old tractors for this guy, and I mean Molines, Olivers, Masseys, Intersmashabels, Dutez and one water pump which is a W9 gas to diesel. The W9 is the next victim which will be most likely a total tear down, but I cannot progress until I get this 720 running along with 3 other tractors in which I am waiting for parts. Oh and there is a fine running 4020 that is for sale as soon as it gets painted.

Sorry to dump so much on any one who reads this but like i said this 720 has tested my limits and I just might have stepped over :arrow:

Any help will be very appreciated thank you!!!!!!

ps: Is this tractor a positive ground?
 

The coil high voltage side has to match the wires. It's either a coil with a spike and a bare wire pushed down over the spike. Or the premium coil with the internal spring clip on the high voltage side which uses a matching plug wire having a crimped on metal tab.
The plugs should not spark to chassis. It's a waste spark system that jumps two plugs together in series at the same time for the electrical path..
What voltage of battery ?
 

Now that explains things.....humm? Well the guy said that it did run. That means the pony is turning the wrong way and why the positive as it is now seems to be coming from and going to ground as in the block....I been thinking it was a positive ground but did not want to burn anything up changing it.

Well that is the first thing I will do when I arrive at my insane asylum. Maybe it is better days now?

cool thanks RLA :)
 

battery is 6.53.
wires and coils are the little pointy coils with wires to match, they are installed correctly.
I am not following your two wires spark plug reference though. Are you saying that both cylinders on one side fire at the same time?
 
The pony is NOT turning the wrong way! LoL. The starter does
not care which way it's grounded, it always spins the same
way. You stated that the capacitors are not the same on the
meter- obviously at least one of those is bad. Also- you cannot
measure the voltage to the spark plug unless you have an
expensive 'capture' meter. cheap meters will not measure 15-
20,000 volts without damage. Also, do not manually power the
coils because they burn out very easily/quickly due to low
resistance/high current flow. They're not robust like the Ag guy
is telling you. And they're expensive.
 
The pony starter motor still turns the same direction REGARDLESS if its Pos or Neg Ground.

The switch originally had a small ballast ignition resistor on its back which dropped about 1.5 volts so the pony coils saw around 4.5 when running but the full battery voltage when she was cranking.

The coils have dual HV output leads, one fires one plug at TDC on its compression stroke while the other lead fires (same time) another piston on TDC of its exhaust stroke, called waste or loss spark.

The LV Primary winding on a (either) coil has one lead to the terminal so it sees voltage when IGN is on while the other end of the LV primary goes to the points. If you place a test lamp on the points terminal (where condensor attaches) it should flash ON when points are open but off when they are closed. She don't fires unless that's happening and when it does fire BOTH plugs (ouit of that coil) fire at same time, one on a pistons compresstion top other on another pistons exhaust top of stroke.

My Ignition procedure is more for a regular coil ignition tractor but most still applies here.

John T
 
The pony engine starting system is a 6 volt system. Running 12 volts to the coils will burn them out quickly. Might be too late for this info.
 

High voltage windings on those coils are like most two cylinder motors with a set of points and sparking two cylinders at the same time.
The Buick 3.8 , 3800 and other vehicles also used the same system. There would be several coil packs, each shared by just two cylinders .
There is no electrical connection between the primary and seconds ready windings. Unlike the ordinary NAPA coil on anordinary four, six or eight cylinder distributer equipped engine.
The spark from one high voltage terminal of the starting engine coil jumps one plug, through the starting engine casting, jumps across it's companion plug and back to the other high voltage terminal on the same starting engine coil.
 
Now there's an idea ! Hook up buick 3.8 coils to these old beasts. That will fire the plugs in oil.
Wonder if someone has done it ? While there at it make it electronic ign. and fuel injection too. I bet they would sell a lot of conversion kits.
I have seen several with 4 car coils on them. And maybe some with coils form onan engines ?
 
I have rigged up some automotive coils but the problem on many Farmer Billy Bob auto coil set ups (Ive seen em with 4 auto can coils) on those ponys is the points are switching a lot more current then standard and they burn up pretty quick.

John T
 
(quoted from post at 17:07:25 09/26/14) I have rigged up some automotive coils but the problem on many Farmer Billy Bob auto coil set ups (Ive seen em with 4 auto can coils) on those ponys is the points are switching a lot more current then standard and they burn up pretty quick.

John T

OK guys!! not to worry I didn't do anything stupid. I went to check out the positive ground and found that the diesel side had a neighoff(?) alt. which is 12V neg. ground. So I popped the positive off of the battery and did a continuity test, bell rang. So pulled the nose and the tiller, off with the hood and chased the wires. sad to say still no spark!

funny though ground for the alt. was taped up and has no continuity but BATT lead has resistance to ground at .945, so alt is defective, BUT this sill don't supply spark.

here is what I just don't get. The cam lobe has the points grounded and therefor passing juice to ground until the lobe lifts the points breaking and then collapsing which allows the voltage spike to fire the plugs. This is to me a positive ground because when something is so energized for so long of a duration then it must be a positive ground BUT every thing else shows the opposite.

K, I went to the counter and they printed out a 6, 12, and a 24 volt electrical diagram, I would say schematic but I guess back then they didn't think that way... those papers didn't/don't read right.....

yeah I think I am off to the loony bend because i just cannot reverse engineer this set up to my brain.

ps: oh I get it!!!! waste spark is actually when it fires retarded. funny though which cylinder does that particular wire go too?
 
I had a bad ammeter on a 720. No lights, no
spark, etc...I bypassed the ammeter and
everything worked.
 

so amp meter...k, I can do that. I found out along time ago that there are three charging systems for an alternator. It would seem the one I missed was the .75 ohm resistance light (think idiot light) that actually let the alternator do its charge and lope.

thank you I will look into it.
 
You may want to try some basic's. take a test light and ground one end of it and touch the probe to the wire going to the points. Crank the engine and watch the light go on and off. If it does go off completely and then back on, look past coil and points for your problem. If the test light goes dim then bright or stays bright, clean points or ground distributor. If no light at all verify voltage coming into distributor, If its good there, ohm coil looking for an open.When checking spark, pull plug and ground, then crank and watch for spark. Good luck
 
Based on all your follow ups below, Id advise taking it to someone who understands the electrical system. Were all trying our best to help but this is jumping all over the place so much I don't think we can help unless we were actually there, but hey were trying our best to help (which now sounds impossible over the net)

You say it has a 12 volt alternator! Well 12 volts applied to those 6 volt pony coils will fry them (plus points burn quickly) unless some sort of series voltage dropping ballast were added. What battery does it have now?????? If the alternator is Neg ground I trust the battery is configured that way??

The tractor was originally Pos ground now it sounds like its Neg ground to accommodate the alternator. The coils will still produce a HV spark regardless of polarity, its just that they are more efficient if used correctly

Best we can do,

John T
 

I think you have it right there John. When I first looked at the tractor I observed the 12V alternator and asked why it was there when there was a 6V under the seat. I was told it was because of the toggle cutoff to the points and coils, and that the twelve was to run the lights. Perplexing as it is I just don't see how this alternator can charge the six volt without a type of Isolator such as one finds in a 24V/12V system. When I pulled the hood and traced the wires I found they ran right to the instrument panel, so there is no voltage step down device. So I guess the 720 has been farmer-ized and just needs un-farmer-ized.

I will follow up and chase down all advice given here and "quote" post a report "un-quote" and, I will do this because some one else is going to run into the same predicament as I eventually.

I would at this time like to thank every one who has responded with advise.
 

Well the owner came out a day later and told me he got it to pop and that was all he wanted, so, so I guess it is fixed....? I'll just have to trust him on this....

thanks every one who offered advice, it was much appreciated.

I am now heading to the cornbinder or intersmashable forum
 

The tractor has an electrical system with jackleg farmer fixes. It will have problems and be frustrating until repaired.
Agri services has some of the best wiring harnesses that I have seen and used. 6V generators are no problem to locate.http://www.wiringharnesses.com
 

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