JD 70D Pony Motor Spark

Okay, so I did something pretty stupid, and I'm looking for help. Many years ago, the mechanic at the local JD Dealer told my dad that it would be okay to run a 12 volt battery on our John Deere 70 Diesel pony motor to give it more cranking speed and spark, as long as the switch was not on for a long amount of time. Right, wrong or otherwise, this is what we did and have been doing for years with never a problem This was when the coils for these motors were not being made aftermarket and were quite expensive and they figured that the spark was getting weak and this would help. So, a few days back, we had a rather nice day and I was going to move the 70 to get the 4020 with snow blower moved to the front. The 70 had not been started for a month or so, and the battery was too weak to start it. I jumped off the tractor and hooked the battery charger up and went to do something else. When I came back after about an hour, I could smell something extremely hot. That's when I noticed that I had left the switch on. Of course, when I tried to start it, there was no spark. The plastic housing of the coils in the distributor was melted in places. I ordered 2 new coils and installed them tonight with the same result. No spark. Is there a certain way that the little metal bar that the parts list calls the coil core, goes? I notice on this part that the one side has two, what I guess you would call flat 'springs', that the coils slide over. On my part, one end of one of these was not touching the bar, but I figured once the coil was slid on, it would push it against the core. Could leaving the switch on have also burned out the condenser and points? Are there any other parts that this could have affected? I will say right here that I am not real knowledgeable in the working of an engine. I can replace parts and such, but am looking for guidance on what to check next. I believe the switch is okay, as when I turn it on, the amp gauge does jump into the negative side. I feel terrible as my dad (gone now for almost 15 years) bought this tractor in the 60's and both me and my son love taking it to the local farm show. I'm hoping someone can steer me in the right direction. And I will say that the pony motor is now running very well and I have been contemplating putting a 6 volt battery back in. I sure wish I would have before the other day!!! Thanks for any help.

all i got to say is why would a person torture that little engine with 12 volts i just put them coils in my 70 in july and 6 volts wheels it
over so fast i sure dont see what this 12 volt stuff is about. my coils where so bad they were just falling apart. yes leaving the switth on
can burn the points if they are closed. i sure dont think when this 6 volt stuff was new the engineers would be thinking it might be abused
with 12 volts. and the thing is there was no 12 volt battery's. the 12 volt tractors used 2 6 volt battery's . talking in the early 1950's

Leaving the switch ''ON'' wouldn't affect the condensers, but sure would burn one or both sets of points if they were ''closed'' at the time.

Is there voltage to the ''distributor'' when the ignition switch is ''ON''?

Applying 12V to the coils increases the amount
Of heat by 4 times .
6V is sufficient as the starter and ignition system were designed for 6V .

Running 12 volts through the coil box is a definite no no IMHO. The system was designed that when the starter was engaged you should have 6 volts going to the coils and when the engine started the voltage was reduced to about 3 volts through a resistor on the back of the ignition switch. However, if the starter button was changed it may not have the side terminal to give the coils 6 volts when engaged. Secondly, the ignition switch internal contacts are usually badly corroded after 60 years and you may not be getting sufficient voltage to the coils. Check the voltage at the side terminal on the coil box when you turn the switch on and when the engine is running. Also make sure the red indicator light on the dash is working so you know when there is power going to the coils and if you left the switch on when the engine is not running. Once you fry the coils they are difficult to remove from the center core bar w/o damaging the bar. It is soft material and any hammering on the ends will mushroom the ends. When you try to slide the new coils on the bar the sharp edges of the bar will skive off the insulation inside and you will have a direct short. Replace the core bar when you replace the coils. Also, make sure your spark plug wires and the new coils are matched. Don't buy the foreign made coils and use spark plug wires with bayonet terminals on the end that insert in, and clip, into the American made coils. I have found that the lead wires on the foreign coils are brittle and will break due to vibration.

(quoted from post at 12:25:12 10/27/22) Once you fry the coils they are difficult to remove from the center core bar w/o damaging the bar. It is soft material and any hammering on the ends will mushroom the ends. When you try to slide the new coils on the bar the sharp edges of the bar will skive off the insulation inside and you will have a direct short. Replace the core bar when you replace the coils. Also, make sure your spark plug wires and the new coils are matched. Don't buy the foreign made coils and use spark plug wires with bayonet terminals on the end that insert in, and clip, into the American made coils. I have found that the lead wires on the foreign coils are brittle and will break due to vibration.

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I believe I'm okay as I did get the old coils off of the coil bar and back on without too much trouble. If I would replace the coil bar, any idea on where I could find one? The coils I bought were from All State Ag, so I don't know if they're US made or not. I just figured they would all be China produced. Also, any idea where I can find the correct plug wires with bayonet ends?

I plan on replacing the 12 volt battery back to 6 volt.

This post was edited by ROSchell on 10/27/2022 at 04:57 am.

If you look inside the coils where the plug wires are inserted, the foreign coils will have a barbed post that plug wires are connected to. The American made coils will have two spring clips that the bayonet terminals snap into. That is why the wires and coils must be matched. If you fried the coils the ignition points are probably also damaged. That would cause you to not have spark even though you have new coils. If the new coils slid on the core bar easily you may be ok. You could probably find a new coil bar on Ebay (???)

Spark plug wire set is available from The Brillman Company (Va) for about $25.00 + shipping

Coils from ASAP cost $48 for foreign coils and $86 for American made.

You asked of the leaf springs on the metal bars (the coil cores). Those are only there to make the coil not be loose or subject
to damage from vibration due to a loose fit on the core. If your coil is loose on the bar you can just slide a small wood wedge
between the coil and the core to prevent vibration.

You also mentioned no spark..... Hope you realize these coils send a spark current out one wire with the hope that the spark
jumps the gap of the first plug, goes into the engine's head and then over to the second plug where it again jumps the gap and
returns to the OTHER plug wire port. The spark has no real desire to go to the frame of the tractor but only to the other coil
port! If you're checking for spark from either coil, you must have that coil's other plug wire either grounded or connected to
the other plug. Both front plugs are in series (in a loop) with each other and the rear plugs are also in series with each
other via the other coil. Said another way, if you are checking the rear coil, you won't see any spark from either port of that
coil unless the other port is either grounded or at least connected to a plug unless the coil has an internal fault and a new
coil should not have any such fault.

Hope that helps.

(quoted from post at 12:25:12 10/27/22) Check the voltage at the side terminal on the coil box when you turn the switch on and when the engine is running.

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Okay, I checked the voltage at this terminal on the coil box.



When I checked it on the battery, it read 11.5 volts. When I checked it at the terminal, it was at .95 volts. I'm guessing the switch should probably be replaced along with the points?

(quoted from post at 15:25:46 10/27/22) You also mentioned no spark..... Hope you realize these coils send a spark current out one wire with the hope that the spark jumps the gap of the first plug, goes into the engine's head and then over to the second plug where it again jumps the gap and returns to the OTHER plug wire port. The spark has no real desire to go to the frame of the tractor but only to the other coil port! If you're checking for spark from either coil, you must have that coil's other plug wire either grounded or connected to
the other plug. Both front plugs are in series (in a loop) with each other and the rear plugs are also in series with each
other via the other coil. Said another way, if you are checking the rear coil, you won't see any spark from either port of that
coil unless the other port is either grounded or at least connected to a plug unless the coil has an internal fault and a new coil should not have any such fault.

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I guess I'm not following you here. When I checked for spark, I took the front left plug out and hooked the wire to it and grounded it to the frame and there was no spark. I also tried one of those in-line spark testers and there was nothing. If everything was working, should I have been able to see spark?

That is a problem. I would immediately get rid of the 12 volt battery to make sure your new coils are safe. Change (or clean) the points, put in a good 6 volt battery and then temporarily run a wire from the battery or the starter terminal to the coil box side terminal. Make sure you have 6 volts at that terminal. If the pony motor starts then change the ignition switch and eliminate the resistor on the back of the switch. Make sure your red dash light is working to know when the coils have power going to them. What is the serial number of your 70 and does the starter button on the starter have a side terminal with a wire running to the coil box?

(quoted from post at 12:50:17 10/28/22) That is a problem. I would immediately get rid of the 12 volt battery to make sure your new coils are safe. Change (or clean) the points, put in a good 6 volt battery and then temporarily run a wire from the battery or the starter terminal to the coil box side terminal. Make sure you have 6 volts at that terminal. If the pony motor starts then change the ignition switch and eliminate the resistor on the back of the switch. Make sure your red dash light is working to know when the coils have power going to them. What is the serial number of your 70 and does the starter button on the starter have a side terminal with a wire running to the coil box?

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I'm going to pick up a 6 volt battery today. The red light stopped working a few years ago, so I will try to fix that.

If you look in my picture above, the only wire coming to the distributor box is the one in the picture which comes from the switch, I believe.

Serial number is 7041698, a 1956 model.

I picked up a 6 volt battery and I have new points on the way. Does anyone have any pointers on how to set the gap? Particularly how to stop the points when they are in the open position?

Also, what about the voltage through to the switch? With the 12 volt battery, it read just under 1 volt, and there were comments about how that was a bad thing. What should the voltage be at the connector on the side of the distributer box? Do I need to also replace the switch?

Thanks for all the input so far!!!

This post was edited by ROSchell on 10/31/2022 at 06:34 am.

You can set the points by removing the pony engine flywheel cover on the left side of the tractor and turn the flywheel manually until the points are at the top of the cam lobe on the end of the crankshaft. Set the first set @ .020 and then move the flywheel until the second set of points is at top of the cam lobe and set them @ .020. With the six volt battery you should have 6 volts at the coil box side terminal. Run a temporary wire from the battery or starter terminal (to bypass the switch) to the coil box to see if the pony starts quickly. Then connect the wire from the switch to the coil box to see if it will start...I'm guessing it will not. Probably need a new switch (~$85) and it must be specifically for pony start diesel tractors. It has a resistor on the back of the switch. Where are located???

(quoted from post at 15:29:18 11/01/22) Where are located???

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North Central North Dakota, 40 miles Northeast of Rugby in a small town called Mylo.

By the way, thanks a bunch for your help. I'm hoping to try and get the tractor running before cold weather, but with work and deer season starting Friday, I don't know if I can get it done.

Do you know Jeff Greig from Rugby? I bought a 70 diesel (7042418) from him many years ago. I think he moved to Texas but is back in North Dakota again (???)

(quoted from post at 02:22:30 11/02/22) Do you know Jeff Greig from Rugby? I bought a 70 diesel (7042418) from him many years ago. I think he moved to Texas but is back in North Dakota again (???)

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I do know who he is. He's certainly a wheeler dealer. He's in the Rugby area again as far as I know.

I knew he was a tractor jockey, but he was always fair with me. I think I bought 2 or 3 tractors from him maybe 15 years ago.

(quoted from post at 15:29:18 11/01/22) You can set the points by removing the pony engine flywheel cover on the left side of the tractor

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Is this little cover on the top the pony motor flywheel cover that you're talking about? Sorry I'm just not real familiar with some of these parts.

This is not my photo or my tractor. I just don't want to run out to the quonset and take a picture right now!!!

This post was edited by ROSchell on 11/01/2022 at 06:38 pm.

Thats the one!