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12 Volt Ignition Conversion For Simplicity 725

by Ken Williams

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This covers the conversion of my 1963 Simplicity 725 to a 12-volt ignition system.  Thanks to the numerous posters on this site who helped me with this information, especially RobS and his instructions. 

Reason for conversion - When I got my 725 the spark plug lead was so old it was cracked and brittle.  I found it would short to the metal parts of the tractor and had to put a piece of plastic sleeve on it so I wouldn’t loose the spark before it got to the plug.  A coil change was needed.  I found there were four things I could do.

  1. Replace the coil with a new one from Briggs.  This would require removing the flywheel and drive shaft plus make or buy a flywheel puller. 
  2. Install an electronic ignition.  You can get a universal ignition module that replaces the points and condenser.  Jack’s has them for $13.50 and they are part number 440-065.  If you have points problems or a problem with a worn points plunger this can work as a solution.  It however uses the standard coil in the flywheel so if you have a coil problem as I did this may not be the solution.  Note, you cannot use this electronic ignition with an external coil.
  3. Magnetron ignition.  This is a replacement for the coil that may require repolarizing the flywheel.  I believe Briggs does this for free?  It still requires the flywheel to come off but apparently works really well when properly done.  I have not done this but have heard good things about it.
  4. Convert to a 12v coil.  This is really simple and, for me, was the best option.  It does not require the removal of the flywheel and gives the engine a solid hot spark even in the winter.  The downsides include it is not original if you want the tractor to be pure and if the flywheel key shears you will not know it until it is too late.

Click for a closer view

Parts - You can get the parts from a car junkyard or even stuff you may have laying around.  I chose to go to Jack’s Small Engine on the net and order parts.
  1. A coil with an internal resistor.  I ordered a Kohler coil from Jacks, part # 460048, $25.50 and it included the mounting bracket.
  2. 12v condenser, again from Jack’s, #455147, $3.85.
  3. Spark plug wire to go from the coil to the plug.  The inside of the plug wire must be copper, not graphite as is found in car wires.  I bought this from Jack’s #135178, $11.90.  It came with 2 connectors, a 90 degree and a straight.  You choose which one you need depending on how you run the wire.
  4. #14 stranded wire
  5. A universal ignition switch.  I got one from Trak auto for $4.99.  You could just use a plain switch but I wanted a key switch so it looked like the one I was taking out.

The Condenser - Remove the cover to the points.  Take out the old condenser.  Also remove the secondary wire from the coil (it is usually shielded by a small metal cable).  This is from the coil inside the flywheel and will no longer be used.  I taped the end and folded it back under the engine out of the way.  Leave the cover off.  Also remove the black wire that hooked up to the old ignition switch and whose purpose was to ground the points to shut off the tractor when the ignition key was turned to off. 

I mounted the condenser in the same spot and even used the bracket that came with it but I did need to trim the bracket for a perfect fit.  Leave the cover off for the “running the wires” section. 

Mounting the coil - The coil should be close to the plug so the plug (primary) wire is not too long.  I mounted mine on the top tube frame just under the hood behind the grill. 


Click each picture for a closer view

Here is a picture of my frame before the coil is mounted.  I did need to drill two holes into the frame for the coil bracket. 

Here is the coil after it was mounted.

The Ignition switch - The old switch on the tractor was a grounded switch and had only one terminal, one wire to the points that would short out the points to ground when turned off.  Using this on your 12v set up would drain the battery so a new non grounded ignition switch was installed.  The universal one fit right into the same hole as the old switch and looked like an original.  The new switch had 3 terminals, battery, ignition and accessory.  I did not use the accessory terminal.

Running the wires - Now that everything was in place it was time to run the wires.  I used #14 copper stranded.

  1. Run a wire from the “battery” terminal on the new switch to the battery but don’t actually connect it to the battery yet until all the wires are run so you aren’t running a hot wire.
  2. Run a wire from the “ignition” terminal on the new ignition switch to the “+” terminal on the coil.  I used crimp on eyelets to make the connections and I soldered them as well.
  3. Run a wire from the “-“ terminal on the coil to the points.  This is connected to the same terminal the condenser is connected to.  Be careful you do not change the gap while doing this or you will need to regap the points.  Also be careful none of the wires are grounding out.  To fit the points cover back on you will need to grind a small V or hole where the new wire runs in.  Once the new cover is on it is a good idea to seal this hole with flexible gasket so oil doesn’t get in and gunk up the points.  As long as I had the points cover off anyway I ran a sliver if paper between the closed points to remove any oil mist on the points surface.
  4. Hook up the hot lead from the battery to the “battery” terminal on the ignition switch.

Spark plug wire - I removed the old plug wire and installed the new one.  The new one I got from Jack’s it had two tips that could connect to the plug.  One was straight and the other had a 90 degree connection.  I used the 90 degree connector after trimming the wire to length.  I used the excess straight connector to wrap the old plug wire tip, the one coming from the coil in the flywheel.  I then thoroughly wound electrical tape around it and tucked it under the engine. 

Testing and Adjustments - OK turn the key on and hit the starter button to see how it works.  I don’t believe you always need additional adjustments but I did.

  1. I had to readjust my low speed set screw.  Perhaps it was due to a better spark at low speed.
  2. I also readjusted my points.  I did this while running.  This is a sure and positive way to adjust them.

 Good luck….hope this helps!

 Ken Williams



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