TO-35 Ignition, Coil Polarity

I’m having trouble with the ignition on my TO-35, is the polarity of the coil important?  November 5, 2016.

I am assuming the starter turns over, but the engine will not fire. If that is true there is probably no problem with your ignition switch. If your starter will not turn over either then it is possible you have a bad ignition switch.

Yes, the marks on the coil terminals matter. If your tractor is a 6 volt positive ground system the (-) negative coil terminal should be connected to the wire going to the ignition switch. If your tractor is a 12 volt negative ground system the (+) positive coil terminal should be connected to the wire going to the ignition switch. The remaining terminal should be connected to the terminal on the side of the distributor.

Some additional things to check include the wire going from the center of the distributor cap to the center of the coil. Check the terminal ends, and inside the towers. Remove any corrosion. Check the wire with a multi-meter. Inspect the distributor cap, particularly the carbon button center contact. If there are cracks in the cap, corroded terminals or the carbon button is worn, replace the cap. Check the rotor. If the springy center contact is worn or loose replace the rotor. Check the point setting, and run a small piece of paper between the contacts to remove any oxide that may have accumulated on them. Check the side terminal on the distributor to make sure it is not touching the metal housing, and make sure the insulators are intact. If the insulators are cracked, replace the assembly. Check the wire between the side of the distributor and the coil with a multi-meter. If it is bad, replace it. Sometimes the wire can break just inside the crimp on terminals.

When you replaced the coil did you purchase one with an internal resistor?  Ferguson ignition systems use ignition coils with internal resistors.

Check the ignition system with a spark tester, or pull off a spark plug wire and prop it about a quarter inch away from the plug terminal. Crank the engine and see if you see a spark jump between the wire terminal and the plug terminal.