Dave, I gotta first educate you in the basics concerning coils, then we can troubleshoot your problem::
1) A 6 volt coil will be labeled "6 volts"
2) A full true straight 12 volt coil may be labeled "12 Volts" OR "12 Volts NOT for use with external ballast" THEY BOTH REQUIRE FULL 12 VOLTS (no ballast resistor) TO WORK CORRECT
3) HOWEVER a coil labeled "12 Volts requires external ballast resistor" is in reality only a 6 volt coil AND INDEED NEEDS THE EXTRA EXTERNAL BALLAST JUST LIKE IT SAYS
4) If your tractor is 12 volts you must use EITHER a) A full true 12 volt coil OR b) A 6 volt coil PLUS an external series voltage dropping (12 to 6) ballast resistor. If its a 6 volt tractor you must use a 6 volt coil AND NO BALLAST OR THE SPARK WILL BE WEAK
5) What some people call a 12 volt "internally ballasted coil" is in reality a full true 12 volt coil (intended for 12 volt tractor unballasted) which has a primary winding resistance in the neighborhood of around 2.5 to 3.5 ohms or so BUT (for MANY coils at least) NOT BECAUSE IT HAS ANY INTERNAL DISCRETE RESISTOR STUCK IN, BUT THE RESISTANCE IS DUE TO MORE PRIMARY WINDINGS/LENGTH/COILS OR HIGHER RESISTANCE WIRE. Its true some coils were made that actually had an internal discrete "resistor" but MANY of the tractor vintage coils were dealing with that are 12 volt that people call "internally ballasted" DO NOT have a seperate "resistor" inside simply more primary wire/length/coils or higher resistance wire to arrive at the 2.5 to 3.5 ohms primary resistance. They used coils with internal resistors in early automotive days but most tractor coils in the years were talking about didnt have any seperate internal pure resistors......CAVEAT its still possible some internal ballast (OR THERMISTOR OF SOME SORT) might be designed into a coil such that if the ignition were left on the points dont draw as much current and the coil NOT overheat and burn up, but Im unsure if NAPA has such a unit, I would have to see their specs to determine that buttttttttt that does NOT change the above about 6 and 12 volt coils in general.
Whewwwwwwwww
Next if the coil fires when you have voltage on its high input (from switch) terminal with the output grounded but then when you unground the output (interrupt curent flow) it fires out the top tower (via coil wire to within 1/8 or so from ground) BUT THERES NO FIRE OUT ANY CAP TOWERS THERES A CAP OR ROTOR PROBLEM,,,,,,,,,
Since your question is so wide open to save time I will copy n past my Ignition Troubleshooting Procedure which will diagnose the cause of no spark.........
TROUBLESHOOTING A BATTERY POWERED EXTERNAL COIL TYPE IGNITION SYSTEM:
PRELIMINARY CHECKS:
(A) To see if it happens to be a cap n rotor problem and to see if at least the coil is firing, remove the coil wire from the distributor (leave coil end intact) and place its bare end to within 1/8 inch from tractor iron, turn her on n crank her over, and see if she jumps that gap with a good visible blue spark?????? If so but the plug wire ends (from wire end to 1/8 inch to frame) or the plugs themselves don’t fire, its a cap n rotor or plug wire problem. If the coil wire isnt even sparking, see below.
(B) Next open the cap and see that the points are gapped correct and indeed opening and closing as the engine is cranked and the distributor shaft rotates and MAKE SURE THEY ARE NOT BURNED OR PITTED OR CARBONED UP BADLY !!!!!!!!!!!! If so, running a point file between them to clean them up might make her run again HOWEVER that’s only a temporary cure, so if that cleaning makes her spark, INSTALL N GAP NEW POINTS. In the event they appear good but only gray oxide coated, non abrasively clean/buff/polish them using say a dollar bill or shop cloth etc. and see what happens.
MORE TROUBLESHOOTING IF ALL THE ABOVE STILL FAILS TO MAKE HER SPARK
1) THE VERY FIRST THING YOU GOTTA HAVE is voltage to be present on the coils high supply (NOT to distributor) terminal when you turn the Ignition switch ON. If not she cant ever fire, but in the event the ignition switch or circuit/wire down to the coil or any Ballast Resistor is bad or open, you can HOT WIRE it by jumping a hot ungrounded battery voltage source to the coils high input supply (NOT to distributor) side n see if she runs then???? If she fires hot wired, you could have a bad ignition switch ((That can happen, when Ignition is on, the switches IGN terminal must turn hot)),,,,,,,or an open Ballast (if it has one) or a bad/open wire from switch to coil.
If the switch is good, if you turn the ignition switch on and place a test lamp on the coils high (NOT to distributor) terminal SHE MUST LIGHT UP. If not again, look for an open Ballast Resistor (if it has one, it should read around 1.25 to 2 ohms across its terminals) or bad/open wires from the switches IGN output down to the Ballast (if it has one) and distributor.
2a) When the Ignition switch is turned on, voltage should appear on the coils high input side. That would be 6 volts on a straight 6 volt system or 12 volts on a 12 volt non external ballasted system, or around 6 volts on a 12 volt system that used a 6 volt coil plus an external Ballast Resistor and the coil is good and the points are closed and they and ALL wiring is good.
2b) To insure the coils low voltage primary winding is not bad/open, use an ohmmeter and measure its DC resistance between its lil + and -terminals. If its an open circuit (no continuity) its bad/open and will NOT work. It should measure around 1.25 to 2 ohms or so if its a 6 volt coil and maybe 2.5 to 3.5 if its a 12 volt internally ballasted coil. NOTE CAUTION have all leads and any voltage source DISCONNECTED FROM the coil for this simple primary winding continuity test.
3) Next, place your voltmeter or test lamp over on the coils other low to distributor terminal side, turn her on and crank the engine over.
4) A test lamp there should flash ON (when points are open) and OFF (when points are closed) as the engine is cranked slowly.
5a) If the lamp never comes on there, the coils primary is bad/open,,,,,,,,,,or the points are never opening,,,,,,,,,or theres a shorted/bad condensor (remove its lead to points and see if lamp comes on, if so, bad shorted condensor or its wiring),,,,,,,,or the points wire is shorted,,,,,,,,or the distributors side pass thru stud is grounded (use ohm meter to test that),,,,,,,,,or the points may have a shorted spring.
5b) If the lamp never goes off as engines cranked, the points are not closing or are bad,,,,,,,,or the wire or circuit is missing from the distributor to the points,,,,,or the distributors not well grounded to the tractor.
She cant fire the coil unless its low side is getting a conductive ground return path via closed points and then the circuit is open when the points open.
Be sure the condensor or its wiring is NOT shorted out and see if the lite comes on (when points open) with the condensor disconnected. If removing the condensor makes her spark, replace the condensor.
SUMMARY
Be sure the points are closing fully and open on high cam and ARE NOT BURNED OR PITTED OR CARBONED UP BADLY,,,,,,,theres voltage present on distributors high side at all times when ignitions on (or its a bad switch or open ballast or bad wiring to col),,,,,,,voltage on coils low side flashes on and off as distributor is cranked,,,,,,,,,condensors not bad/shorted,,,,,,,,no shorts in wires to points and no shorts in pass thru side out distributor stud,,,,,,,,coil has continuituy.
You may luck out n just need a new set of points. If the coil wire fires (see above) and the plug wire ends to 1/8 from frame but NOT the plugs, they are badddddddddddd. Check them BOTH.
Good Luck n God Bless, post back any questions and your findings and any questions.
John T Nordhoff in Indiana, retired electrical engineer who usually lurks over on the Mother Deere boards versus over here on the “dark side”.
