12v 9N - no spark

this is a 12V conversion by someone else that has been dependable for 15 years. Front Mount Distributor.

so, with my fathers assist ( 79 year old Mech & Elec. Engineer) we changed the points, condenser, cap, plugs ( exhaust maifold gaskets while we were feeling ambitious) and a 12V coil.

when we hit the starter, NO SPARK. Voltage at teh coil was 6v. Don't know if the points were open or closed.

since i saw 6v at the coil, i assumed i needed a 6v coil and returned the 12v coil for a 6v coil. we now have 2v at the coil with NO Spark.

i've got 40 years of replacing points and the like and i am lost as to why no spark.

thanks for any help

1) A full 12 volt rated coil with no ballast. A 12 volt coil has around 2.5 to 3.5 ohms in its primary as measured between its lil + and - terminals.

ORRRRR RRRRR

2) A 6 volt coil and ahead of it a series voltage dropping ballast resistor which reduces 12 volts down to 6. They are around 1.5 to 1.85 ohms at around 30 watts rated. A 6 volt coil has around 1.5 to 2 ohms primary resistance.

NOTE on some tractors theres a ballast by pass starting circuit which feeds full unballasted battery voltage (by pases ballast) to the coil ONLY while shes cranking. This helps in cold weather starting.

John T

after repositioning the lug i had spark and she fired right up. i've never heard this engine operate so quitely with the new manifold gaskets and tailpipe/muffler.

thanks again...Bob

the frustration stems from the fact the the engine ran prior to the tune up. I changed the points condenser and coil.

with a 12 volt convesion ( Battery, starter & Alternator) should i be using a 12v coil or 6v coil?

thanks

NOTE: That assumes you have and its correct a 6 volt coil !!!!! if not ignore this n post back.

PS see my poat above about 6 and 12 volt coils

John T

1) If its a 12 volt system that uses an external voltage dropping ballast resistor and a 6 volt coil and the points were closed (usually are) and they are working correct, I would expect: There to be 6 volts on the coils high input (from switch) terminal and near zero volts on its low (to distributor) terminal. If it was converted from 6 to 12 and the old 6 volt coil used, it would have needed an external ballast resistor, otherwise the coil runs hot and the points burn up sooner (excess current).

2) If its a 12 volt system using a full 12 volt rated coil, theres NO external ballast and I would then expect 12 volts on the coils input and again, near zero on its output.

3) The coils (regardless which voltage or set up) output should read near zero when the points are closed, cuz the points are supposed to be a switch thats tied to ground when closed to conduct coil current. Then when they open the current flow is interrupted causing the coil to fire.

4) Place a test lamp or voltmeter on the coils high input (from switch) side, turn the ignition on, and it should read 6 or 12 volts (depends on if it has a 12 to 6 series ballast that should then use a 6 volt coil). NEXT place the lamp or voltmeter over on the coils other low (to distributor) terminal and crank her over. THE LAMP THERE MUST FLASH ON (points open) AND OFF (points closed) for her to fire.

a) If the lamp stays ON (on coils low side) its EITHER, the points arent closing (CHECK POINTS GAP),,,,, or the wires open from coil to points,,,,,

b) If the lamp stays OFF, its EITHER the coils primary (between lil + and - terminals) is bad/open,,,,, ,,,,,or the points arent opening,,,,, ,,,or the points spring is shorted to ground,,,,, ,,,,or the condensors dead shorted,,,,, ,,,,or theres a short like where the terminal passes thru the distributors side or in the points n condensor wiring.

For her to fire you have to have voltage to the coil and then the points serve as a mechanical switch to conduct (closed) then interrupt (open) coil current. To see if the coil is firing independant of any possible cap or rotor problems, remove the coils wire out of the distributor cap (leave coil end intact) and place its bare end to within 1/8 inch from the engine or frame, and each time the points were closed but then you open them (can manually using an insulated probe etc) the bare end of the coil wire ought to spark across that gap. You can accomplish the same (to see if its a points or condensor problem) by turning her on and manually jumping the coils low side terminal to ground to conduct coil current then open the circuit and the coil wire should fire.

If you suspect a shorted condensor, remove it from the circuit n see if she fires then???

Some of my best guesses (assuming points are gapped correct and indeed open n close??? check that FIRST) are theres a short in the points spring or the points n condensor wiring or in the pass thru terminal of the distributor (where wire form coil attaches).

Check easy simple things first like BE SURE POINTS ARE GAPPED PROPER N OPEN AND CLOSE AS DISTRIBUTOR SHAFT ROTATES,,,,,then check points n condensopr wiring for shorts,,,,, ,,,insure condensors not shorted,,,,, ,,,,try the direct coil wire tests above to see if the coil fires but its a cap or rotor problem???

John T Nordhoff in Indiana, retired electrical engineer who usually lurks over on the Mother Deere boards versus over her on the "dark side" lol

This is a Front mount distributor with a top mount coil. I am checking voltage at the lug where the wire connects to the coil.

with the 12v coil in place i read 6v at the lug

with the 6v coil in place i read 2v at the lug

in each case i have insufficient voltage to power the coil?

John T