Re: 12v 9N - no spark

Posted by John T on August 30, 2005 at 13:30:45 from (66.244.90.98):

In Reply to: 12v 9N - no spark posted by Bob N. on August 30, 2005 at 12:14:32:

Bob, for the voltages you mentioned to be able to help me diagnose the problem, I need to know where they were measured (what side of coil) and if the points were open or closed. However, I can give you some pointers based on theory and my best guesses.

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

Follow Ups:

• Re: 12v 9N - no spark Bob N. 13:44:24 08/30/05 (1)
  • Re: 12v 9N - no spark John T 16:10:44 08/30/05 (0)

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