According to, https://www.carparts.com/classroom/ignition.htm . I have two wires that go to positive side of coil, one from resistor / ballast and the other from ignition switch, is this correct? My question is, what is the expected voltage supposed to be when key is in ON position and expected voltage when cranking on these 2 positive wires? I am working on a truck that will not produce spark. Orange positive wire to coil has 10v when its in ON position and the other black positive cable(I assume is a resistor) has 1v. I do not remember how much it had when cranking, but I believe it was 12v, I will double check on that. Could I be missing voltage and thus causing no spark?

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  • It is my understanding, the resistor should knock the voltage down to ~6v. If you're only getting 1v, I'd suggest the resistor is bad. The power from the resistor should only be "on" while the engine is being cranked. During run, it should be ignition voltage. At least that's my understanding. Aug 18, 2019 at 13:34
  • @Pᴀᴜʟsᴛᴇʀ2 I think the second part of your comment is backwards. When cranking the coil should be getting the full battery voltage, but because of the high current through the starter the battery voltage may drop from 12 to 9 volts or less. When the ignition is "on" but not cranking, the ballast resistor drops the coil voltage to 6 to 9 volts, to stop the coil overheating when the battery is being charged and the voltage is 14 to 15.
    – alephzero
    Aug 18, 2019 at 13:59
  • @alephzero - Could be ... been a while since I worked with the resistor on a coil. Aug 18, 2019 at 14:03
  • Yeah, this is all irrelevant for electronic ignition. But for mechanical ignition, if the points happen to be closed when you switch on the ignition before starting, with no ballast resistor the full battery voltage would be going continuously through the coil until you start cranking, which would not be a good idea if you left the ignition on for a hour while trying to diagnose some other electrical fault! Electronic ignitions have timers to prevent that situation.
    – alephzero
    Aug 18, 2019 at 14:13
  • @alephzero Those of us fault-finding other faults for extended periods of time would disconnect the ignition feed to the coil, and remember to put it back on when finished... :) Also, for the early electronic ignitions overheating the coil was also a concern as some were very basic. One of the best I used was the Lucas Optronic system that I pout on a v8... replaced the points with an optical sensor.
    – Solar Mike
    Aug 18, 2019 at 14:14

1 Answer 1


The “standard” or at least early versions of ballasted coil ignition systems had a resistor that dropped the voltage supplied to the coil to 6v or 9v so a 6 or 9v coil was used.

On starting, that resistor was by-passed so the coil was supplied with 12v, basically to help provide a better spark when the battery was under load from the starter. The battery voltage does tend to drop a bit when the starter is operated.

The 12v starting supply was either via a second wire from the ignition switch or from a terminal on the starter motor - usually from the starter as that was a shorter cable run...

  • In the on position, is it normal to have 10v? Doesnt seem right to me because the coil gets very hot and actually the first coil it had blew up and had to be replaced. Aug 18, 2019 at 14:26
  • Perhaps the orange wire is the ballasted cable and the black is 12v when the starter is operated. You will need to check the wires and perhaps the wiring diagram. On the cars I used to work on I knew the color codes... yellow being ignition (fused) while white was ignition (unfused).
    – Solar Mike
    Aug 18, 2019 at 14:28
  • I just noticed the comment from @alephzero, its a mechanical ignition. Its really hard to figure out why there is no spark. Voltage appears to be correct. Aug 18, 2019 at 14:31
  • @AlejandroH and that is the system I described to you...
    – Solar Mike
    Aug 18, 2019 at 14:32
  • I had already replaced condenser and the 2 closing points, the only thing left that I could think of is the timing. Aug 18, 2019 at 14:35

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