Total resistance for ignition coil 12 volt system

C

Capuano

Member
Changed my 1940 flathead to 12 volt system. The ignition coil purchased had a 1.5 ohm primary resistance. I bypassed the original ballast resistor and wired the coil directly from the ignition switch. Some of the threads indicated the total resistance to the coil should be 3 ohm total resistance. I measured the primary coil resistance and it was 1.5 ohms. What fails the primary or the secondary coil. How do you determine if the secondary failed do to the incorrect resistance to the primary of the ignition coil?
 
Have you experienced a coil failure? You can run a 1.5 ohm coil on 12v for a while (no idea how long), but not the best idea. Just about all 12v ignition systems need somewhere between 3 and 4 ohms resistance. 12v and a 1.5 ohm coil would require a 1.5 ohm ballast resistor.
 
If you look at the Ford specifications in the manuals, from 1957 to 1974 they used (about) a 1.5 ohm primary winding ignition coil coupled with (about) a 1.5 ohm ballast resistor that was switched in during engine RUN.

If you pencil it out, in a nominally "12 volt" system, this 3 ohms of primary circuit resistance limits the current flow (heat) in a contact point ignition to around 3 amperes. Limiting the voltage is a thing too, but I think it is excessive current flow is the issue. Notice what happens if the ignition switch is set to RUN without the engine running. If the points happen to be closed, Bad Stuff happens after 10 or 15 minutes.
 
Yeah, current flow is the real issue, it's just easier to talk about resistance (unless you are into electronics). I have some charts on detailed calculations somewhere. But for most cases it is simpler to just shoot for 3 to 4 ohms in a 12v system.
 
Thank you for the responses. I guess I am still trying to know what fails Bad. The primary of the coil or the secondary winding for the high voltage to the spark plugs. I can still read 1.5 ohms on the primary.
 
Well it's probably more likely the secondary windings in the coil will be damaged first, this is made up of many many turns of very fine gauge enameled copper magnet wire. This can be tricky to diagnose, as even just a few burnt or shorted turns in the secondary windings might not be accurately measurable with an ohmeter, or it might only become evident when the coil is hot at normal operating temperature, but this kind of defect will cause attenuation of the spark duration. The MoToRs Manuals talk a little bit about this. The spark at the plugs "looks" good to the naked eye, in the open air, but it isn't what it should be.

With all that, I wouldn't lose sleep over it, unless there's something to lead you to that? If you're running points & condenser, about 3 ohms primary circuit resistance should keep you off the side of the road.
 
easy check the specs by part number with the coil manufacturer bosch /lucas/nippondenso/ marterelli all vary they will give you the readings and whether you need to run a ballast resistor or not
 
What happens is that the coil overheats and the car stops running. Sometimes it may recover after cooling down and you might be able to get home. But for safety's sake you ought to view that coil as compromised and replace it.
If you don't want to use a resistor, get a coil spec'd for a 12V Volkswagen beetle. The Bosch blue coil is the preferred one. I just bought a variant of the blue coil for my sedan.
No resistor required.
Or use the coil you have and fit a resistor. I don't know if the old 6V resistor would be of a suitable value.
 
again talk to the supplier the manufacturer will tell you if and what rating resistor you need just don't slap a resistor in regardless as that causes more problems than it cures also ask what condenser is required [critical to coil and point life but often overlooked]
 
Back in the day;
http://http://www.myrideisme.com/Blog/points-are-cool-msd-6a-with-points-benefits/[/URL]
This worked very well on a boat I had and on my flatty before I went to a Chrysler electronic pickup and GM HEI module.
 
Mart said:
Or use the coil you have and fit a resistor. I don't know if the old 6V resistor would be of a suitable value.
Click to expand...


The early yblock ceramic 6v resistor and the yellow top yblock coils was just a adaptation for the v6 ignition. Up to at least 57 the dizzies were 6v, even though they converted the vehicles to 12v in 54.
 
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