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Subject: 12v ign system voltage & resistance test?

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Rusty 2N    Posted 04-09-2018 at 12:41:30 [URL] [DELETE]        [Reply] [No Email]  
  • 12v ign system voltage & resistance test?
  • I have searched and can only find instructions for doing a 6v system, this is 12v. Seems to be going through points.1950 8n side mount. I know the condensor is spose to help prevent this. So I need to make sure all ohms and voltage is correct going through this system. Had a Falcon do this to me once and it turned out to be a shorted wire behind dash. Checked all the wires, all good there. So I figure if I test to make sure all is right before I start just throwing parts at it.
    I will go over it all again and again till I figure this out. But need the process and correct readings to do it right.
    Any help, advise, pointer will be appreciated. Thank you Rusty

    Bob in KS    Posted 04-10-2018 at 09:33:48 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • "1950 8n side mount"

    Since this is a side mount distributor which should have the round can coil, do yourself a favor and replace your 6 volt coil with a 12 volt - no external resistor required coil, such as NAPA IC14SB or equivalent.

    This will operate directly off of 12 volt system and eliminate the extra resistor. Should save a lot of confusion.

    I would also replace wire harness if wires are old and cracked.

    -- post on coil to side of distributor for negative ground, + post to side of distributor for positive ground. Same points and condenser for 6 or 12 volt.

    Rusty 2N    Posted 04-10-2018 at 10:32:54 [URL] [DELETE]        [Reply] [No Email]  
  • Re: 12v ign system voltage & resistance test?
  • This ole girl was 12v when i got it but I put a Just 8ns harness as to JMores pictograms found on this site for tractors,after ser. Number 263844. No resistor any wheres. 12v coil included.
    I have always disliked the front mount dist. After 20 some years of cussing them I did manage to get to appreciate the simplicity of them. Always thought a side mount would be the greatest thing ever!
    As much as the Ns are same there is sure a lot of differences. Sometimes I do miss my 2N.
    Oh well, another day and life goes on!!! Rusty

    Bob in KS    Posted 04-10-2018 at 11:38:07 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • "12v coil included"

    Which 12 volt coil?? "12 volt- no external resistor required" (approx 3 ohms across + and - primary) or "12 volt - external resistor required" (approx 1 1/2 ohms across primary)

    As indicated the second one requires resistor for 12 volts. It could be used without resistor on 6 volt system.

    Rusty 2N    Posted 04-10-2018 at 15:11:03 [URL] [DELETE]        [Reply] [No Email]  
  • Re: 12v ign system voltage & resistance test?
  • If I understand this correctly I need the coil with 3 ohm reading? I do have one of them!!! Tests out good. According to all the learning I have crammed into my old brain today. This makes sense the more I think about it. Thank you Bob!! Rusty

    Bruce (VA)    Posted 04-10-2018 at 15:30:34 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • Yep.

    If your side coil reads 3 ohms or better, no resistor needed at ll.

    Mike Stanaway north mi    Posted 04-09-2018 at 18:13:29 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • I had the same problem. Talked to my local parts guy (Auto Value) he gave me a ballest resister to keep my coil voltage below 13V and a good quality set of points. Been good so far 6 months. Sorry couldn't find the box for the resister.

    Bruce (VA)    Posted 04-10-2018 at 07:25:51 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • Thank you for the tutorial.

    Lets start with your assumption that reducing voltage to the points will increase their longevity.

    It wonít.

    The points will operate just fine on 6 or 12 volts; if they wouldnít, you would see different part numbers for 12v points or 6v points.

    The issue isnít voltage to the points, itís voltage to the coil, and specifically, that 1932 designed front coil.

    Ford put the front coil on his first V-8 in 1932 and on the 9N in 1939. The first time I ever saw one was in the early 1960ís helping the neighbor work on his 40 Ford. Those coils have a poor design by todayís standards. The oem tractor coil had an internal resistance of about .5 ohms; that allowed max current to the points for start up on a cold morning when battery voltage was reduced. But, while running at 7.5 volts, the low internal resistance of that square coil would result in over heating. Ford solved the problem by adding a ballast resistor in the circuit w/ a value of .3 ohms cold and 1.7 ohms hot. So, at start up, you got about 6.8 amps of current to the coil. But, as the resistor heated up, running current was reduced to 3.4 amps.

    I asked about the values of the coil and resistor because it is very easy to add too much resistance. If your tractor didnít have the oem ballast resistor and you added one, thatís not a problem. But, if you added the usual ceramic resistor, it could easily have too much resistance in the circuit, depending on the internal resistance of your coil.

    The good news is that your coil will last forever; the bad news is that a slightly discharged battery on a cold morning wonít get you enough voltage to the coil to fire the plugs.

    The key to points longevity as you noted is quality points. But randomly adding resistance in the ignition circuit could easily result in a weak spark.

    Bruce (VA)    Posted 04-09-2018 at 20:09:07 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • What is the internal resistance of your coil?

    What is the value of the resistor?

    Round (side) coil or square (front) coil?

    Mike Stanaway north mi    Posted 04-09-2018 at 22:00:48 [URL] [DELETE]        [Reply] [No Email]  
  • Re: 12v ign system voltage & resistance test?
  • Mine is a top coil on a 2N. I couldn't tell you the resistance of the coil or the resister. My parts guy grabbed a resister off the shelf. Think he said it was for lowering input voltage to 13v. told me my points will be here tomorrow. And i put it together.Best advice is find a good set of points and a condenser. Try that if it still burns them up buy a matched voltage coil. Don't get stuck in the diagnoses. When I was in prototype design we called it paralysis of analysis. Our grand parents fixed this stuff in the mud with limited parts in only daylight with breadsacks on there feet walking for tools uphill both ways. You can do it too.

    Bruce (VA)    Posted 04-09-2018 at 13:06:50 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • 12v coil? Should be 3 to 3.25 ohms internal resistance.

    Alternator should be putting out 14.5 volts.

    Yea, a bad condenser will cause pitting/metal transfer on the points, but the likely suspect for 'going through points' is cheap points. Use Wells, Bluestreak or Echlin.

    See tips 66,67 & 68.

    Rusty 2N    Posted 04-09-2018 at 14:08:57 [URL] [DELETE]        [Reply] [No Email]  
  • Re: 12v ign system voltage & resistance test?
  • Thank you Bruce. Have only used Blue streak or Echilin for many years. Havent bought a Champion spark plug since it became impossible to get 6 good plugs in a box, 25 years ago. I have heard some good about Champion lately but live to far from parts store for return trips on bad parts. Rusty

    Jack - Iowa    Posted 04-10-2018 at 07:21:13 [URL] [DELETE]        [Reply] [Email]  
  • Re: 12v ign system voltage & resistance test?
  • I see no reason for your testing procedure to change, 6V to 12V.

    Be suspect of any readings you take while the engine is running IF you are using a Digital Meter. They are terribly susceptible to engine electrical noise.

    Bruce has given you the resistance for a "pure" 12 volt coil. There are also coils sold as 12 volt which are actually lower and require a ballast resistor. Maximum current in the coil primary circuit should not be more than 4 Amps.


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