N Board Forum - Expanded Thread Page

Subject: Another look at the resistor

[Back] [Return to Top of Forum]

Farmer Dan    Posted 01-25-2019 at 07:27:07 [URL] [DELETE]        [Reply] [Email]  
  • Another look at the resistor
  • Been some chatter lately about the ballast resistor and why its needed. Even I was not very clear on it. Take a look at this youtube video. Not the same as the N wiring but pretty darn close, same principles apply.


    What this guy is saying is that a 12 volt coil can not take 12 volts without frying (after an undetermined amount of time) same with the 6 volt. So why call it a 12 volt coil? Guess we can't have everything in life but that explains the resistor need.

    Tim Daley(MI)    Posted 01-29-2019 at 04:34:30 [URL] [DELETE]        [Reply] [No Email]  
  • VALUE?
  • OK, so what is the Ballast Resistor value supposed to be when measured static, can it be measured static? If you can test the coil static, should be able to test the resistor as well. Most resistors have a +/- 10% or 20% rating. Is that factored into its design? All front mounts are wired with the coil wire connection going to one of the top two terminals on the ballast resistor. The other top terminal has one of the ignition switch wires attached.

    Tim Daley(MI)

    Farmer Dan    Posted 01-29-2019 at 05:39:51 [URL] [DELETE]        [Reply] [Email]  
  • Re: VALUE?
  • I believe the original resistor is 2ohms if you comvert to 12volts and still use the 6volt coil you need to add another 1ohm.

    JMOR    Posted 01-29-2019 at 11:56:01 [URL] [DELETE]        [Reply] [No Email]  
  • Re: VALUE?
  • 12250 res varied about 5 to one depending on temperature. It glows red & will ignite paper when touched to it before it gets hot enough to reach 2 Ohms.

    Bob in KS    Posted 01-25-2019 at 15:35:42 [URL] [DELETE]        [Reply] [Email]  
  • Re: Another look at the resistor
  • The video gives an excellent description of the resistor bypass starting circuit. However, it is only applicable to a 12 volt system.

    The Ford N series front mount distributor operates on a similar principal with it’s 6 volt system.

    Ohms law states that current = voltage divided by resistance (do an internet search for further explanation)

    Most old tractor distributor ignition coil primary windings are designed to operate at about 4 amps. With a 6 volt battery, a coil with 1 1/2 ohms of primary resistance would give 4 amps. With a 12 volt battery 3 ohms of resistance would give the 4 amps. A coil is a transformer with one side of primary and one side of secondary tied together.

    In the front mount N series ignition, the coil is designed to require a special resistor. It is a temperature variable resistor, ie, lower resistance when cold and resistance increases as current through the resistor causes it to heat up. When first started, the resistor with lower resistance would give a slightly higher current to give a hotter spark for starting, then the resistor would increase in resistance and lower the current to 4 amps. The coil can stand this increased current for the shorter starting duration but not over an extended time.

    Round can coils are different. A “6 volt” or “12 volt external resistor required” coil have approximately 1 1/2 ohms primary resistance. In a 6 volt system either one will run at 4 amps, no extra resistance required or allowed. They can both be used in a 12 volt system by adding another 1 1/2 ohms of resistance between the output of the switch and input of the coil. 1 1/2 ohms of the coil plus 1 1/2 ohms of resistor equals 3 ohms.

    There is also a “direct 12 volt” or “12 volt no external resistor required” coil that has 3 ohms of built in primary resistance. This coil is designed to operate in a 12 volt system with no added resistor. This coil is wound with 3 ohms of resistance in the coil primary, there is no internal resistor in series with the primary.

    Some automotive systems have used different coil primary resistances which is why the video is saying 8 – 9 volts . That is true of that coil, but not universally true of all coils.

    But here we are talking old tractor systems. The specific coil used will determine if and how much external resistance is required.

    Many later tractors did use the starting bypass system described in the video. This would only be used in a 12 volt system with 1 1/2 ohm coil, it would not be used in a 6 volt system. The starting bypass described in the video will not work with a direct 12 volt 3 ohm coil.

    For convenience we think of coils in terms of them being 6 volt or 12 volt. Coils really don’t care whether a 6 or 12 volt battery powers the system. Their only concern is with maintaining the 4 amps of current passing through them. The 4 amps is maintained by the combination of battery used and external resistance that may be required for that specific application.

    Farmer Dan    Posted 01-25-2019 at 13:54:37 [URL] [DELETE]        [Reply] [Email]  
  • I've got it now
  • The 12 volt coil is really 8 volts and the 6 volt coil is really 4.5 volts. Thanks guys.

    steveVa    Posted 01-25-2019 at 12:42:58 [URL] [DELETE]        [Reply] [No Email]  
  • Resistance is futile!!!
  • I am reluctant to reinstate discussions on resistance.
    I have always thought of a ballist resister as a shock absorber. It allows higher current when needed and reduces current when not needed.
    I am guessing that the main purpose of a ballist resistor was and is to extend the life of your coil. Once your engine is up and running it requires less current to run properly, and when cranking an engine the starter takes a lot of the current.
    I am also guessing here, but I always thought that a coil fails from overheating. Those fine little winding wires get too hot and their coating breaks down and they short out, or the little wire actually breaks. I always figured that an intermittant coil has a broken wire that touches together when cool and then looses its connection when it heats up. The intermittant ones are the ones that drive me crazy. I also had a spark plug that did that once. It was fine for just a minute or two and then stopped working. Drove me nuts...

    Jack - Iowa    Posted 01-25-2019 at 07:56:44 [URL] [DELETE]        [Reply] [Email]  
  • Re: Another look at the resistor
  • Sorry Dan,

    That is not a Ballast Resistor! Just a straight dropping resistor. The true Ballast Resistor is as I explained. Low resistance when cold, higher resistance when hot.

    Anyone using a 6 volt coil on a 12 volt system needs both.

    Neither coil can take full stated voltage. The old Fords actually had a 4 volt coil on them.

    Hopefully Bruce will jump in and validate what I've written.


    Tim Daley(MI)    Posted 01-25-2019 at 07:45:52 [URL] [DELETE]        [Reply] [No Email]  
  • Re: Another look at the resistor
  • I still haven't seen any in-depth explanation of the ballast resistor, coil, and electrical system. just saying to google positive ground system and such is not the answer. It is too general. We need a definitive Ford 6V/POS GRN System stated in both front mount and side mount terminology.

    Also, FWIW, does anyone else see anything wrong with this diagram? Look at the upper LH exploded view.

    Tim Daley(MI)

    Farmer Dan    Posted 01-25-2019 at 13:53:14 [URL] [DELETE]        [Reply] [Email]  
  • Re: Another look at the resistor
  • drawing is for an 8N, no regulator and no fuse.

    Tim Daley(MI)    Posted 01-26-2019 at 08:17:54 [URL] [DELETE]        [Reply] [No Email]  
  • Re: Another look at the resistor
  • I think you meant to say the drawing I posted is for the 9N/2N Dan. It is FIG. FO70 from the I&T FO-4 manual, see lower RH corner. A voltage regulator, p/n 9N-10505-A, was used on the very first early 9N-10000-A small 2-wire/2-Brush B-Circuit generator. The round can cutout and the 9N-10000-B generator were released to replace the previous electrical components. The 9N-B generator was now a 1-Wire/3-Brush A Circuit Design unit. This setup was used up thru all 9N and 2N production only the generator was revamped with a larger barrel for the larger armature to produce 11.5 amps output on the 9N-10000-C unit which was virtually the same internally as the 2N-10000 unit. The 8N model used a new 3-Wire/3-Brush A Circuit Design generator, output at 11.5 AMPS and a new voltage regulator. At s/n 263844 when the side mount distributor was released, the generator became a 20-AMP, 3-Wire/2-Brush, A Circuit Design unit and moved to the LH side of engine.

    No 'N' ever used any fuse in the electrical circuit. Only the light switch had a 15A fuse and lighting kits were always dealer optional equipment accessories.

    Now, here's my take on the Ballast Resistor. There are two styles, the 9N/2N unit and the 8N unit indeed, but the difference is in the mounting screws only, and nothing to do with its design and function. It is mechanically designed internally on both units. The 9N-12250-B resistor uses two 5/16-18 x 7/16 Flat Head Machine Screws, shown as p/n 24628-S7, to mount to dash. The 8N-12250-B resistor uses two #12 x 5/8 Flat Head Self Tapping Screws, p/n 41843-S8. I'm not 100% certain but the hole spread may be a bit different as well. I have a half dozen used resistors around somewhere so will look to see if I have one of each to compare. The way each one is wired into their circuits is different because the 9N/2N used the 2-wire cutout, the 8N used the 3-Wire VR; the 9N/2N used a 1-wire, no solenoid starter motor, the 8N used a 1-wire starter cable plus the solenoid had 3 wire connections. Nothing changed with the front mount distributor and coil assembly. The same wires are used for coil to resistor, ignition switch to resistor, with each model wired from their circuits. This is how I interpret the two resistors and don't see why each would have its own or require a different internal design. You could take a 9N resistor and use it your 8N and vice versa as long as you wired each as shown in their diagrams and could find a way to mount them. Just because each is wired a bit differently does not make them different parts, in my opinion. I could be wrong here, my wife says I often am, so if Bruce, JMOR, Soundguy or anyone else read this, please chime in.

    Tim Daley(MI)

    Farmer Dan    Posted 01-26-2019 at 13:32:02 [URL] [DELETE]        [Reply] [Email]  
  • Re: Another look at the resistor
  • could you sell me one of those used resistors cheap? I need to put one on my tester.

    Tim Daley(MI)    Posted 01-27-2019 at 06:36:24 [URL] [DELETE]        [Reply] [No Email]  
  • Re: Another look at the resistor
  • No but I'll give you one next time I come down...


    Tim Daley(MI)    Posted 01-27-2019 at 08:20:25 [URL] [DELETE]        [Reply] [No Email]  
  • Re: Another look at the resistor
  • If you have an old ballast resistor, you can simply purchase a new resistor without the mounting plate for it as long as the terminal posts aren't stripped and/or the phenolic plate isn't cracked. Look at Carpenter or Just8Ns for p/n 18-12250, $15, for the OEM ceramic resistor used on both the 9N and 8N blocks. If the block is still good, only the resistor needs to be replaced and at half the cost. There is a generic aftermarket replacement part sold for about $6 too.


    Tim Daley(MI)

    Farmer Dan    Posted 01-25-2019 at 08:53:15 [URL] [DELETE]        [Reply] [Email]  
  • Re: Another look at the resistor
  • It is showing an 8N resistor.

    Jack - Iowa    Posted 01-25-2019 at 08:11:47 [URL] [DELETE]        [Reply] [Email]  
  • Re: Another look at the resistor
  • Tim,

    Did you read my previous post explaining the Ballast Resistor?

    I'm not seeing what you are in the posted diagram. Not positive wire colors are correct.

    Y large - Battery
    Y small - Lights
    Y Brn Trace - Voltage to Switch
    R Brn Trace - Switched Voltage
    R - To Coil

    What am I missing?


    PS I still have radio schematics. Need Model numbers

    Tim Daley(MI)    Posted 01-25-2019 at 09:41:15 [URL] [DELETE]        [Reply] [No Email]  
  • Re: Another look at the resistor
  • Never mind, my bad. My initial impression was the wiring to top active resistor terminal posts were backwards. The RED coil wire should be to the LH post, the ignition switch RED w/BLACK TRACER wire to the RH resistor post, as viewed from front of tractor. Then I realized it doesn't matter electrically as resistors are not polarized. I think it does matter in a real life scenario as to which tractor model, which wiring harness is used, as the wiring harnesses were different depending on location of key switch. The early 9N had the ignition switch on the upper RH dash panel. It got moved to the LH side and then to the steering column. The 8N switch is on the lower LH dash. Wiring Harnesses don't allow much wiggle room as they are so having the correct one seems more important. I've worked with aftermarket harnesses and had issues with one or two wires coming up a tad short. The OEM 9N/2N switch wires are BLACK w/RED TRACER to LH resistor post and YELLOW w/BLACK TRACER wire to the resistor bottom junction post. My 8N MPC shows the ignition switch wires as YELLOW, to bottom junction, and RED w/GREEN TRACER, to RH top resistor post. Not a big tech issue, just saying. Also, the only differences between the 9N ballast resistor and the 8N unit are the mounting screws. I don't believe there are any electrical value differences -shouldn't be anyway.

    Tim Daley(MI)

    Jack - Iowa    Posted 01-25-2019 at 14:26:25 [URL] [DELETE]        [Reply] [Email]  
  • Re: Another look at the resistor
  • Tim,

    You are not the only one to make an error! I referred to my previous post but that was on another thread! Below is what I posted:

    The purpose of the ballast resistor is to drop the voltage to the coil. It is constructed of a wire with a high temperature coefficient.

    When cold, the resistor is quite low allowing maximum voltage to the coil for starting purposes.

    It soon warms up which lowers the voltage to the coil and thus reduces the current through it and the heat it must dissipate.

    Some coils are built with internal resistance which in effect does the same thing. Those coils generally are marked: NO RESISTOR REQUIRED or something like INTERNAL RESISTOR.

    If your coil does not have those markings you likely need the ballast resistor to keep the coil from burning up.

    Many years of automobiles were built with a switch section on the starter switch or solenoid which by-passed the ballast resistor for starting. You can do the same thing on your N by holding a quarter across the resistor terminals.

    Hope this helps to clear up some of the confusion and misunderstanding.


    [Back] [Return to Top of Forum]