[Farmall] Ignition Systems 101

[Stan Bass]

Maybe I over simplified a little too. I understand the use of ballast
resisters and the V = IR equation. My observations have shown factory
coils (on the common subjects of this list) at about 1.2 to 1.5 ohms.
And since the ballast resisters come in optional values, they need to be
spec'd. I use a rule of thumb at 6V and 1.5 ohms to get the 4Amps
(mentioned below.  At 1.2 ohms it would be 5Amps) that needs to be
maintained to the coil; and when stepped up to 12V, I usually target
2-1/2 to 3 ohms either through use of a ballast resister or just a new
coil. But I realize that these calc's are just maintaining balance in
our ignition systems, and that's different from understanding how these
numbers were selected by the designers (ie: 4Amps through the points and
a 1.2uF capacitor?).  I was thinking specifically back to this
comment...

> I could go into more detail as to the proper microfared capacity matching of the condenser to
> the number of windings that are in an ignition coil...

and some of my measurements/observations ...

> > but I recall the last condensers I checked were about 1.2 uF
> > for the Farmalls and about 0.2 uF for the Cub Cadets? And coil
> > selection (1-1/2, 3, or 4-1/2 ohm coils). Our old tractors seem to draw...

I have to add one update. Working on an older cub cadet today turned up
my first 4-1/2 ohm coil! So maybe there is something to that comment I
read somewhere about the single cylinder motors...

If this is getting too picky/detailed, then I'll drop it. Just always
intrigued at the chance to learn something new!
Stan(VA).


cliff king wrote:
> 
> Weellllll...LOL. If you go by the laws of resistance/ohms/volts it's natural for the resistance to go up as the amount of voltage increases. Coil saturation time is controlled by crankshaft rotation degrees, commonly known as "dwell". In a 360 degree rotation of the crank, the points will stay closed for approx. 30 degrees of that. Almost all 12 volt systems have a ceramic resistor installed inline. Chrysler has used them for years. If you convert a tractor that is 6 volt, to avoid burning up the points one must install one of these. Ford Motor Co. used an inline distributor lead to knock the voltage down to the points. So an answer to your question about the condensor...if you do the proper conversion, be it a ceramic resistor or the inline wire between the coil and points, it still drops the voltage used to 8 or less. I know this is long winded, but it would be a good idea to cut/print this info. And one must keep in mind that when most tractors are converted, most guys !
 us!
>  e an
>  alternator which puts out 14.5 to 15.3 volts at full field charge.Naturally, when you slam 14 or more volts into a contact set...POOF the MAGIC DRAGON appears..or was that Puff...they get blue lookin and go up in smoke. Or burn together and it won't start. Back to the question about the condensor, if you put the resistor in there, the system operates just as if it were the original 6 volt. I can go into more detail if anyone needs it.
> 
> Stan Bass <headlight at erols.com> wrote:Cliff,
> Do you mean literally to go off the resistance on the primary to select
> the condenser, or the current drawn? ie: If we covert to a 12V system do
> we need to spec a new condenser? The primary resistance goes from 1.5 to
> 3 ohms, but that is to keep the current the same at about 4 Amps.
> Stan(VA).
> 
> cliff king wrote:
> >
> > Not a problem guys..I am always willing to give assistance or thoughts, however, I tend to take the technical route because that is the way I was taught. I can't recollect the correct ratio of condenser to coil capacity..but I would sure research it if needed. I have it in my collection of tech manuals somewhere around here. You have to go off the primary winding of the coil due to the magnification in voltage developed in the secondary winding needed to fire the fuel ignition device( spark plug). 90% of the time I work on/tear up/repair diesel engines so I tend to forget some of the basic ignition stuff required to ignite gasoline. I didn't intend for the message to be sarcastic..but I deal with wanna-be parts changers on a daily basis (most of whom have no earthly idea what the hell they're doing, let alone knowledge of how it's supposed to work in the first place).
> >
> > Cliff
> >
> > Stan Bass wrote:
> > I would vote to see your thoughts on this, Cliff. Now this is from
> > memory... but I recall the last condensers I checked were about 1.2 uF
> > for the Farmalls and about 0.2 uF for the Cub Cadets? And coil
> > selection (1-1/2, 3, or 4-1/2 ohm coils). Our old tractors seem to draw
> > about 4 Amps, so I know our old 6V coils are about 1-1/2 ohm and the 12V
> > ones are about 3 ohms; but I read somewhere (wish I had saved it off)
> > that single cylinder motors like the Kohlers in IH Cub Cadets use a
> > 4-1/2 ohm coil (but I've checked several and all so far have 3 ohm
> > coils?). Trying to remember what it was based on, voltage, current,
> > cylinders and RPM range? Don't worry about going over anybody's head,
> > that's how we learn. My only comment from doing this in the past, is
> > please state when things in an explanation are over-simplified in the
> > interest of brevity. That way if someone is researching it further on
> > their own, they know where the back holes are ;)
> > Thanks,
> > Stan(VA).
> >
> > cliff king wrote:
> > >
> > > In my garage, which I operate as a business, we don't guess. Put yourself in the customer's shoes standing there listening to someone tell you why the bill is outrageously high for a bunch of parts installed that didn't fix the problem. Use a test light and do some diagnostics instead of being a "parts changer at someone else's expense". I would lay odds on the condenser being bad. Practically every mag I overhaul has to have the coil and condenser replaced. The condenser is a miniature storage capacitor for the coil. The point set is simply a mechanically operated switch that turns the voltage stored on and off. I would say that 9 out of 10 mags have points that are ok, just need the mountains filed off to operate at the proper clearance. I could go into more detail as to the proper microfared capacity matching of the condenser to the number of windings that are in an ignition coil...but I'm afraid I would lose some of the more simple minded folk on the list.
> > > Cliff
> > >
> > > "soffiler at myeastern.com" wrote:
> > > In the trade, that's called "plug and chug". Take an
> > > educated guess, replace parts, repeat as necessary until
> > > problem is solved. It is generally considered a less
> > > desirable problem-solving technique versus doing some basic
> > > diagnostic work. I realize that sometimes it's just more
> > > convenient, or less intellectually taxing, to use the plug
> > > and chug approach.
> > >
> > > Steve O.
...