My coil's okay BUT....

can leaving an ignition in the on position overnight really fry points ?
Something caused her to stop dead on the road the other day, with no spark. It happened a few days AFTER I charged the battery when the wife left the ignition on overnight.
The points were black and lacked spark.

So I just installed another set of new points, performed checks for continuity, checked connections, and wala, she fired right up.
Thanks to your tips guys, I eliminated the coil as the culprit.

Just wondering what really happened.....



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Yup.The points can definitely get fried with key left on if the points are in closed position.

OF course it WILL. You have to understand a little about coils and ignitions. A coil in normal operation (running) is NOT a DC load. It is an AC circuit, because the points switching act sorta like "an inverter" or more correctly a chopper. This means that you have a pulse effect, more like AC than DC, and that the duty cycle is lower.

Second part is more difficult to understand. A coil has "inductance" and exhibits "inductive reactance" It is in fact the coil and condenser together that make the spark. If you ever see a tune-up pattern on a 'scope, you'll see the 'ringing' during the spark time. These oscillations are caused by the coil and condenser alternatingly trading off energy to each other--first the charged condenser (capacitor) discharges through the coil, which forms a magnetic field. Then the magnetic field collapses, generating electrical energy in the coil, which recharges the capacitor. This ringing goes on until electrical losses deteriorate to nothing, or else the points close again as the engine turns. The point is, that as an engine runs, a coil does not draw the same current (because of this inductance) as the points open/ close as it would if you replaced it with a pure resistive equivalent.


What all this nonsense boils down to is that a running coil DOES NOT draw current WHILE RUNNING in the same manner that it does when NOT RUNNING. The 100 percent draw (cureent/ wattage) of a coil when stopped is incredibly more than when running.

The only other thing that (on many engines) is that it may have a "ballast" or "coil resistor" which WILL help to mitigate some of this current, I.E. the more current that the coil tries to draw, the less the ballast allows it to draw.

In older motor vehicles, a coil/ resistor could actually cause a fire. Visualize the mid-late 60's or early 70's. Visualise the usual family truckster, with 1/8" of grease and oil all over the engine room. A hot resistor not a good thing.

The points overheat because they have poor contact from wear, oxidation, pitting, etc. Like any poor or loose connection there is resistance to current flow. Resistance equals heat, heat equals more resistance .... pretty soon points fry same as any other inadequate contact.

Points that are fully closed will usually not heat up and burn, as they are making a good contact.
What gets you is when the engine stops with the points just on the verge of opening or closing. They are still in contact, but without enough pressure to make a good contact. The poor contact allows the points to heat and burn.

Depends on who you ask, I'd lean closer to pulsed DC rather than AC. Even though the spark will "ring" or resonate a cycle or two with points. On an electronic system, little to none.

Thanks for clarifying all this, theory and practical. Makes a lot more sense now. But why doesn't my wife feel any remorse ? Somehow it's all my fault.

Maybe I'll post this picture with the red light on in the kitchen......as a reminder.

Of course I'll be on my own for dinner, can't win

If the points are good n clean and closed well with with no oxidation or carbon accumulation or pitting etc leaving the ignition on wont harm them much HOWEVER if theres resistive carbon thats setting there with the ignition left on such will heat up n maybe cause even more carbon accumulation n damage so YES it can "burn them up" as you state it. Of course, with age points can accumulate some carbon and yes such can cause heat build up if the ignitions left on not running

With the ignition left on long periods n the tractor not runing the coil can get hottttttttt cuz its always conducting current versus the cycling of current that normally occurs with the tractor running

John T

Well in the future if/when she leaves it turned on again and the coil burns out like they can some times with a sound like a shot gun going off she might remember to turn it off. Yep coils can/will blow up and when they do they make a big mess and sound like a gun shot. Don't ask how I know

Actually, a spark can still be produced WITH NO CONDENSER WHATSOEVER in the ignition primary circuit, albeit not as intense as it is if a proper sized condensor is indeed located in the circuit in paralell with the points.......The condensor in addition greatly extends the life of the ponts ......

The coil still draws straight DC (I = E/R) current when the points are closed and the inductive effect has diminished, its just that the current is constantly being switched on then off then on then off as opposed to alwaysssssss being on when not running with ignition ON (assuming points closed) therefore more heat is generated. The inductive reactive effect a coil has is evidenced at current turn on time and current turn off time but is NOT evidenced during steady state DC conditions, at which time its a pure resistive load with I = E/R. Impedance (for AC) is dependant upon frequency while resistance (at DC) is not.

The purpose of a voltage dropping current limiting resistor used in many tractors and autos etc is two fold, yes it reduces coil current (to avoid overheating) but it also reduces the amount of current the points are forced to switch so they dont burn up prematurely

Hope this helps the original popster better understand the good old fashioned Kettering Ignition system

John T

Another reason for the ignition resistor is it allows hotter spark for starting.
When most automobiles changed from 6V to 12V in the mid 50's, the kept using the 6V coils by adding a resistor in the feed wire to the coil.
They were wired so the ignition resistor was bypassed while cranking, so the 6V coil was fed 12v to produce a much stronger spark for better starting. once the engine started, the resistor was switched back inline to reduce coil heating and prevent burning the points.

Youre correct, even one more ballast advantage

John T

Yeah, I useta blame that kinda thing on my wife too. She finally kicked me out so I started blaming it on my son. Now he is grown up and gone so now I have to blame it on my dog.

Ya need a 0 - 30 psi oil pressure gauge on that VAI. Bet that 0 - 100 psi gauge barely lifts off the peg.

Joe

I know and it bothers me to this day, but I didn't have the cash to buy CASE reproduction or NOS gauges.

This is the original, and indicative of the condition I found this ol' girl in.

That's pretty funny about your dog, but my wife DID leave the switch on...........I think.

(quoted from post at 14:16:51 03/29/09) Actually, a spark can still be produced WITH NO CONDENSER WHATSOEVER

John T

Click to expand...

Most vehicles WILL NOT RUN with no condenser. While the ignition may still "produce a spark" it will normally be of such low intensity as to be useless

but my wife DID leave the switch on...........I think."

Ooooops, that's good enough for me, she did it alright. Can't question definite ironclad irrefutable proof like that.";^)