Re: tractor engine coil question

Posted by John T on December 06, 2013 at 06:47:41 from (216.249.76.176):

In Reply to: tractor engine coil question posted by INCase on December 06, 2013 at 05:34:58:

See below for an answer to your specific question, but this background information may help others..................

First of all, MANY (NOT all, a person can often find an exception) garden variety of typical points and condenser ignition coils used on old farm tractors circa 30's through 60's.

IM NOT TALKING HIGH ENERGY OR ELECTRONIC IGNITION OR AFTER MARKET PERFORMANCE COILS OR AUTOMOTIVE APPLICATIONS. I'm NOT talking about High Voltage or Performance Coils like say Mallory or Accell or Pertronix Flame Thrower etc etc. IM TALKIN OLD TYPICAL STOCK FARM TRACTOR COILS!!!!!

That used what's commonly referred to, labeled as such, designed and sold for 6 volt nominal operation "6 VOLT COILS" and MANY (NOT all) had a typical LV Primary winding resistance in the range of 1.25 to around 2+ ohms (1.5 typical value). They could produce a sufficient spark at reduced voltage say around 4.5+ to 5 range when cranking and still work and not overheat if charging voltage was elevated to 7+ volts THEY ARE KNOWN AS, LABELED AND SOLD AS "6 VOLT COILS".

If they were designed, labeled, known as and sold for 12 volt nominal operation "12 Volt Coils", MANY (NOT all) had a typical LV primary resistance in the 2.4 to 4 ohms range (3 typical). They could produce a spark if voltage was reduced to say 9+ to 10 while cranking and still produce a spark and not overheat if charging voltage was 14+ THEY ARE KNOWN AND SOLD AS 12 VOLT COILS.

NOTE 1: Many manufacturers USED THE SAME 6 VOLT COIL ON 6 OR 12 VOLT TRACTORS. Its just that if used on a 12 volt tractor they added an in line series voltage dropping (12 to 6) Ballast Resistor that dropped the 12 battery volts down to 6 for the coil.

NOTE 2: Coils labeled as "12 volts" or "12 volts NOT for use with Ballast Resistor" are designed and intended for 12 volt nominal application. HOWEVER a coil labeled as "12 volts for use with Ballast Resistor" or "12 volts requires Ballast Resistor" MEAN WHAT THEY SAY and operate (actual coil voltage) at closer to the 6/7 volt range NOT full 12.

NOTE 3: If you got to a farm store or NAPA and ask for a generic old farm tractor coil, likely the very FIRST thing they may ask is if its for 6 or 12 volt. Of course, they could sell just a 6 volt and tell you if used on a 12 volt tractor to add the series voltage dropping (12 to 6) ballast resistor.

NOTE 4: Some (NOT all) later automotive application coils and some electronic type ignitions used more of a voltage spike/pulse to fire the coils instead of the old style points operation where closed points conducted coil current and then when they broke open the field collapsed and the coil fired. THEIR RESISTANCE MAY BE CONSIDERABLY DIFFERENT THEN TYPICAL OLD FARM TRACTOR 6 OR 12 VOLT COILS!!!!!!!!!!!!

SO FINALLY TO ANSWER YOUR QUESTION "Do these reading sound normal for a "good" 12V coil?

While that's not what Id normally expect for a typical old farm tractor points and condenser ignition coil THAT MAY BE FINE FOR THE SYSTEM THEY WERE DESIGNED FOR......... However they may still "work" (not saying how well if they aren't designed for the tractors ignition, could yield a weak spark!!!) but if they had a resistance much lower then 2 to 3 ohms and you used points (instead of elec ignition) to switch coil current, the points would burn up prematurely....

Finally, a low voltage low energy ohm meter test can tell you if a coil is definitely bad (open winding) but it cant tell you if its necessarily good as many failures take place as HV breakdown and some only after the coil is warmed up.

NUFF said lol Best Wishes n God Bless all

TRY the coils, they may (and may well NOT lol) possibly produce a sufficient spark, but typical old 12 volt stock farm tractor points and condensor ignition coils were more in the 2.4 to 4 ohm range.

John T

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