tractor engine coil question

have 3 extra coils from tractors i've taken appart over the years. need to use one for the FIL's tractor.

2 are Delco Remy's. one with 266/BR stamped on the side and the other has 293/BR on the side.

Both measure 10.45ish ohms

The 3rd one has no brand but 540 and made in usa printed on the side. it measures about 7.5 ohms.

Do these reading sound normal for a "good" 12V coil?

Thanks.

The resistance reading across the primaries should be .75 to 4 ohms. Maybe you've got a wacky ohm-meter?

A low ohm coil is typically for a 6 volt system or 12 volt if an external resistor is added. A high ohm coil is typically for 12 volts only with no resistor added.

Here are some random coil specs:

1992 Subaru 12 volt breakerless: 2.4 ohms

NAPA IC-12 Echlin 2.1 ohms primary, 8.9 ohms secondary

NAPA IC-27 Echlin 2.2 ohms primary, 6.38 ohms secondary

A500 12 volt 2.6 ohms primary, 9.8 ohms secondary
Echlin IC something 4.2 ohms, 4.82M secondary (no ext resistor)

Deere 6 volt (as used on 430, 1010) 2.6 ohms prim, 7.12 secondary

Hot Spark brand - HS06HEC - HEI, CDI and electronic ignition systems requiring a coil with 0.6 Ohms primary resistance. 11.6 K ohms secondary.

Mallory 29219, 12 volt coil - primary 1.4 ohms, secondary 9.8K ohms. 58K volts

Mallory 29217 12 volt breaker-point coil, primary 1.4 ohms, secondary 9.8K ohsm, 58K volts,

Accel coil for points, 1.4 ohms primary resistance, 9.2 k ohms secondary resistance, 42,000 volts maximum voltage

NAPA* Echlin* ICR11 (1.35 Ohms)
NAPA Echlin ICR23 (1.2 Ohms)
NAPA Echlin ICR34 (1.4 Ohms)
Lucas* 3BR (1.3 - 1.4 Ohms)
Mopar* DCC-4529795 (1.4 Ohms)
Accel* ACC-150250 (1.35 Ohms)
Standard* RU-4 (1.35 Ohms)
Standard RU-23 (1.2 ohms)
Standard RU-37 (1.4 Ohms)

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

Touch the meter leads together and read the indication. Odds are the zero is high.

Or his reported 10.45 is really 10.45K Ohms & he is measuring to HV tower.

Another thought: As an engineer and electronic hobbyist of over 50 years I've owned several ohm meters and found some cheapies aren't all that accurate at the low ohms range!!! Regardless and speaking in general, however, those 7 to 10 ohms readings ARE NOT in line with the more typical 2 to 4 ohms in old stock farm tractor 12 volt coils. They may be in the range for more modern automotive or pulse fired or elec ignition or high energy ignitions, hard to say from here.

PS youre talking about the LV Primary resistances right??? That's what my discussion below was referring to

CHECK YOUR METER AND FINALY HEY JUST TRY THE COILS OUT its hard to sit here and know the answer.

Let us know what you find

John T

no i was actually checking across the secondary. Couldn't get the meter to read the primary's but i may have had the dial in the wrong range. will change the range and try again tonight.

Meter isn't a "cheapy" but its no high end fluke either.

I think the 263 DR coil came of the non-running Case 400 (little) one came from a non-running Case 730 and the other non-Delco came from a running Case 800. I think the one that says 12V on the back is the one from the 800.

with some googling i see the 293/B-R coil was used on late 60's GM cars. Does anyone know what the BR stamped under the number means?

Thanks again.

"no i was actually checking across the secondary"

AH HAH makes more sense, I think another JMOR below also suspected such!!!!!!!!

Fun chattin with ya and thanks for the feedback

John T

does the secondary resistance tell us anything?

(quoted from post at 14:12:34 12/06/13) does the secondary resistance tell us anything?

Click to expand...

imited. If open, then wire broken & trash. If too far on the low side, as in Ohms instead of thousands of Ohm's, then likely windings shorted & again, trash. The huge range in between, means it has a chance of being OK.

yeah.

the values i put in my 1st post should be KOhms not Ohms which explains why i wasn't getting a reading on the primarys. it was late and cold out.

will check again tonight.

I presume the higher the primary resistance the more likely 12V or 12V without need for external resistor?

Thanks

ya.

the values i put in my 1st post should be KOhms not Ohms which explains why i wasn't getting a reading on the primarys. it was late and cold out & i was in a hurry.

will check again tonight.

I presume the higher the primary resistance the more likely 12V or 12V without need for external resistor?

Thanks

Buying a meter and knowing how to use it are different things.Many posters that have a meter dont know its proper use.I have an old vom that has zero to 1 ohm divided in 4 divisions.I can pick out a shorted scr that has a diode in parallel.Saves unsoldering a good part.You need a good battery on low ohms readings, not a weak battery that drifts on low ohms readings.Needle point test prods are best on circuit boards.