Ignition: Bench Testing Ignition Coils - What Resistance Is Required?

[2R2]

I have a collection (!) of ignition coils. I would like to test them to confirm they would be good spares for my 259 '60 convertible and my '63 R2 Lark. When bench testing the coils, what resistance range should I look for on both the primary and secondary side to confirm if the coil is good? I am setting my ohm meter at 200 for the primary winding, and 20K to test the secondary windings.
I am not seeing any specifications in the shop manual, so I am hoping someone can help.

Thanks!

[Big Dan]

Here's a link to a site for Studebaker coils that looks pretty good. I'll let you read it and take from it what you will! I'm sure someone else will come along that may offer something more specific!

(Ignition Coils) (studebaker-info.org)

[2R2]

Thanks, this was a big help!!

[Bud]

A 6 volt coil will have a primary resistance of about 1 ohm and a 12 volt coil will have a nominal resistance of around 1.5 ohms give or take a couple of tenths. The secondary resistance should be at least 8,000 ohms and can go as high as 12 to 13 thousand ohms depending on the coil. Stock 12 volt coils will be on the lower end of the spectrum and high performance coils will be on the high end and those numbers are the secondary resistance numbers. 12 volt coils meant for use without a ballast resistor will have a primary resistance of about 3 ohms with the secondary resistance being the same as the other coils. Bud

[jcharlestc]

....just retrieved a brand-new--as in "never been used; still in the box"--Borg-Warner E-70 coil (12-v) from the garage.
Used a very good multimeter, and got the following readings:

Primary: 1.5 Ω
Secondary: 7270 Ω
"Use with an external resistor" printed on the coil (that's how we know it's a 12-v coil)

I'm not sure I'd worry too much about specifications. It's been my experience that most all 12-v coils will work in any "non-boutique" (a specialty) ignition system, i.e., on any car, in any 12-v application.

That same experience led me to expect the readings on this coil to be what I found them to be.
It also led me to state, in an earlier post (somewhere), that external ballast resistors for 12-v ignition coils are in the 1Ω range.

The Kettering ignition system is one of THE most basic of electrical/electronic systems to generate high voltage (a coil fed by contacts opening and closing), and there's really only one way to do it economically; to "light' the spark plug (generate 30kV-50kV) under the normal range of pressures found in almost every internal combustion engine.
This fact is what led me to use, very successfully, a wide range of coils and ballast resistors in my '87 Jaguar (don't ask), and never notice any difference in performance.

[Bud]

jcharlestc I agree completely with your statement. I've used stock Ford yellow top coils on my 63 Avanti and 62 GT engines for years and I've never had any ignition related problems with either of them. I did however have problems with the Avanti tach and an MSD coil. The tach would drop to zero above 3,000 rpm with no loss of ignition. If a stock or NORS coil is in good condition, they will work as well on most engines as the voltage needed to light the spark plugs is well within the range of what a stock coil can produce. I have an engine analyzer so I can test different coils to compare output voltage. I will say that the old stock Ford yellow top coil while not the highest in output voltage, does have about the longest spark duration that I've seen. Bud

[Jeffry Cassel]

A lot of new coils are kinda wimpy. I like 1.2 to 1.4 Ohms on the primary and 10,000 to 11,000 Ohms on the secondary windings. Most of the new coils I have checked have checked have been in the 7-8,000 Ohm range. Tho I did find a Delco off a Corvette that tested > 11,000 Ohms so I put it on my 64 Hawk. Can't argue with Ford yellow top coil being reliable. I have had Delco, Autolite, and Pertronix fail. I just don't know that MSD is all that reliable so I avoid it.

[jcharlestc]

One has to be careful when using secondary resistance as a 'figure of merit' of an auto ignition coil.
Consider that all the primary resistances you see above are remarkably close in value. There's a good reason for this: it takes a large current (through the primary) to generate a high voltage in the secondary winding, and no coil manufacturer would skimp on the size of wire used on the primary.

The high secondary voltage is generated by a very large number of secondary turns.

Now, I will readily admit that I don't have any quantitative data (hard numbers); but if most coils generate about the same magnitude of spark voltage (which my experience says is--approximately--so), then this means that most manufacturers of ignition coils use approximately the same turns ratio between primary and secondary, which translates to approximately the same number of turns for the secondary winding.
If this is so, why would one coil have a higher secondary resistance? Very simple: one manufacturer uses a lighter gauge wire on the secondary winding (which is less expensive, and means higher resistance than a heavier-gauge wire) than a manufacturer whose coil shows a lower resistance. The use of a lighter-gauge wire in one coil vs, another could explain a difference in reliability between two different manufacturers' products. Could; Might.
(The secondary winding doesn't handle much current at all; its main purpose is to generate very high voltage at almost no current whatever---unlike neon sign transformers--typically 12 kV, 20 mA-40 mA (that's 240 to 480 watts; 1/4-1/2 KW) and they will kill you; don't play with them.)

Regards...and remember: whatever works best for you is best.


" Pain make man think. Thought make man wise. Wisdom make life endurable."--Sakini, Teahouse of the August Moon

[Bud]

There is also inductance in coil windings and that goes for any coil of wire. I've found that ignition coils with a higher secondary winding resistance have more turns of wire and a higher inductance which equates to a higher winding turns ratio between the primary and secondary windings that will generate a higher output voltage. The so called high output coils have a higher turns ratio but use a lower resistance primary winding to achieve a higher output voltage which works well with electronic ignitions but not so well with points as the current draw goes up causing shortened point life. There is a difference in output and spark duration between different coils as I have done testing with my engine scope, but for most Studebaker applications a stock coil will give good service. I have found that many of the new production coils especially the cheaper ones are wound with aluminum wire which is not as good as a coil wound with copper wire. Bud

[jcharlestc]

Thanks, Bud. It never occurred to me that aluminum wire would be used; that, most definitely, would result in an increase in resistance, over a copper-wire coil.
Actually, it never occurred to me that any manufacturer would DO THAT! (My mind is still "boggled")
(You are certainly correct about manufacturers playing around with turns ratio, primary resistance, etc...somebody always has an idea to increase spark voltage of the coil. Seems as though us customers always go back to the basic decades-old, long-life design.)
Thanks again.

[jcharlestc]

...see very important comment (#8) on using aluminum wire from Bud, above---

[Bud]

There isn't a beer can style coil that I've found that will do more than 38 kilovolts. The design of the coil limits the output voltage and it doesn't matter who built the coil. I look at the ads from MSD, Pertronix etc. touting the big increase in performance over a stock coil. As far as I'm concerned, that is advertising hype and does not equate to real world engine performance. There are coils being produced that will put out a higher voltage and higher current that are meant for racing applications but aren't needed in most street applications. I just saw another Pertronix coil fail on a Ford recently because the car's owner believed the hype from Pertronix about their coils not needing a ballast resistor. The coil finally had enough and failed due to overheating which is now at least the fifth one I've seen fail do to overheating. Bud

[jcharlestc]

A manufacturer telling you you don't need a ballast resistor with HIS coil---another case of a manufacturers' hype (I would have said BS, but this is a family publication so I won't say BS).
What this does is generates a much higher output voltage (which is NOT needed) but a much shorter life of the coil--which Bud has documented. Moral: there ain't no free lunch (in this case, what you get on the front-end isn't needed and not useful anyway).
What would be much more useful--for more compete fuel burning (hence lower emissions) --is if it were easy to generate a longer high-voltage pulse (see comment #5 from Bud). It AIN'T easy. It's easier to tell you to throw away your ballast resistor, and then point to the warranty's time limit when the coil fails.
You can get the same results by throwing the ballast resistor away when using a low-cost coil. It won't cost as much.

“A learning experience is one of those things that says, 'You know that thing you just did? Don't do that again.” ― Douglas Adams, The Salmon of Doubt

[Bud]

I've done testing with Pertronix coils using my engine scope and found that without the use of a ballast resistor, the output is around 5 kilovolts higher than when the Pertronix coil is used with a resistor. I agree 100% with jcharlestc's comment that a longer spark duration is just as important as a really high voltage spark especially in an engine that isn't used for racing . The longer spark duration does a better job of lighting the fuel air mixture which will improve things such as a slightly smoother idle and a bit better fuel economy. Most Studebaker engines are not high compression or high revving so a really high output coil is not necessary to get them to run well. Do not under any circumstance use a coil without a resistor in a 12 volt system if the coil is meant for use with a resistor on a point type ignition as the current through the points is substantially increased and will cause an early point failure. I also agree with jcharlestc's comment about there ain't no free lunch and that comment applies to many things other than ignition parts. Bud

[jcharlestc]

Bud--perhaps you can give the answer to a question which I've had for a long time, and I'm sure a lot of people here would find useful--

Is there really any difference between a 6-v coil a 12-v coil, except for the need for using a ballast resistor in a 12-volt application?

It occurred to me several years ago that, if the answer to this is, "Not really", then this means that one need only keep one good, high-quality 'bog standard' coil on the shelf for use in both 6-v and 12-v cars.

Am I way off base on this?
----------------------------------------------------------------------------------------------------------------
[Speaking of 'bog standard', what's a good value for inductance of the coil's primary , and an average value of a 'standard' condenser (across the points)? ]

[Bud]

6 volt ignition coils typically have a primary winding resistance of around 1 ohm and a 12 volt coil has a primary resistance of around 1.5 ohms give or take a couple of tenths. I have a coil that is branded Atlas which was sold by both Exxon and Standard/ Chevron years ago and from the looks of the coil it was produced by Prestolite. It is marked for both 6 and 12 volts if a ballast resistor is used for 12 volts. I have used 12 volt coils on 6 volt ignitions and the engines started and ran fine. I remember years ago I had a 1965 Volkswagen with a 6 volt electrical system and I found that the engine actually ran better with a 12 volt Ford yellow top coil than it did with the original Bosch coil. I would recommend keeping both a good 6 volt and a 12 volt coil on the shelf just in case. There is also a 12 volt coil that has an internal resistor and its primary winding will be around 3 ohms. Some of the earlier 12 volt 6 cylinder Studebakers used those coils. Condensers should have a capacity of .19 to .22 UFD for most Delco systems but I've seen some Prestolite replacement condensers with a capacity of .25 UFD and the points live a long and happy life. I'll check a few coils for inductance today and get back with some numbers. Bud

[jcharlestc]

Thanks for the answer, and confirmation of what I've thought for a long time. I'm sure that a lot of people here will find this to be a real 'eye-opener'.
Thanks also for the data on condensers; I'll be on the look-out for the info on coil (primary) inductance.

[Bud]

jcharlestc, I have some inductance numbers for you. A stock 12 volt Delco coil has 7.4 mh primary inductance with 1.5 ohms resistance, Ford yellow top 12 volt 6mh inductance and 1.5ohms resistance, Accel yellow can high performance coil 6.6mh inductance and 1.4 ohm resistance, Echlin IC1700 high performance coil 10.3 mh inductance and 1.4 ohm resistance, stock Prestolite replacement Echlin IC17 7.5mh inductance and 1.66 ohms resistance, Atlas badged Prestolite 6 volt 6.1mh and 1.1 ohms resistance, Echlin IC8 6 volt coil 5.5mh inductance and 1.1 ohm resistance, Standard Motor Parts 12 volt internal resistor coil 16mh and 3 ohms resistance. Last but not least, the MSD Blaster high performance coil 4.5 mh inductance and .64 ohms resistance. The MSD coil should not be used in any ignition system that has points due to the low primary resistance and resulting excessive current draw through the points. The only coils I tested that aren't NOS are the Delco, the Echlin IC8 and the MSD coils. They are used but tested good. I hope those numbers have answered your question about coil inductance. Bud

[jcharlestc]

You have more than answered the question. and your diligence, hard work, and knowledge are deeply appreciated, not only by me, but by *everyone* here--you can be certain of that.

Warmest regards from all of us...