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Ignition coil resistance specifications


JimD

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From a post on Cadillacforums.com:

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

"Ignition coil primary resistance = 0.5 to 0.9 Ohms

Ignition coil secondary resistance = 5K to 10K Ohms"

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

Today, I measured the secondary of one of my coils (did not remove the coil from the engine) and I measured 6.24K Ohms from tower to tower.

No continuity to chassis ground. Obviously within the range someone published on the other board.

It's interesting to me that there is no chassis ground return for a secondary winding center-tap. They have to be using the capacitance of the spark plug gap to form a tank circuit.

If someone has a defective OEM coil taking up space on their bench, I would enjoy having it to play with.

Jim

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  • 4 weeks later...

Jim I just measured what I believe is a bad coil

Between the ignition wire towers 8.26 k ohm

Between the two connectors on the bottom .5 to .6 ohm

I was not sure what was primary and what was secondary

Do these measurements make any sense? Mike

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....Between the ignition wire towers 8.26 k ohm
That is the "secondary winding" of the coil (transformer). The DC resistance value is within the specifications.

Between the two connectors on the bottom .5 to .6 ohm
That is the "primary winding" of the coil (transformer) and is also within the DC resistance value specifiactions.

I was not sure what was primary and what was secondary

Do these measurements make any sense? Mike

Yes they do. The values meet the specifiations; what is important is the"ratio".

Look at it this way. The ratio between 0.5 Ohms (primary winding) and 8,260 Ohms (secondary winding) is approximately 16,520 to 1.

Therefore, the ratio of primary Voltage (12 Volts) to secondary Voltage would be 16,520 to 1 which yields a maximum secondary Voltage of 198,240 (theoretical) Volts at the plugs. But the plug would fire long before the Voltage reached 198 KV.

The ratio of 16,500 to 1 will also reflect the "turns ratio" between the primary and the secondary windings of the coils (transformers).

The coil you have in your hand should operate as intended.

Jim

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That's interesting stuff Jim, thanks. That coil is out of my x's Monte Carlo, she was getting a misfire code but she had an intake manifold leak also, so maybe the coil is ok. Let me ask you a question, why was it that when I attached the meter leads to the primary it read higher at 1.5, then 1.2, then 0.8, then it settled on .5 to .6? I thought that that was odd behavior, is there a reason for that meter behavior with a coil?

I am working on an electronics course. Its a 28 volume home study Electronic Course prepared by the Cleveland Institute of Electronics copywrited 1969 by Wayne Lemons. I searched the internet and he is very well known. A friend of mine had the course and gave it to me, it has never been used and came with the lab equipment. I want to do this course with an oscilliscope and hope I can borrow one.

Thanks, Mike

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That's interesting stuff Jim, thanks. That coil is out of my x's Monte Carlo, she was getting a misfire code but she had an intake manifold leak also, so maybe the coil is ok.
As far as the Ohmeter reading will take you, that coil passes the test. However, an insulation breakdown due to an invisible crack in the tower could allow the coil to arc over to a convenient ground instead of delivering the high voltage to a spark plug.

Let me ask you a question, why was it that when I attached the meter leads to the primary it read higher at 1.5, then 1.2, then 0.8, then it settled on .5 to .6? I thought that that was odd behavior, is there a reason for that meter behavior with a coil?
Not unusual at all. Measuring the DC resistance of a coil with an Ohmeter requires you to apply a small current (from the meter battery) to the coil while the meter "reads" the voltage developed across the coil by that small current. Your meter reading is momentarily influenced by the (normal) transformer action of the coil as it reacts to the small current flowing through the primary winding. Magnetic flux in motion. There are other events taking place, but the detailed theory would have me typing for days.

I am working on an electronics course. Its a 28 volume home study Electronic Course prepared by the Cleveland Institute of Electronics copywrited 1969 by Wayne Lemons. I searched the internet and he is very well known. A friend of mine had the course and gave it to me, it has never been used and came with the lab equipment. I want to do this course with an oscilliscope and hope I can borrow one.

Thanks, Mike

Very good, Mike. Fast forward to the "Meters" section of that material and get a sense of what a meter can tell you and how the measurement is accomplished.

Melding electrical/electronics terminology with automotive terminology is tricky in some cases. And ignition applications is one case. In the automotive world it is a coil. In the electrical world it is a transformer. Same thing, same principle, same behavior; just different terminology. And I sometimes slip into language that crosses over and can lead to confusion.

Jim

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  • 2 weeks later...

Mike D had a mis, and it was gone when he changed his coil. See this thread <a href="http://caddyinfo.ipbhost.com/index.php?sho...c=17247&hl=" target="_blank">http://caddyinfo.ipbhost.com/index.php?sho...c=17247&hl=</a>

I asked Mike to send me the coil so I could check the resistence readings of the primary and secondary windings.

Here are my findings

Secondary (resistence across the plug towers) ZERO, as if the secondary was OPEN

Primary --- 0.6 ohm (normal)

So we have a definitive test to determine a bad coil. Test the Primary and Secondary windings with an ohm meter. If it is OPEN or not the multiple that JimD describes above, replace it. The two coils above (JimD's and mine) are good coils, I would expect that a normal coil would fall within those parameters utilizing a digital ohm meter, if not, swap in a 'known good" coil to see it your symptoms go away.

As soon as it warms up, I am going to measure my own 4 coils and report the readings, if anyone else wants to contribute their resistance readings, I think this would be a worthwhile exercise to establish 'norms', thanks. I also want to determine if its possible for a coil to operate but not perform to 100% and whether its possible for the windings to be slightly shorting out thereby diminishing a coils output. Jim is that possible?

Thanks, Mike D for sending the coil and contributing to our research, Mike

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....I also want to determine if its possible for a coil to operate but not perform to 100%
Most certainly possible.

....and whether its possible for the windings to be slightly shorting out thereby diminishing a coils output. Jim is that possible?
More likely in the secondary winding; it is possible to have faulty varnish cover on one or more layers of wire which would effectively remove 2 or 5 or x% of the secondary turns. You would not be aware of the condition in a street vehicle.

You could also lose turns as above in the primary winding; but there are fewer primary turns to lose so the resulting lower secondary voltage might be obvious. Also, shorting between layers in the primary will increase the current draw and at some point of coil failure, the IGN fuse will be blow.

Jim

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Jim, could the conditions you described above cause a miss? thanks, Mike

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Internal insulation breakdown might or might not cause a miss; it depends on which two layers of wire are talking to each other. You can usually hear internal breakdown with a stethoscope.

Picture two hundred feet of monofilament fishing line wound on a fishing reel. Thats how the wire inside the coil is wound.

Jim

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Internal insulation breakdown might or might not cause a miss; it depends on which two layers of wire are talking to each other. You can usually hear internal breakdown with a stethoscope.

Picture two hundred feet of monofilament fishing line wound on a fishing reel. Thats how the wire inside the coil is wound.

Do you think if a coil was shorted, that our resistence readings would affected or would it be too small to measure?

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

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That would be a tough call.

Let's restate the primary this way; the primary DC resistance should be 0.7 Ohms, plus or minus 0.2 Ohms. That is a range of plus or minus 28.6%!

The secondary DC resistance should be 7.5K Ohms, plus or minus 2.5K Ohms. That is a range of plus or minus 33%!!

About the only way you could say for sure that a suspect coil had shorted turns would be if you knew precisely what the resistance was before the failure and now the resistance measures less than before the failure.

Fortunately, it is much easier to identify an "open" circuit.

Jim

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That would be a tough call.

Let's restate the primary this way; the primary DC resistance should be 0.7 Ohms, plus or minus 0.2 Ohms. That is a range of plus or minus 28.6%!

The secondary DC resistance should be 7.5K Ohms, plus or minus 2.5K Ohms. That is a range of plus or minus 33%!!

About the only way you could say for sure that a suspect coil had shorted turns would be if you knew precisely what the resistance was before the failure and now the resistance measures less than before the failure.

Fortunately, it is much easier to identify an "open" circuit.

Hmm, so testing coils via an ohm meter is not going to be effective it seems. Logan referred to a meter, in Mike D's original thread, that tests Kv output of coils, and that is probably the best method, but for $150, its a tool that probably wouldn't get much use, but when you needed it, it would be a terrific thing to have. Do you agree?

Pre-1995 - DTC codes OBD1  >>

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An Ohmeter can be an effective tool. A coil that measures outside the specified DC resistance range should be replaced.

I try to avoid dedicated tools wherever possible. A 'scope with an inductive HV pickup would be a more versatile tool to have.

Jim

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Would it be correct in assuming that since one went bad then the others are just around the corner? Should I go buy another to keep on hand for the next failure? $25 at autozone sounds good. My dad could have got the delco for $36 plus $8 for shipping but I was impatient since I had to go on a trip. 11 mpg on the highway at 70mph was killing me...Thanks again guys, Mike d

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99 STS 111,000 Miles

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Would it be correct in assuming that since one went bad then the others are just around the corner?

No.

Jim

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Use your cell phone.

CHOOSE ONE !

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  • 2 weeks later...
Here are my findings

Secondary (resistence across the plug towers) ZERO, as if the secondary was OPEN

Primary --- 0.6 ohm (normal)

So we have a definitive test to determine a bad coil. Test the Primary and Secondary windings with an ohm meter. If it is OPEN or not the multiple that JimD describes above, replace it. The two coils above (JimD's and mine) are good coils, I would expect that a normal coil would fall within those parameters utilizing a digital ohm meter, if not, swap in a 'known good" coil to see it your symptoms go away.

Well I bought a multi meter. Turned it to the 200 ohms scale and all my top posts showed an open circuit... What am i doing wrong? Is the scale too low for top post?

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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Well I bought a multi meter. Turned it to the 200 ohms scale and all my top posts showed an open circuit... What am i doing wrong? Is the scale too low for top post?

Mike:

Not knowing what brand/model multimeter you are using makes this about impossible to answer with any certainty. There are just too many folks making/selling multimeters that assume anyone buying the meter would know how to interpret the displayed value. No offense intended.

I would expect the "200 scale" is too low to measure a resistance in the range of 6,000 Ohms. But it depends on how your specific multimeter handles/displays 'out of range' values.

Try higher-than 200 scale selections and see if you find a scale selection that displays a value closer to 6.0, or 6.0K, or 6,000.

Jim

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JimD, I sent you Mike D's old coil and my old ISS speed sensor yesterday, it will be to you in about 5 days, Mike

Pre-1995 - DTC codes OBD1  >>

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JimD, I sent you Mike D's old coil and my old ISS speed sensor yesterday, it will be to you in about 5 days, Mike

Thank you for taking the time/effort/expense to do that. Count on me to share whatever I learn from playing with those parts.

If you sent the package to my 30277 address, it will take a few extra days to find me. And there is a possibilty it will not find me at all. Do not be surprised if the package is returned to your address.

Jim

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JimD, I sent you Mike D's old coil and my old ISS speed sensor yesterday, it will be to you in about 5 days, Mike

Thank you for taking the time/effort/expense to do that. Count on me to share whatever I learn from playing with those parts.

If you sent the package to my 30277 address, it will take a few extra days to find me. And there is a possibilty it will not find me at all. Do not be surprised if the package is returned to your address.

Jim I sent it to your 30265 address, I hope that was right, Thanks, Mike

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Jim I sent it to your 30265 address, I hope that was right, Thanks, Mike

Whoops!

I'm betting I gave you that address prior to Oct., 2007. Your package might find me, but I would not count on it; I expect the USPS will deliver it back to your mail box.

Sorry for the mixup.

Jim

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Try higher-than 200 scale selections and see if you find a scale selection that displays a value closer to 6.0, or 6.0K, or 6,000.

Yup it was the 20K scale that worked....

Here is what I found....

Coil pack #6 & 7 was 5.6 ohms

Coil pack #1 & 4 (my new one) was 5.8 ohms

Coil pack #3 & 8 was ZERO

Coil pack #2 & 5 was 5.6 ohms

Two bad ones in 3 weeks... Hmmm...

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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I got the new coil and swapped it out. The car is still missing and running bad. I'm not sure what's next. I can't get a compression check done here (without paying out the back end). What would a bad cam sensor or crank sensor feel like? No codes with it right now...

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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  • 6 months later...
Jim I just measured what I believe is a bad coil

Between the ignition wire towers 8.26 k ohm

Between the two connectors on the bottom .5 to .6 ohm

I was not sure what was primary and what was secondary

Do these measurements make any sense? Mike

Yes they do; I say that coil should work. The towers are the secondary winding and the blade sockets on the bottom are the primary winding.

On Mike D's coil, I found the secondary was open circuit. That would cause a miss on two cylinders.

And on the primary side, I was showing an open circuit at the blade connectors. I carved away the molded plastic to get to the primary winding wires and was able to measure the expected 0.5 Ohms.

I did find evidence of corrosion within the molded plastic. And that might explain the lack of continuity when measured at the blade connectors.

At least I can confirm the values in Post #1 are correct (or at least close enough).

Jim

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