Measuring ignition coil primary resistance

I get asked this a lot, so here is a quick video on how to do it!

[@frosthoe]

Thank you for explaining the ohms to output relationship, and why you need /or not, a ballast. Im not a mini guy but this covers all similar coils so its great. Ty again! I'll keep this knowledge filed away forever, very useful info!

[@Ozcrazy49]

Simple & concise tutorial. Thanks heaps.

[@markgutter8443]

I learnt something good from all of your videos. Thanks for taking the time to share.

[@jamesws3922]

Have learnt so much from these videos! Thanks for sharing!!

[@ACDodd]

Excellent please share!

[@georgehrycyk971]

It's a good video especially working on a Ford flathead in a 31 roadster Hot rod

[@ericgeber6145]

Makes it totally CLEAR as always! Changed my coil for 0.8 with high energy powerspark dizzy and it IGNITES ..never thought its that good😮

[@onepairofhands]

your videos are really helpful - thanks for posting

[@ACDodd]

No problem I try to aim them at problems I see. Please share!

[@GPZ_Biker]

Loving your videos. Really educational. Thanks

[@dirtyspanners]

great piece of information on the coils their cheers

[@tapp1t]

Thanks, great vid, thorough and concise.

[@MiniMattTV]

Excellent information. Just for clarity, if you are fitting an electronic points replacement kit, would you stick with the same coil as for actual points? Most adverts say no other modification necessary. Do these kits change your plug gap recommendations also? Cheers

[@ACDodd]

Stick with the same setup. However if you were using a 3 ohm coil you would be better off converting to a 1.5 ohm at the same time and including a ballast resistor. Better still fit a kit that can use a 0.8ohm coil and the you can fire 0.035” plug gaps

[@ACDodd]

Coil type determines plug gap. Aftermarket electronic ignitions have different requirement depending the age and who makes them.

[@grahamswain7356]

This is so interesting , something I had know idea about ,but some time ago I had a rancho martra, and it came home .The following morning it was dead so I suspected the condenser and with out much success so I attached a nother one one the outside and iwas back in business ,in instantly I had now multimeter at the time .

[@johnnymees5927]

Great video , I'm looking for a small coil to replace The standard 12 volt 'Lucas' type . Must be 3.5 or more ohm primary resistance ............. Any ideas ?

[@ACDodd]

Why more than 3.5ohms?

[@plumini1275]

Best Information ever about Coils and Minis, write 101% to your logbook, extremely well explained, thanks you very much. Quick question, what will be the best coil if I use a "12G4180MS DISTRIBUTOR PRE A PLUS ELECTRONIC 1275cc" ? Do I need a 0.9 ohms Coil ?

[@ACDodd]

Contact minispares for this info.

[@stevesmith2214]

Wish everyone would watch this

[@ACDodd]

Share it Steve!

[@nicholaskennedy1748]

Hi AC Dodd, I've been thoroughly enjoying watching your videos lately. I needed some clarification from yourself regarding coils - I have fitted the Powerspark 65DM4 distributor, with High energy coil (0.7/0.8 ohms, part number replaces DLB 198) and silicone leads. I have seen on the powerspark website and the video with Twin-cam that it should be the correct rating coil. However, on one of the mini forum threads, you have advised somebody to use the 1.5ohm coil instead, as using coils with lower resistance causes premature module failure and reduced part throttle torque. Would you be able to advise which information is correct? Could you have just meant if the plug gaps were increased, putting wear on the coil? I am by no means disputing your excellent work and advice, but I just want to know what I have fitted is correct before I start the engine Regards, Nick

[@ACDodd]

Use a 0.8ohm coil and run 0.035” Plug gaps. That previous info was when I was having a few issues with modules. Now this issue has gone use the as above.

[@markleatherland3792]

Hi AC - useful info again Thanks. You showed a coil damaged by running too big a spark plug gap for a ballast system. Is there any issue or consequences of running too small a gap with an electronic system i.e. 65DM and 0.9 coil I need to check mine as I'm not sure I realised or read the right info when I fitted my kit car with 65DM/0.9coil from an MG Metro- cheers, Mark (Note - I've also just changed my air filter after watching one of your on the road sessions where you said the filter fitted was no good which was the same as the one I had)

[@ACDodd]

Too smaller gap does not do any damage as far as I have experienced however you can light off leaner fuel mix using the larger gap

[@kkaplin23]

hello and thanks for the video. i have a coil with primary resistance that tests within specs but a secondary resistance that tests a bit high. (secondary range is supposed to be 2.4k-3.5k, coil is testing at 4.37k). is there any problem with running this coil? what impact might it have?

[@ACDodd]

No I think the ones I use are much higher than that.

[@paulstevenson7273]

Very good advise brings me back in time when I was a home tune man amazing the amount of times incorrect coils been fitted and ballast resisters being missing and cold start feeds not there when cranking ,can I ask why you don't scope anything to look at ignition rotor air gap or ht kvs interesting to see you only use lambda don't you use a four gas

[@ACDodd]

I used a 4 gas for years, to slow to respond and I can sense when the engine is not running correctly. I have also used scopes but after tuning a few thousand engines I can tell a bent dizzy shaft or a poor coil and have ground scopes to be of little use now.

[@michalchlup6758]

Great information. Maybe it would be usefull to say what spart plug gap you recommend for each application as well? 0.025 for point distributor and 0.035 for electronic one? (as most sources say, or do you have other opinion based on your experience?)

[@ACDodd]

You have it correct. 3 ohm and 1.5ohm ballasted types 0.025”. 0.8 is ohm with the 65d 0.035”

[@ACDodd]

3 and 1.5 ohm coils 0.025”, 0.8ohm 0.035”. Standard new plugs come at 0.032” so the gap is too wide for engines that are not using the 0.8ohm coil and needs to be reset at 0.025”. All new plugs need to be correctly gapped before fitting.

[@ianroberts8692]

So for my electronic ignition I should have a 0.8 ohm coil ?

[@ACDodd]

@@ianroberts8692 what electronic ignition have you got?

[@ianroberts8692]

@@ACDodd replaces the points in the dizzy

[@bernardwarr4187]

Just to add to your excellent information. MSD in the US do a coil with primary resistance of .7 OMS and a ballast resistor of .8 OMS as a high performance kit, for points cars. What’s your thoughts on that?

[@ACDodd]

Yes that’s ideal for engines with more cylinders.

[@Schlipperschlopper]

Ballast coils take up to 80 Watts from your battery not good in the case of a tired alternator or generator fail, so on old cars with low power DC generators only use 3 Ohm or higher ohm coils because you might empty your battery when also using wipers and lights, also the hotter 1,5 Ohm ballast coils overstress old ignitions switches and locks and are heavier on breaker contacts, I would say they are for motorsport or more modern cars with relais assited starter switch and 55a alternator.

[@ACDodd]

Actually ballast coils are only 30 to 40 watts not 80. If you are not interested in performance or emissions then yes you can stay with older lower powered ignitions. However, if you need performance and need to pass emission standard there is no excuse for using older systems.

[@keithgarland3404]

Good, helpful information.

[@ACDodd]

Please share!

[@herbnalis3723]

All new cheapy multi meters are shipped with the 9 volt battery not fully connected. The plug is pushed on 1 terminal & just touching on the 2 terminal. That's why it wont zero when probes connected. It's a good idea to fit a new 9 volt battery as well.

[@ACDodd]

My £450 bluepoint meter is no different

[@Texasmini]

Thanks for the lesson. I have a 65D, is there a problem to use a 1.5 instead of the .9 you suggest? Much easier to source.

[@ACDodd]

You can use a 1.5 ohm coil, but you need to reduce the spark plug gaps to 0.025”. The point of the 0.8ohm is more spark energy, that means you can run 0.035” plug gaps.

[@davidanastasi]

Excellent videos clear explanation. Have you ever set up a 1400cc 74mm race engine , forged Omega pistons 12.5 :1 cr, all steel bott end, SW23 cam. 1.5 rockers Twin HS6 1 3/4 carbs. 23d4 electronic dist. What max advance should I be seeing at 6000rpm.? What needles would you recommend? Would appreciate an approx. Idea . David Malta

[@ACDodd]

Race engines need to be dialled in under load, Anything else is guesswork.The maximum advance is determined by using a rolling road. I can’t answer that without measuring it!

[@Conor.Twomey]

Hi AC would you recommend converting a CSI Ignition and 3 Ohm coil to a 1.6 Ohm coil and ballast? Would you also keep the 0.025” gaps on the spark plugs? Thanks in advance

[@ACDodd]

Yes and yes.

[@Conor.Twomey]

@@ACDodd thank you.

[@davidpackphoto]

What coil would you use for a ballasted harness 998 that has been converted to electronic ignition 1275?

[@ACDodd]

What electronic ignition type?

[@davelinkson1]

My 1972 MGB (non ballasted)is having some problems with coughing and spluttering and cutting out when hot in stop start traffic, am using CSI electronic dissy with Lucas sport coil (gold) oil filled, am thinking of replacing with a dry coil to your instruction in the video. Thanks.

[@mickellis9624]

What ohms setting did you change your multimeter to for the secondary test please

[@ACDodd]

Mine is autoranging

[@SouthernIdaho]

Sharp guy great info, thank you

[@ACDodd]

Thankyou

[@ianwilliams4787]

I bought the so called Lucas coil recently to run with points ignition. Worked well enough for a few weeks. Recently the acceleration was struggling. When I opened the bonnet to look at the distributor i touched my arm on the coil body and it was red hot !!

[@ACDodd]

Check the primary resistance

[@JimboJangle]

Great stuff.Thanks.Appreciated 👍🇬🇧

[@MitzvosGolem1]

Coils when hot often change dramatically in Ohms.

[@alhamedi66]

I subscribed channel deserve it. Clear clean organized professional work with passion. What we do in the past all wrong because when coil faulty they replaced any other coil without checking all this information. How I know the right coil for my classic car. Best regards,

[@bernardwarr4187]

Where can we find your other video on coils and ballast resistors?

[@ACDodd]

Just look in my list of videos; kzbin.info/www/bejne/pHmygq2fYq-YZ6ssi=_oPGwsb66HlmsBeG kzbin.info/www/bejne/aYitpnptm9etns0si=TWvv0im33xcHzkDM

[@paulhotson5820]

AC, What is the difference between a 1.5ohm and 0.8ohm coil when using a 65dm4 non ballast? Is it just that you can increase spark gap to 35thou with a 0.8ohm? I have a 65dm4 no ballast with a 1.5 40,000 output coil and gapped plugs at 32thou, should I reduce this to 25? Any advice is appreciated

[@ACDodd]

Some coils will run fine with the larger gap, try both and see what runs best. However I have noticed that some coils breakdown when firing wider plug gaps, and hence using the correct rated coil for the job

[@paulhotson5820]

@@ACDodd Ok thanks for the advice,I will reduce gap at 25 just to be on the safe side .

[@fozzee08]

awesome learning here Thx

[@jamespro1957]

Could you just unhook the negative battery terminal when checking the coil ?

[@ACDodd]

Yes

[@ronschlorff7089]

Good information I have starting problems with my 200 six in 64 Ranchero, runs well but needs help staring, could it be a mis match between the HEI distributor type ignition and the coil, a standard, not a "flame thrower" type. I did measure the resistance and it checks out to spec in the range, about 1.5 ohms. I will check and replace new plugs and gasp them per you recommendation soon's I get them, I'm looking at a rebuild of 1bbl Holly crab too, plus and new fuel pump. Thanks for the vid, very valuable if you want to get down the road in your old cars! :D

[@ACDodd]

Check to see if it has a ballast resistor still fitted, it might be that the connection to short this out during cranking is not working this reducing the ignition power while cranking.

[@ronschlorff7089]

@@ACDodd thanks for the tip, I will check that.

[@mydreamarewheels]

for a mini cooper rsp 1275cc 1990, witch coil its better ? you can tell me please?

[@ACDodd]

The standard coil is perfect it’s a 0.8ohm unit.

[@mydreamarewheels]

Briefly, can you tell me the symptoms of this part being damaged?

[@ACDodd]

@@mydreamarewheelsa damaged coil usually causes misfires or not running at all. Can also change with heat.

[@claymunsamy]

Curious to know the secondary reading for the lucas coil

[@chrisbeard5884]

Excellent. I've been looking into changing from 24v coil on Willy's Hotchkiss jeep. Running a voltage dropper to 12v then 12v coil still on points and condenser. Now im not sure which coil ohm's i should use. I was looking at Remax oil filled. Interesting but confused.

[@ACDodd]

For a willys engine a stock 3 ohm 12 volt coil is fine it’s does not rev hard enough to warrant a high power coil, I also doubt you could calibrate the carburettor suitably to take advantage of a high power coil setup.

[@chrisbeard5884]

@@ACDodd thanks nice to know that there are still some Good old school gents around.

[@neilphilip2320]

very informative!

[@Ozzy3333333]

Question: I will use round numbers to make it easy, If the primary is driven with 10V, the the secondary is outputting 10K volts, whats is the ignition turns ratio?

[@ACDodd]

1:1000

[@Ozzy3333333]

@@ACDodd Sorry, wrong. 90:1 in the ones I have checked. A ignition coil does not work like a regular transformer. It charges the primary winding, then opens the circuit and this happens. 1. The secondary is open circuit (10's of megaohms) as the spark plug has a air gap (and in the cylinder the air is compressed, making it more resistance than atmospheric pressure). 2. with a very high impedance load on the secondary, the voltage sky rockets upwards (its called CEMF or BEMF). high enough to jump the air gap of the plug. If you run the primary with say 1Vac sine wave and look at the secondary you will see ~90Vac for a 90:1 ratio. I am sure they are not ALL 90:1, but somewhere around there.

[@ACDodd]

@@Ozzy3333333everyday is a school day!

[@charlesmarsh9608]

Thank you.

[@matthewrichardwitts8651]

Mr.Dodd I have an accuspark conversion with no ballast resistor what coil should I ohm coil should I use

[@ACDodd]

Contact accuspark direct.

[@anomalous9448]

Does the coil have to be checked disconnected from the car?

[@ACDodd]

It needs to be disconnected from the electrical circuit to be able to check it. Only one side of the coil needs to be disconnected to make the measurement.

[@billpayne3983]

One day someone will post a video on how to test the amount of VOLTAGE going through a coil. It seems to be one of those blindingly obvious things that no one has yet thought of.

[@ACDodd]

Maybe you should check out other videos of mine, for just that; kzbin.info/www/bejne/gHy2qnWtp7t8qNUsi=_msKx3Au46oJyPB9

[@AdrianShawDrummer]

Well actually voltage doesn’t go through a coil, current does. A useful analogy to understand basic direct current electricity is a hose pipe. Voltage is the water pressure, current is the rate of flow of water through the pipe and resistance is the stricture of the hose resisting the flow of water. It’s simplistic but works well in understanding the basics.

[@StefanMiniCooper]

Thanks for the video AC! I have in my aplication an 123 ignition with a Bosch blue coil (3.3 ohm). Is this setup ok?

[@ACDodd]

It’s lacking energy at higher rpms

[@StefanMiniCooper]

​@@ACDoddlate answer from me. I see you use the 1.5 ohm balasted Viper coils at the 65D electronic ignitions. Will this be the one that I need for my 123 ignition and will be there a improvement? Thnx

[@ACDodd]

@@StefanMiniCooper no I have now reverted to the 0.8ohm viper I had a couple of coil failures with the 1.5ohm units running the bigger plug gaps.

[@ACDodd]

@@StefanMiniCooper whichever dizzy you fit you want to be using the highest coil power you can.

[@StefanMiniCooper]

@@ACDodd ok, thanks for your reply! 123 says that I need a coil with a minimum from 1.0 ohm, so 0.8 ohm can damage it I'm afraid. So maybe better to look for another 1.5 ohm coil?

[@Darth_Chicken]

Can you check coils with them still wired in the car?

[@ACDodd]

Simply disconnect one side of the coil To make the measurment.

[@johndokic987]

Doesn't the ballast coil , has it embossed on the bottom of the coil??

[@ACDodd]

No

[@chuckmaddison2924]

Something that can catch folks. If you look around under the hood of some cars, you won't see the usual resistance block. It's built into the harness as a wire.

[@notwocdivad]

So would you say a 4.5 ohm coil would not be suitable for a NON ballast points ign system? This is actually for my RE Bullet motorcycle butI have heard so many "experts" saying "to high" or "to low" and I know nothing about electrics so I must turn toa real expert for proper advice! Thanks.

[@ACDodd]

How many cylinders?

[@notwocdivad]

@@ACDodd Single cylinder 4 stroke points ignition no ballast as far as I can tell! Cheers

[@ACDodd]

@@notwocdivad single cylinder that should be fine. However if you want to increase the plug gap, then fitting the appropriate lower resistance coil is the way forward. The lower the coil resistance the more current the points will have to contend with and the shorter the life of the contacts. You will also need to adjust them more often.

[@notwocdivad]

@@ACDodd Thank's AC, You have been a great help. The Standard plug gap is .70mm which will do me on a low power engine like the Bullet! I'll replace the HT lead and see if that gets it sparking. Thanks again.

[@DC-dp3hk]

I can see what you meant when you said the reading was no resistance on the failed coil as no reading came up on the meter but it should have been stated as open circuit or out of range for the meter.

[@notwocdivad]

One more stupid question before I leave you in peace, Does a coil outer case have to be earthed ?? So many different opinions out there! Cheers.

[@ACDodd]

No, that’s asking for trouble.

[@notwocdivad]

@@ACDodd Thank you, Your help deserves my sub

[@gerardlacey9384]

Many thanks. How hot should a coil get?, my a35 coil gets very hot to touch and car breaks down until it cools again. I would appreciate your advice.. Many thanks again. G.

[@ACDodd]

What’s your coil resistance?

[@gerardlacey9384]

​@@ACDoddmany thanks for your help, it's a fairly new coil bought from Bull motif, primary 2.8 ohms, secondary 8.39. I also have the old coil which I took off, it's readings are, primary 3.3,secondary 4.31. Thank you again. G.

[@ACDodd]

@@gerardlacey9384 what is the dwell angle reading with the engine running at idle and at 4000rpm?

[@gerardlacey9384]

​@@ACDoddthe dwell is 62 degrees at both idle and 4000 rpm.

[@ACDodd]

@@gerardlacey9384 reduc the dwell to 56, this will take the edge off the running temp of the coil.

[@tonywood6332]

Which would be for a 1980 998cc mini van with electronic ignition AC?

[@ACDodd]

What type of electronic ignition is fitted?

[@tonywood6332]

Accuspark I believe

[@ACDodd]

@@tonywood6332 part number?

[@tonywood6332]

45D

[@ACDodd]

@@tonywood6332 no the accuspark part number

[@troypowell1671]

i have a 93 cooper converted back to a hif44 carb with a pertronix flame-thrower distributor 59D A+ w/vacuum advance and my coil is 0.8 flamethrower II coil. just wanted to know if that is right. the car was rewired with a YMC10654 new harness for WIRING MAIN LOOM FROM 1989 SERVO BRAKES from minispares my coil has from the loom the 2 white/black to neg and a yellow and a pink/white to pos side. when plugs fire the spark looks week, any idea why. i would have to come tune mine but i am in the US. oh plugs are gapped at 35 and are NKG BPR6ES

[@ACDodd]

You need to see if the dizzy is a variable dwell unit, by measuring with a dwell meter. You also have a ballast feed to the coil which is not needed with a variable dwell dizzy

[@troypowell1671]

@@ACDodd I'll see if I can find a dwell meter. And let you know

[@troypowell1671]

@@ACDodd yes it vas the pertronix ignitor II with variable dwell

[@ACDodd]

@@troypowell1671 ok next contact Petronix and see if the module is rated to run with a 0.8 ohm coil with no ballast resistor.

[@troypowell1671]

@@ACDodd yes he said i can get rid of the resistor wire he said the modules are rated at a certain volts and no matter how many you try to put through it it will only put out what it is rated at but you restrict it and you end up with weak spark

[@ACDodd]

Check out this video if you want to Find out why 1.5ohm ballast coils perform better than 3 ohm units. kzbin.info/www/bejne/gHy2qnWtp7t8qNU

[@martinvestergaard6707]

hi. what is the part number for that viper 0.8 ohm coil?

[@ACDodd]

VCE

[@spoofrider]

If I am using a fiesta electronic distributor what is the best coil.

[@ACDodd]

What does the manual say should be used with it?

[@erdemir5641]

You didn't say why you need a ballast resistor for that resistance

[@ACDodd]

It will overheat the points if you run a 1.5ohm coil with no ballast.

[@giuseppe896]

Or you can look at the side of the coil and it will say if it's a 6 VDC coil or a 12 VDC coil

[@ACDodd]

No, that’s not possible in a lot of cases, the insulation on a 6 volt coil will inhibit its use in 12 volt applications. Most coils in the uk are not voltage marked

[@WilsonPendarvis-tn3wm]

It is assumed that we know what normal resistance should be. Which is futile anyway

[@ACDodd]

Why would you not know when it is in your workshop manual. As someone who tunes these cars it is not futile, in my experience.

[@subatomicparticle6535]

When starting a car with a 12 volt battery you would never get the full 12 volts directly to the coil by removing the ballast resistor from the circuit because the heavy load the starter puts on the battery drops the voltage from 12.? to somewhere in the range of 9.? volts.

[@ACDodd]

You are correct, that is the benefit of a ballast system. However in a high power 0.8ohm system, you get the same effect. But better still you have twice the energy at higher rpm.

[@mikeadler434]

👍👍

[@erdemir5641]

Why don't you say what the resistance should be

[@ACDodd]

In what respect? There are 3 applications, 3 types are needed for these applications. If you want to know what’s best, then that’s easy, the 0.8 ohm coil wins evertime. However if you have an original 1960’s vehicle this may it be the right one for you.

[@alhodge4051]

brevity

[@AllyhajiissaIsss]

Oh me am take know this subject

[@petenikolic5244]

Strikes me this is a viper commercial sorry but that is how it comes over

[@ACDodd]

I don’t work for Viper or Powerspark, I just use there components.

[@gratiamerancio9589]

𝐩яⓞ𝓂𝓞Ş𝐦 🙌

[@ptibbitts-nu9jr]

Interesting, but a bit er, and um, & erm,y - could do with editing those pauses out please, becomes annoying to keep hearing them, eh?

[@ACDodd]

Thanks for the feedback, being dyslexic and short in time means that that’s just about the best I can do.

[@bernardwarr4187]

I have know idea what he is talking about, it’s a great video, presentation and content!

[@peterpace2463]

Why bother?