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Ignition coil primary resistance #669009 09/26/16 2:00 pm
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Andy Haldeman Offline OP
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There are specs printed everywhere for the secondary resistance of an ignition coil but through my books and searching online I can't find anything on primary resistance except for an old thread where mr Pete thought they should be somewhere above 5k. Basically everyone just says if it makes a spark it's fine...however my right side coil reads about half what the left does and I wonder if it's contributing to my less than perfect idle/low range running. So for background:

T140e
12v coils, both get 12v from the battery. All wiring is brand new Lucas repro.
Left reads 2.3ohms secondary, 8500 ohms primary
Right reads 2.1ohms secondary, 4100 ohms primary

Bike runs like a champ throughout midrange and top end but the lower rev range is weak and the right carb has to have the idle set markedly higher for that cylinder to "keep up" with the other


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Re: Ignition coil primary resistance [Re: Andy Haldeman] #669011 09/26/16 2:11 pm
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L.A.B. Online Content
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Originally Posted by Andy Haldeman
There are specs printed everywhere for the secondary resistance of an ignition coil but through my books and searching online I can't find anything on primary resistance except for an old thread where mr Pete thought they should be somewhere above 5k. Basically everyone just says if it makes a spark it's fine...however my right side coil reads about half what the left does and I wonder if it's contributing to my less than perfect idle/low range running. So for background:

T140e
12v coils, both get 12v from the battery. All wiring is brand new Lucas repro.
Left reads 2.3ohms secondary, 8500 ohms primary
Right reads 2.1ohms secondary, 4100 ohms primary



You appear to have got your 'primaries' and 'secondaries' mixed up?

Lucas type 6V coils would normally be around 2.0 Ohms primary, but around 3.5 Ohms for 12V coils.

Are there any markings on the coils (usually stamped on the base if there is)?

Last edited by L.A.B.; 09/26/16 2:23 pm.
Re: Ignition coil primary resistance [Re: Andy Haldeman] #669017 09/26/16 2:59 pm
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gavin eisler Offline
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Andy, if it runs OK at the higher rev range sounds like the coils are good, usually sparks will die with revs if something is not good with the coil..
Measuring resistances in high number turns of windings is something I used to to a lot of , I worked with HV alternators and Transformers, in effect you are measuring the inductance and the resistance here which gets tricky, the most useful measurement is HT hot end to coil casing, this is the resistance check for the coils insulation, these should be similar but, moisture ingress to the coil can change this easily, as long as its in the thousands/ K ohms range there is nothing wrong.
A more robust test is the electrical breakdown strength of the insulating medium, which is oil in the stock coils, they are impossible to sample being sealed , so thats out. Next best thing is a decent k ohm reading for HT to coil outer frame.

if you are checking resistance from one LT terminal to the HT end at the lead socket and you get a very different reading this suggests that one of the coils is shorting between turns , coils are pretty cheap, substitution would be the simplest way to go.

A superior resistance check would be performed with a 5 KV megger,again HT plug to coil chassis earth this puts the HT side under real electrical stress and gives a better picture, still reads in Ohms . But is tailored for HV conditions, these are not generally available to the public, due to the Dangerous voltage involved, strict earthing requirements must be enforced if using a megger, test gear and item MUST be earthed after taking the readings or a nasty belt is the result. For HV equipment the resistance is read after 1 minute ( R1) of stress, then again after 10 minutes R2. R2/ R1 gives the "Polarity Index" , typically 1.5 - 2, bigger numbers are not good. Typically healthy gear shows the resistance climb fairly quickly for the 1st minute then slowly rise after that, sudden dips indicate failing insulation. This is an insulation check since most HV gear is robust enough not to have inter turn faults, bike stuff is different and much more delicate.

it doesnt take much to change these readings, any carbon on the outer plastic surface will allow surface leakage as soon as KVs are involves , HV gear needs to be cleaned scrupulously and fairly regularly to stay in optimum condition, a cotton bud with some meths can be used to clean around the coil top to minimise this path.
i no longer have a megger , but I have a VOM, I will measure a few coils and post results for a comparison.

Last edited by gavin eisler; 09/26/16 3:22 pm.

71 Devimead A65 750
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Cagiva Raptor 650
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Re: Ignition coil primary resistance [Re: Andy Haldeman] #669025 09/26/16 3:45 pm
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gavin eisler Offline
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Old 6 volt coils Lucas OIF style. Using cheap VOM on 20K range for HT and 200 range for LT readings.
Across primary LT conns 2.1 ohms, across LT to HT, 4.9 Kohms, both coils similar withing less than 5%

New last year coils, same spec . Both coils similar within less than 5 %.
Across primary LT conns 2.3 Ohms, across LT to HT , 8.8 Kohms
All coils HT to chassis was infinity, correct.

The bike ran fine on the old coils I changed them as a precaution. Interesting to note the New coils LT to HT readings are double the old coil. Presumably this is the oil breaking down over the years, what I would expect.
Another factor, the old coils are stored in a damp shed.
The new coils are warm and dry.

Although your coils are 12 volt, so have higher primary resistances the disparities in the readings HT- LT do not seem out of line suggesting one coil is maybe damper or less fresh than the other, as a precaution replace the low reading coil would be my recommendation, this may not cure your fault, but it wont hurt it..

Cheers
G


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: Ignition coil primary resistance [Re: Andy Haldeman] #669034 09/26/16 4:23 pm
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Try swopping the coils over and see if the "weak" cylinder changes to the other side.
If it does then it points (no pun intended) pretty conclusively to the coils.
Also has the advantage of costing nothing.
Gavin--you are a scots guy--thought you would be onto that one! (Brit humor).

Re: Ignition coil primary resistance [Re: Andy Haldeman] #669037 09/26/16 4:38 pm
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Andy Haldeman Offline OP
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You're correct LAB I had my terms backward. I have not checked the stamping on the bottom of my coils but remember seeing in the past that they are genuine Lucas. They're 12v because they're run in parallel, so most likely they are 60-2203 or Lucas 17m12. I guess the first thing I should be concerned about then is the low primary reading in the 2 range. I will do some more investigating when I get home tonight, but as for now I'm planning on just taking the dive for a new set. Any comments on the quality of the Taiwanese replicas that seem to be everywhere?

Re: Ignition coil primary resistance [Re: Andy Haldeman] #669049 09/26/16 6:07 pm
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gavin eisler Offline
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My new coils are Lucas Wassel, maybe from the far East , cheap, seem to work OK. IMO coils are service items, particularly on old brits, I am on my 3rd set, Change before failure.
1st set one gave up, would run when cold , then die as it heated up. Next set were red 12 V"special ones " 2nd hand/free, the Boyer didnt like them ,swapped for 2nd hand 6 volters mid 90s, much better, renewed them last year . Coils are best fresh, , pretty cheap insurance in the great scheme of things. Vibration and damp get them in the end, its very damp where I live.

I still think bigger coils are better, i would fit something else if there was space. The best longest lasting HV stuff at the power station was always built like a brick outhouse, even so, the business does not expect more than 25 years out of an alternator, and transformers hardly ever get past 40,anything operating in damp shaky conditions is always more prone to failure.
Ive see a few burst solid insulation double outlet coils otherwise I would reccomend them, probably best get something with Bosch written on it.
Good tip about swapping them over, couldnt see the wid fir the trees.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: Ignition coil primary resistance [Re: Andy Haldeman] #669053 09/26/16 6:35 pm
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quinten Offline
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Some primary resistance numbers
for Lucas coils
from a Lucas work shop manual

LA6 ___ 1.0 - 1.1
LA12 ___ 3.0 - 3.4
MA6 ___ 1.8 - 2.1
MA12 ___ 3.0 - 3.4
17M6 ___ 1.7 - 1.9
17M12 ___ 3.3 - 3.8

.

Last edited by quinten; 09/26/16 7:45 pm.
Re: Ignition coil primary resistance [Re: Andy Haldeman] #669059 09/26/16 6:58 pm
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Andy Haldeman Offline OP
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Good info! I think I'll ditch the digital MM and dust off the old Simpson 260 for another test tonight to confirm my findings.

Re: Ignition coil primary resistance [Re: Andy Haldeman] #669072 09/26/16 10:26 pm
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http://www.tioc.org/boyercoildata.htm

I know it says Boyer coil data, but it has data for Lucas 12V too.....

Steve


'77 T140J
"Vintage Bike". What's in your garage?

"The paying customer is always right."

Fitting round pegs into square holes since 1961...
Re: Ignition coil primary resistance [Re: Andy Haldeman] #669075 09/26/16 11:23 pm
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Andy Haldeman Offline OP
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Ok Simpson says left coil 3.8ohm right 3.0ohm, with the same secondary findings as before. Confirmed Lucas 17m12. I'm just gonna get a new set

Re: Ignition coil primary resistance [Re: Andy Haldeman] #669083 09/27/16 2:18 am
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Stuart Online Content
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Hi Andy,

Originally Posted by Andy Haldeman
T140e
12v coils, both get 12v from the battery.
Confirmed Lucas 17m12.

Just to be clear, this is a '78 T140E, with points ignition, not a '79-on T140E with electronic?

Originally Posted by Andy Haldeman
Left reads 2.3ohms secondary, 8500 ohms primary
Right reads 2.1ohms secondary, 4100 ohms primary
Simpson says left coil 3.8ohm right 3.0ohm,t

The second set of readings suggest the first meter needs a new battery ...

Just as a matter of interest, when you tested the secondaries - between one LT terminal and the HT terminal - did you test separately between the HT terminal and each LT terminal? Reason I ask is a single-output coil has one end of the secondary windings attached to one LT terminal; that could be the reason for the discrepancy between the secondary readings of the two coils?

Originally Posted by Andy Haldeman
I'm just gonna get a new set

Be careful what make you buy; current suggestion is either PVL or Tri-Spark are the best and least-likely to give you trouble in the short- to medium-term.

Be aware that PVL '12V' coils might have higher primary readings - a little higher than 4 Ohms - and secondary readings exceeding 7 KOhms.

Hth.

Regards,

Re: Ignition coil primary resistance [Re: Andy Haldeman] #669211 09/28/16 1:14 am
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Andy Haldeman Offline OP
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Indeed that meter did have a failing battery. I realized that also once I retested with the trusty Simpson.

Well I bit the bullet and ordered a pair of 6v pvl coils and a Pazon sure fire unit. I've been happy enough to keep after points but must admit that it's always in the back of my mind whether they're to blame for this and that. The benefits really seem to outweigh the negatives. Was thinking about a tri spark but they are pricey and I should have plenty of room to mount the Pazon black box under the tank, and I once again leaned on the good old exchange rate and ordered my parts from Rex's in the uk which I've done business with before and was very pleased. Even after "P&P" as you call it I saved quite a bit over ordering the items from a stateside distributor. Especially the coils, they go for big bucks over here and the pair from the uk cost about the same as what a single would have over here.


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