# question about ignition coil resistance measurements



## cityboy (Sep 13, 2011)

My 357xp doesn't want to start and in the process of troubleshooting it I measured the coil resistance. The results sound strange to me, but before I throw away $60 on a new coil + shipping I'd like to be fairly sure the coil I have is worth replacing.

The Husky service manual claims the primary resistance should be 0.2 ohms, mine's reading 135. The secondary resistance isn't spec'd in the Husky manual, but is 1600 ohms on my coil. I think a typical winding ratio on a good coil is in the thousands isn't it?? My ratio is about 12, which seems like the coil is shot. However, when I talked to a guy at Bailey's today he seemed to think he's measured around 120 ohms on the primary before on perfectly good coils, so he wasn't sure it was really bad.

So can anyone give me any advice here as to what the proper coil resistance should be.

According to Bailey's they use the same coil on everything from the Husqvarna: 334, 335, 336, 338, 339, 340, 345, 346, 350, 351, 353, 357, 359, 362, 365, 371, 372, 385, 390, 570, 575

If anybody has one of these Huskies already apart, or if you feel like removing the starter cover and measuring it, I'd appreciate it. 

thanks everyone.


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## scottr (Sep 13, 2011)

I have a known good Husqvarna ignition module that reads 200 ohms primary resistance and 2000 ohms secondary resistance to the ground connection. I don't know what saw it came out of. The turns ratio is lower due to the higher capacitive discharge voltage in the primary circuit compared to the old points/condenser/magneto system.


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## Roanoker494 (Sep 13, 2011)

Is your coil blue or black? Not sure it will make a differance but I have one of each here that I can get a reading on in the morning, I am positive that both are working coils. Could someone refresh my memory on how to get these readings? It has been quite a while since I had a coil related issue and needed to measure one. Why do you suspect a coil issue?


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## scottr (Sep 13, 2011)

Roanoker494 said:


> Is your coil blue or black? Not sure it will make a differance but I have one of each here that I can get a reading on in the morning, I am positive that both are working coils. Could someone refresh my memory on how to get these readings? It has been quite a while since I had a coil related issue and needed to measure one. Why do you suspect a coil issue?


 
First set meter to ohms X1K or 1000 then zero your meter by connecting the test leads together and adjust if necessary. Next check your ignition module with the killswitch wire disconnected. Check the primary to ground then check the secondary to ground.


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## cityboy (Sep 13, 2011)

scottr said:


> First set meter to ohms X1K or 1000 then zero your meter by connecting the test leads together and adjust if necessary. Next check your ignition module with the killswitch wire disconnected. Check the primary to ground then check the secondary to ground.


 
Just to clarify a bit, for secondary to ground you can attach one lead to the spark plug boot electrode, and the other to the cylinder (ground). For primary there's a little tab on the coil where the wire lug attaches, attach one lead to that and the other to ground. The frame of the coil is ground, as is anything else attached to the engine case (like the cylinder).

thanks scott for the info, I think you saved me from replacing the wrong part.

the reason I suspected the coil was it won't start, not even a sputter.


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## Somesawguy (Sep 13, 2011)

Are you sure its not 0.2k ohms? That would be 200 ohms. 
Any time you are changing scale on an analog meter, you will need to re-zero the meter. 

Unless your coil is arcing internally, those values don't sound too far fetched. Are you sure the capacitor is good, and that the stop switch isn't stuck in the "off" position? Sometimes the contacts will stick together if they've been sitting long enough. 

Disconnect the capacitor and the switch and check the separately. 

cap: on 10k ohms setting check the resistance, It should briefly tick up and then settle to a high / infinite resistance reading. It still may be bad at higher voltages, but you might see some leakage.

Switch will usually be open circuit in the ON/Run position and closed in the OFF position. Check for resistance in the run position, you should have infinite resistance.


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## scottr (Sep 13, 2011)

cityboy said:


> Just to clarify a bit, for secondary to ground you can attach one lead to the spark plug boot electrode, and the other to the cylinder (ground). For primary there's a little tab on the coil where the wire lug attaches, attach one lead to that and the other to ground. The frame of the coil is ground, as is anything else attached to the engine case (like the cylinder).
> 
> thanks scott for the info, I think you saved me from replacing the wrong part.
> 
> the reason I suspected the coil was it won't start, not even a sputter.


 
Glad to help. You could have a spark plug that has a carbon path from ground to electrode. Did you try a known good plug?


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## cityboy (Sep 13, 2011)

Somesawguy said:


> Are you sure its not 0.2k ohms? That would be 200 ohms.
> Any time you are changing scale on an analog meter, you will need to re-zero the meter.
> 
> Unless your coil is arcing internally, those values don't sound too far fetched. Are you sure the capacitor is good, and that the stop switch isn't stuck in the "off" position? Sometimes the contacts will stick together if they've been sitting long enough.
> ...



I'M AN IDIOT !!! 

I re-read the manual page I looked at (a little too fast apparently) and realized that while its instructing you to measure the resistance across the ignition coil, the purpose of that test is to check the kill switch, not the coil. The 0.2 ohm spec is for the kill switch, not the coil primary resistance.


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## Somesawguy (Sep 13, 2011)

cityboy said:


> I'M AN IDIOT !!!
> 
> I re-read the manual page I looked at (a little too fast apparently) and realized that while its instructing you to measure the resistance across the ignition coil, the purpose of that test is to check the kill switch, not the coil. The 0.2 ohm spec is for the kill switch, not the coil primary resistance.


 
Ahh, that makes more sense. 0.2 ohms is pretty small especially for any sort of coil. 

Keep us posted. opcorn:


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## Roanoker494 (Sep 14, 2011)

I will check my coils in the morning and post what I find, both are oem and the black one is brand new. To tell everyone the truth I only ever run one test on a coil, if you have no spark with the kill wire disconnected then the coil has to be bad. If you have fire but it still will not start I check the fuel system first and then the flywheel key. If you are getting fire and fuel at the right times, then it is going to be a compression issue.


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## Roanoker494 (Sep 15, 2011)

I finally check my coil this morning but I had a small issue with the meter settings, mainly my digital meter did not have a 1K setting so I used the 2k. Hopefully someone will be able to make since of these numbers.

Plug wire to kill switch tab 1.66
Plug wire to coil mount 1.57
Coil mount to kill switch tab .088

I believe you will need to times my numbers by 1000 which would make my readings
1660
1570
88


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## scottr (Sep 15, 2011)

Roanoker494 said:


> I finally check my coil this morning but I had a small issue with the meter settings, mainly my digital meter did not have a 1K setting so I used the 2k. Hopefully someone will be able to make since of these numbers.
> 
> Plug wire to kill switch tab 1.66
> Plug wire to coil mount 1.57
> ...


 
That's correct. It might help if you gave the saw model or the coil part number.


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## Roanoker494 (Sep 15, 2011)

scottr said:


> That's correct. It might help if you gave the saw model or the coil part number.


 
It is a oem limited coil for the saws listed in the first post. It is marked as 13600rpm, made in 2007 and it came off my 2007 372xp.


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## jerrycmorrow (Sep 15, 2011)

*oly coil*

gonna piggyback on this existing thread and ask one more of my simpleton questions. 

coil is electronic for the olympic 264. 

i measured resistance for both the primary and secondary circuits and got readings of approx 2400 and 160 ohms. i also took the primary wire out of the coil and it shows 0 resistance. the question is: given that neither circuit shows an open why else would the coil not produce spark given that everything else - connection to case, gap to flywheel, etc. is correct.


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## scottr (Sep 15, 2011)

jerrycmorrow said:


> gonna piggyback on this existing thread and ask one more of my simpleton questions.
> 
> coil is electronic for the olympic 264.
> 
> i measured resistance for both the primary and secondary circuits and got readings of approx 2400 and 160 ohms. i also took the primary wire out of the coil and it shows 0 resistance. the question is: given that neither circuit shows an open why else would the coil not produce spark given that everything else - connection to case, gap to flywheel, etc. is correct.


 
I'm not familiar with your saw but most coils are potted. How would you take the primary wire out of the coil ?


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## jerrycmorrow (Sep 16, 2011)

scottr said:


> I'm not familiar with your saw but most coils are potted. How would you take the primary wire out of the coil ?


 
unscrewed it


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## scottr (Sep 16, 2011)

jerrycmorrow said:


> unscrewed it


 
The only wire on a coil that I've ever seen that was threaded into a coil was a plug wire and that is the secondary circuit. If your primary is 160 ohms to ground , the secondary is 2400 ohms to ground, and you know you have a good plug wire, then the next thing to check is proper air gap. Do you have a known good spark plug ?


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## jerrycmorrow (Sep 16, 2011)

scottr said:


> The only wire on a coil that I've ever seen that was threaded into a coil was a plug wire and that is the secondary circuit. If your primary is 160 ohms to ground , the secondary is 2400 ohms to ground, and you know you have a good plug wire, then the next thing to check is proper air gap. Do you have a known good spark plug ?



yeah you're right on the plug wire, duh! i had just set the air gap with a feeler gage using the proper gap. also, the spark plug was good. after that coil wouldn't fire i switched it around with a known good one which did fire.


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## scottr (Sep 16, 2011)

jerrycmorrow said:


> yeah you're right on the plug wire, duh! i had just set the air gap with a feeler gage using the proper gap. also, the spark plug was good. after that coil wouldn't fire i switched it around with a known good one which did fire.


 
Are you saying that you switched the coil with a known good coil ?


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## Roanoker494 (Sep 16, 2011)

I would say the first coil is bad then. I can not really comment on the resistance readings because I never really go by them, I had to ask how to get the readings when I tested the coil for this thread. If the air gap is correct and the kill wire is unplugged then there is no other choice but a bad coil, especially since you have a second coil that is working.


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## rover (May 21, 2015)

I HAVE A BOSCH COIL FROM A 064av STIHL SAW THREE MOUNT HOLES DEALER 0004001300 WON"T FIT ANY OTHER COILS FIT IT? HELP


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## cnp (Dec 28, 2020)

Husqvarna's specifications for the ignition module: Primary circuit 0.2 - 3.0 ohms, Secondary circuit 5.0 - 20 K ohms.


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## Old2stroke (Dec 29, 2020)

When you are dealing with an electronic ignition module, due to the semiconductor package that switches the primary, there is really no valid test that you can do with an ohm meter. Only positive troubleshooting is to replace it with a known good one.


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## cnp (Dec 29, 2020)

Old2stroke said:


> When you are dealing with an electronic ignition module, due to the semiconductor package that switches the primary, there is really no valid test that you can do with an ohm meter. Only positive troubleshooting is to replace it with a known good one.


I agree, but the challenge is to obtain a good one. Using those specifications certainly provides you with an indication of problems. Recently I returned two new Ignition modules, Primary : 129 ohms and Secondary 1.55 K ohm, which I bench tested and then installed. I confirmed that it was defective- no spark. It was made by one of the oems SEM . I am now awaiting another unit made by FHP Elmotoren, an oem which I found recently. The vendor tested it and photographed the results of his ohmeter which were promising. It may work. 

Unfortunately most vendors will not provide you with test results of the part number, even if the part is made by an oem.


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## sean donato (Dec 29, 2020)

They typically go by the thought of a new part being good. And most people wont break a multimeter unless they have cause for an issue.


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## cnp (Dec 29, 2020)

If customers ask for such results, and condition their purchase based those results, vendors may change their thinking. Otherwise, purchase and use return shipping labels based upon non-conformance to specifications if that is the case.


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## Old2stroke (Dec 29, 2020)

Ohm meter tests with these modules are so meaningless that it is irresponsible of the manufacturer to give the info and mislead anyone into thinking they are relevant.


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## pioneerguy600 (Dec 29, 2020)

Exactly, even if the readings are within those specs it doesn`t mean the module is any good or will work as in most instances its not the coil part of the module that fails, its the trigger chip embedded in the module that fails most often.


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## Old2stroke (Dec 30, 2020)

In the older points type ignition, it seems like the coils lasted forever, sometimes the capacitor would dry out but the coils kept going. Now with modern electronic ignition modules there is a high failure rate and, as just noted, most (if not all) are due to semiconductor failure in the electronics package that replaces the function of the points. The package switches the current in the primary and as heat is the real enemy of semiconductors, components that handle this current should be mounted on heat sinks but of course have to be packaged with the coil, so they are all potted together in epoxy which makes it difficult to dissipate the heat. The coils are mounted where they get the cooling air from the FW fan but after total lack of cleaning, they get packed in with nice insulating sawdust. A final insult to the coil is when the engine is shut off and the cooling air stops, the heat of the cylinder soaks into the coil and might be what kills it. Most people claim it was running fine when it was shut off and then would never start again. Progress is SO good.


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## KASH (Dec 31, 2020)

Great post that is why I liked the old two piece electronic ignition .The coils very very seldom fail and you can use a module.Maybe I am a dinasoar but I have had chain saws that were used till they were worn out and they still had the original points. I have been running saws for 55 years and have never bought a set of points. I changed alot of sets in outboard motors but that was because we ran a fishing resort and it was faster and pretty well guarantied the motor would run well.I have come across a few bad caps over the years you can usualy tell the cap is bad if the points are burnt heavy on one side. Lots of times the motor will start and idle but will not rev.
Kash
Heat is the biggest killer of electronics in my hop


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