HUSQVARNA 240 CHAINSAW

[JAMES2602]

I have a husqvarna 240 chainsaw that want start compression is 170 psi and spark is weak , it want jump a 6mm spark gap but it will jump a 4mm i can only get 10,000 kv with a new rcj7y sparkplug ohms reading is 222.5 ohms on primary to ground and 3,115 ohms on secondary to ground when i spray fuel in intake it want start carburetor is delivering fuel to the spark plug it is wet my question is i want to know what the ohm reading is for this coil i can not find it anywhere online.oh i checked flywheel key and gap between coil and flywheel is good .

 

[Tiger Small Engine]

I have rarely been able to find any specific specs on ohm readings on ignition coils, only general guidelines. If you suspect weak spark and can buy a decent aftermarket coil for a reasonable price, and have eliminated everything else, go for it. Sometimes you have to throw a little money at it even when not 100% sure.

 

[bertsmobile1]

All modern small engines use a Hall Effect timing chip to replace the points ( remember points ? )
Originally they were in a stand alone unit so easy to test the coils
Then some clever dick worked out they could embed the chip inside the coil which now becomes a "module " .
As these are now unique to that engine the price wen up about 10 time the price of a plain coil + timing chip
As such the "modules" can not be tested , other than the secondary , TH lead & plug cap with a simple Ω meter
You need around $ 12,000 to $ 45,000 worth of expensive gear to test them electronically
And when you do diagnose the coil or chip is faulty, it can not be repaired so testing is a moot point
It either works or gets replaced

 

[JAMES2602]

Some coils that i have tested over years is because of weak spark that looked to be good but turned out to be bad when tested with ohm meter it did not test in range some other coils i have seen spark but not at rite time again ohm meter found out that it was out of spec on a no spark problem you still need a ohm meter to check the resistor inside the sparkplug boot if it does have a RFI risistor in it not all have them and you do need to test to make sure coil is grounded to block i have seen times where just cleaning the mounting post would make the coil spark again if it still doesn't spark after checking both RFI and the ground and you unhook primary wire and checked the magnets on flywheel and sparkplug then i replace coil that is how i test a no spark problem this all pertains to the coil and not the kill lead and equipment side, sometimes you do need a ohm meter to test if you have weak spark or not enough spark or intermittent spark i seen it happen alot over the years this 240 chainsaw has a weak spark and i have a coil order for it the gap tester showed that it was under 6mm gap and putting out under 10,000kv with new plug .

 

[JAMES2602]

I did check flywheel key and compression 170 psi and magnets on flywheel flywheel did not have any cracks and np damage i forgot to mention that i figured someone was going to ask me

 

[ILENGINE]

Take the spark plug out and shine a light into the spark plug hole and look at the cylinder wall above the exhaust port. Is it damaged in any way. I have seen saws over the years show good compression readings and the cylinder walls be scored to no end.

 

[bertsmobile1]

1) IF there is no Ω range in the repair specifications then the coil is not checkable with an Ω meter, simple as that.
and this is the case for the bulk of the coils supplied now days
2) yes you can check the secondary but as there is usually no repairs possible other than replacing the cap

Now I have only been in the repair business for 12 years and in that time would be lucky to do more than 30 or so had held tools a year so I do not have vast experience , however I am reasonably good with circuits so realise without any data on the exact type of Hall trigger fitted know that trying to measure primary resistance is a total waste of time unless one is specified
Even if the primary is within spec, that still does not mean that the Hall effect trigger is in fact working or if it is working that it is opening & closing the primary windings at the right time
And this is excluding other complications like rev limiters

Now I am 72 so when at school got taught to use log tables & slide rules
SO to my generation understanding the theory & principles was considered to be more important than the correct numerical value down to the 10th decimal point .
I went through university without the aid of a calculator for the first 2 years and bought my first one in my 3rd year of a 4 year degree after learing how a computer works but again for the purpose of exams it was slide rules & log tables

The co-hort that were 10 years behind me ( around 60 now ) are the first group where calculators were the norm & things like log tables & slide rules were not taught and they were the start of the innumerate graduates because they no longer understood numbers or their significance

Go back another 10 years ( around 50 now ) and you have the generation with no understanding of fundamental principles of anything all they know is how to get the "right " numbers down the the 10th decimal place
Thus they will post here that the coil has a resistance of 22.35 Ω because their meter is set to 2 decimal places and when I tell them to press the probes harder they can not understand why that gives a different set of numbers , or their battery is 12.71V

To this and subsequent generations the idea that there is not a "correct number " for everything is beyond their capacity for thinking .
To a big extent it is not their fault it is a total failure of the educational system

Now on some coils you can dig the potting mix out to find a screw that holds the HT lead in place but by the time I have done that plus repotted the joint I have spent $ 80+ worth of time fixing a $ 20 coil + $5 for the new wire & cap

Out of curosity I have actually cut open some coils to see what is in them , some have a PCB, some have the individual components to make the Hall effect trigger and one actually that 3 totally individual trigger circuits to compensate for the difference in spark timing needed between cranking speed of 200 rpm to running speed of 12,000 rpm and I think it was a Stihl one that had 2 individual primary windings to do the same job.

Now to electronically test a magneto coil you need a very high voltage output detector that discharges the induced voltage lest it find an easier way to ground and a means of generating a flux in the coil that varies in both magnitude and frequency .
For the mechanic these tools are way out of their price range .
Some ( like me ) do things like mount flywheels in lathes with the coil on a mount in the cross slide, an inductive tacho on the HT lead which then goes into a variable gap spark tool & finally to ground

However again just setting this up takes near an hour testing another 20 minutes plus another hour to run the same test with a known good magneto
And even with my special test pulleys on the lathe the best spindle speed I can get is 8720 rpm way too low to check the rev limiters
To me the very idea that a simple Ω reading can determine if a coil is functioning properly is quite laughable and I would suspect leads to a lot of good magnetos being consigned to the scrap bin .

 

[Tiger Small Engine]

1) IF there is no Ω range in the repair specifications then the coil is not checkable with an Ω meter, simple as that.
and this is the case for the bulk of the coils supplied now days
2) yes you can check the secondary but as there is usually no repairs possible other than replacing the cap

Now I have only been in the repair business for 12 years and in that time would be lucky to do more than 30 or so had held tools a year so I do not have vast experience , however I am reasonably good with circuits so realise without any data on the exact type of Hall trigger fitted know that trying to measure primary resistance is a total waste of time unless one is specified
Even if the primary is within spec, that still does not mean that the Hall effect trigger is in fact working or if it is working that it is opening & closing the primary windings at the right time
And this is excluding other complications like rev limiters

Now I am 72 so when at school got taught to use log tables & slide rules
SO to my generation understanding the theory & principles was considered to be more important than the correct numerical value down to the 10th decimal point .
I went through university without the aid of a calculator for the first 2 years and bought my first one in my 3rd year of a 4 year degree after learing how a computer works but again for the purpose of exams it was slide rules & log tables

The co-hort that were 10 years behind me ( around 60 now ) are the first group where calculators were the norm & things like log tables & slide rules were not taught and they were the start of the innumerate graduates because they no longer understood numbers or their significance

Go back another 10 years ( around 50 now ) and you have the generation with no understanding of fundamental principles of anything all they know is how to get the "right " numbers down the the 10th decimal place
Thus they will post here that the coil has a resistance of 22.35 Ω because their meter is set to 2 decimal places and when I tell them to press the probes harder they can not understand why that gives a different set of numbers , or their battery is 12.71V

To this and subsequent generations the idea that there is not a "correct number " for everything is beyond their capacity for thinking .
To a big extent it is not their fault it is a total failure of the educational system

Now on some coils you can dig the potting mix out to find a screw that holds the HT lead in place but by the time I have done that plus repotted the joint I have spent $ 80+ worth of time fixing a $ 20 coil + $5 for the new wire & cap

Out of curosity I have actually cut open some coils to see what is in them , some have a PCB, some have the individual components to make the Hall effect trigger and one actually that 3 totally individual trigger circuits to compensate for the difference in spark timing needed between cranking speed of 200 rpm to running speed of 12,000 rpm and I think it was a Stihl one that had 2 individual primary windings to do the same job.

Now to electronically test a magneto coil you need a very high voltage output detector that discharges the induced voltage lest it find an easier way to ground and a means of generating a flux in the coil that varies in both magnitude and frequency .
For the mechanic these tools are way out of their price range .
Some ( like me ) do things like mount flywheels in lathes with the coil on a mount in the cross slide, an inductive tacho on the HT lead which then goes into a variable gap spark tool & finally to ground

However again just setting this up takes near an hour testing another 20 minutes plus another hour to run the same test with a known good magneto
And even with my special test pulleys on the lathe the best spindle speed I can get is 8720 rpm way too low to check the rev limiters
To me the very idea that a simple Ω reading can determine if a coil is functioning properly is quite laughable and I would suspect leads to a lot of good magnetos being consigned to the scrap bin .

Sometimes I wish situations in small engine repair were black and white, very definitive. However, often, there is a lot of gray areas, subjective. What is difficult for some people is that not everything fits perfectly in a nice new box. Ignition coil’s don’t go bad that often, but they do get replaced occasionally, often after many years of use.

 

[ILENGINE]

Sometimes I wish situations in small engine repair were black and white, very definitive. However, often, there is a lot of gray areas, subjective. What is difficult for some people is that not everything fits perfectly in a nice new box. Ignition coil’s don’t go bad that often, but they do get replaced occasionally, often after many years of use.

And with the spark advance built into some of the modules on saws it can get very grey indeed. Like having a saw run fine for 30 minutes and then start bogging to the point of dying when the trigger is pulled but will idle. Or when throttle is applied then starts 4 stroking under full throtlle like it is rev limiting, and it is, but you are well below the threshold for that rpm.

Nothing like trying to diagnose a faulty ignition module that never looses fire.

 

[StarTech]

Here is what is in the service manual about the spark.

So if see a spark it is likely the coil is fine.

Now it can have triggering problems like the FS55 I am working on. I had a good spark but just not under compression load. I could crank and it ran a second or two and then no restart. Replace the coil now I can nurse it up to full throttle. Now I have found that the FS55 also has a bad carburetor (beyond just a repair kit problem).

I kinda piss that the Stihl dealer I was using and had parts on order from just shut down and didn't even have the respect to call me about the back ordered parts.