Coil Testing with Multimeter?

[HansFranz]

My Dolmar PS-3410TH has weak or absent spark. Replaced spark plug, nogo. Took switch out of the circuit, nogo. Most of the time it won't fire at all (I pulled plug and tested), but occasionally I can get it to start for maybe a quarter of a second, then it quits. (This is always with a cold saw, since I haven't had it running for longer than 1/4 second in months.)

The parts diagram shows two items labeled "ignition coil" -- items #121 and #130 in diagram below.

These parts are all-but-unavailable online ... although you can get "both" for $110 from an ebayer in Estonia.

Before I waste $110+ on parts I don't need for a 17-year-old saw, is there a way I can test these coils with a multimeter?

I searched "how to test a chainsaw coil" online, but all I'm finding is copypasta gibberish and AI GIGO.

Thanks in advance for any clues.

 

[ZeroJunk]

Loosen and tighten the screws that hold it. The resistance on a good ohm meter between metal on the coil and case should be close to zero. Check resistance between kill wire terminal and case. Should be several K ohms unless the wire is shorted somewhere.
With the kill wire disconnected and the coil laminations common with the case and it still won't fire, doesn't leave much else.

 

[echomeister]

Testing coils is more difficult these days, because technology ....

Old school coils were just two copper winding, primary was a couple hundred turns of wire. Ohm reading 1 - 5 ohms.
Secondary was 10k turns of very fine hairlike wire Ohms 100 - 2k ohms.

But the engineers couldn't stand it. They added electronics inside the guts. Cant do simple tests anymore. I bought 4 ignition coils for my Rav4
cause they ohmed open on the secondary. They did that because there are strings of diodes in them now. $$$$ -> drain.

If you are getting no spark and the magnets are strong on the rotor there isn't much else to be wrong except the coil.

 

[Old2stroke]

Testing coils is more difficult these days, because technology ....

Old school coils were just two copper winding, primary was a couple hundred turns of wire. Ohm reading 1 - 5 ohms.
Secondary was 10k turns of very fine hairlike wire Ohms 100 - 2k ohms.

But the engineers couldn't stand it. They added electronics inside the guts. Cant do simple tests anymore. I bought 4 ignition coils for my Rav4
cause they ohmed open on the secondary. They did that because there are strings of diodes in them now. $$$$ -> drain.

If you are getting no spark and the magnets are strong on the rotor there isn't much else to be wrong except the coil.

Fortunately, no diodes in the secondary of chainsaw coils (yet).
One end of the primary and one end of the secondary are connected together and connected to the laminated core so you can test that with an ohm meter, should be high resistance (over 5k) between plug lead and coil core. If test shows open circuit make sure it isn't just a bad plug lead such as bad connector on end of lead. Test by pushing a pin through the lead and test from it to the coil core. Bad plug leads can usually be repaired or replaced. Thanks to the electronic package that controls the primary current, there is no valid test you can do on the primary with a meter.

 

[HansFranz]

Thanks for your replies and help, guys.

I'm actually starting to think it might be something in the plug wire, since it seems like I only get occasional spark/fire after jacking around with the plug lead (like after testing the plug outside of the engine). I'm thinking it might be making/breaking somewhere in that wire, so I will try soldering in as long a piece of known-good copper stranded wire as I can...and see what I get. (BTW, why do they use such thin conductors on plug wires? To avoid RF interference? I "get it" that it's high voltage/low amperage and doesn't need much AWG, but thin wire is so delicate...seems like it's just asking for ignition trouble...) ((OTOH, if voltage is high enough to jump the plug gap, it ought to be able to jump a sizable break in the plug conductor...))

Will report back...

 

[MacAttack]

Can you replace the plug wire?

Unless I'm not understanding correctly, but most of the ignitions I've worked on the plug wire either unscrews off a threaded spike on the coil, or just pulls straight out.

 

[HansFranz]

Can you replace the plug wire?

Unless I'm not understanding correctly, but most of the ignitions I've worked on the plug wire either unscrews off a threaded spike on the coil, or just pulls straight out.

Oh, cool, thanks for that tip MacAttack. I did not even consider that possibility. (I haven't worked on chainsaws much.)
I will see if I can either unscrew it or pull it out...

 

[MacAttack]

Oh, cool, thanks for that tip MacAttack. I did not even consider that possibility. (I haven't worked on chainsaws much.)
I will see if I can either unscrew it or pull it out...

Sure thing! Just go easy on it until you figure out how the wire comes out.
I can't 100% guarantee it will come out since I haven't worked on every kind of saw, but on at least 3 different brands the plug wires have come out of the coil.

 

[david newbie]

Sure thing! Just go easy on it until you figure out how the wire comes out.
I can't 100% guarantee it will come out since I haven't worked on every kind of saw, but on at least 3 different brands the plug wires have come out of the coil.

Sorry to butt in, do you happen to know if 394xp has a replaceable plug wire?

 

[MacAttack]

Sorry to butt in, do you happen to know if 394xp has a replaceable plug wire?

I haven't replaced a plug wire on a 394 but I think they're replacable. There's plenty of replacement spark plug wires available which would suggest they can be replaced.
I had no idea the wires were removable until I learned it on this site haha.

 

[Bob Hedgecutter]

Sorry to butt in, do you happen to know if 394xp has a replaceable plug wire?

Yes, they unscrew off the threaded spike in the coil outlet.
Easiest way to ascertain such, is to look at an IPL- if it shows a separate lead to the coil body- they are replaceable.

 

[Bob Hedgecutter]

My Dolmar PS-3410TH has weak or absent spark. Replaced spark plug, nogo. Took switch out of the circuit, nogo. Most of the time it won't fire at all (I pulled plug and tested), but occasionally I can get it to start for maybe a quarter of a second, then it quits. (This is always with a cold saw, since I haven't had it running for longer than 1/4 second in months.)

The parts diagram shows two items labeled "ignition coil" -- items #121 and #130 in diagram below.

These parts are all-but-unavailable online ... although you can get "both" for $110 from an ebayer in Estonia.

Before I waste $110+ on parts I don't need for a 17-year-old saw, is there a way I can test these coils with a multimeter?

I searched "how to test a chainsaw coil" online, but all I'm finding is copypasta gibberish and AI GIGO.

Thanks in advance for any clues.

Never pulled that paticular trigger pack apart- so cannot say for sure it is not "hard wired" to the trigger unit and as your own IPL shows it still attached- maybe it is.
It is most often the trigger unit that fails on a two piece coil system- not the main coil body at the flywheel.

 

[Vintage Engine Repairs]

I searched "how to test a chainsaw coil" online, but all I'm finding is copypasta gibberish and AI GIGO.

Don’t worry, I don’t take it personally

 

[HansFranz]

(I was actually hoping you had something like this out there, Tom, but I didn't think to search your channel.) Thanks!

 

[Vintage Engine Repairs]

(I was actually hoping you had something like this out there, Tom, but I didn't think to search your channel.) Thanks!

All good mate, I hope you find some value to it

 

[Okie]

As the guys have said electronic magnetos tests with a ohmmeter cannot be done with any logical results. After all else is eliminated replacement is the test.

Something you might try as a last ditch effort.
Baking your magneto in a oven and contacting standard magneto for a replacement.
Several of the Homie magnetos (blue coil of death) would get moisture inside and a oven bake would bring them around. (and NLA and expensive when found)
A company by the name of something like Standard Magneto makes lots of coils. They now make the blue coil of death for about $50 (magnetos of all kinds, might be worth checking.

I now press the bad blue magneto coil off the laminations and install a $20 coil (just the coil) from another chainsaw but I had a hint from a member on this site about doing such. (replacing the blue coil of death)
Been doing this for about 5 years with good results.

 

[HansFranz]

Curious what you mean about the "blue" coils, Okie.
It just so happens that my Dommar coils are blue...are the blue ones bad?

 

[pioneerguy600]

Curious what you mean about the "blue" coils, Okie.
It just so happens that my Dommar coils are blue...are the blue ones bad?

Your set are completely sealed to the lamination's and cannot be pressed off like the older coils found on the saws like the Homelite XL series.The old coils were made by Prestolite, they had a blue outer cover and soon beame known as the coil of death as they were not sealed up tight moisture could and did get inside the windings and that stopped them from producing spark.

 

[HansFranz]

Thanks for all the great replies, everyone. I'm really gaining an education here.

 

[MacAttack]

Your set are completely sealed to the lamination's and cannot be pressed off like the older coils found on the saws like the Homelite XL series.The old coils were made by Prestolite, they had a blue outer cover and soon beame known as the coil of death as they were not sealed up tight moisture could and did get inside the windings and that stopped them from producing spark.

He's working on a Dolmar with a blue coil though, would be different than the infamous blue coils on a Homelite right?