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...