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Hi all, first post here. I’ve been chasing this for a while and it’s got me and all my small engine guru friends stumped. Few years ago, I bought a used Ariens tractor, made by Husqvarna. 42” hydrostatic drive 20hp BS single. Used it for a season, covered it for the winter and fired it up in spring. Cut the lawn once, then it wouldn’t start. No spark. Buddy put another coil in it and it ran. Cut the lawn twice and, you guessed it, no spark, and the motor would not crank past the compression stroke. We went round and round with it, three different people tried their hand at it. No go. Finally my buddy changed the motor to a 19hp BS he had on his shelf. I helped install it and it fired right up. Took it home and cut the lawn once and, again no spark. In disgust I covered it and pushed it under the deck. So here it is, spring in Maine. So I pulled it out again, got another coil and installed it. It runs! Cut the grass and no spark. I know the wiring been chopped up, all the safety switches removed, but I can’t figure it out. Help?
Clue: it’s always not re-started, that is seems after shutting it off, the coil dies. Should I try disconnecting the wires in kill switch and try a stand alone kill switch to the chassis ?

I suspect that when the key is shutoff it is sending a very brief 12 volts through the kill wire to the coil which is instantly killing them. You can disconnect the small wire on the starter solenoid and then connect a test light between the disconnected coil wire and ground and then start turning the key from off to start and back several times. Briggs recommends 50 times and if the light even flickers the switch is bad. The reason for removing the trigger wire on the solenoid is you don't want or need the engine to crank while testing.

ILENGINE said:

I suspect that when the key is shutoff it is sending a very brief 12 volts through the kill wire to the coil which is instantly killing them. You can disconnect the small wire on the starter solenoid and then connect a test light between the disconnected coil wire and ground and then start turning the key from off to start and back several times. Briggs recommends 50 times and if the light even flickers the switch is bad. The reason for removing the trigger wire on the solenoid is you don't want or need the engine to crank while testing.

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I will try that. Thanks.

I prefer an analog meter myself as the voltage can be too low to light the light. I had one several years that was only passing 1.5 volts but it was enough to disable the coils.

I went with the test light because I knew most people have digital meters, and the sample rate isn't fast enough to pickup the glitch voltage.

IL do you remember the old Simpson 260 meters?

Sorry Star I don't

I've got some of the Simpson 260's and 160's. I can repair and calibrate them oldies. The precision parts are getting scarce from Simpson.
I prefer them analog type meters for Ohms testing but the DMM's for voltage testing. Usually what wipes them out is not paying attention and trying to read voltage with them set on wrong scale, like on Ohms or Milliamps. (It happens fast kinda like sending voltage to the kill wire on a Magneto)

Back to the bad magnetos. If I were you I would just go ahead and replace the ignition switch. Make sure you get the correct OEM one. One size does not fit all even though the plugs interchange. I've seen/found rust inside the Ignition switches and cause such and be intermittent and erratic sending voltage to a magneto.

Thank you, good advice I’m sure. But what about isolating the mag by running the kill wire through a horn button then to the engine block? Will that work?

Just complicates the wiring but some sort of momentary switch to ground the coil is all that is needed.

But what about isolating the mag by running the kill wire through a horn button then to the engine block? Will that work?

Yes,

or disconnect the existing wires at the head light switch (isolated their ends by tapeing) and use ithe on/off headlight switch as you describe as a horn switch as a run kill test for awhile
Most likely replacing the ignition switch will fix your issue. I've seen it happen quite often especially if the tractor is parked outside and the key switch is exposed to rain. Loose rust forms down inside the switch causing intermittent voltage leak through to the mag and oil or contact cleaner is not the cure for such. The kill wire lead at the magneto is actually sensitive electronics inside the mag and any back feed positive voltage damages the magneto electronics.

You obviously have a switch problem
Believe it or not I have seen a similar case where it was ants in the switch
The problem is finding a good ground on mowers where everything is plastic .
If you mower has a carb solenoid and the solenoid works then you can ( for the purpose of testing only ) remove the kill wires , check that the solenoid shuts down the engine then mow like that for a few weeks to see if the problem goes away
Just be very careful because without the kill wires in place you can get off the mower with the blades running and new feet are a lot more expensive than new magnetos .

bertsmobile1 said:

You obviously have a switch problem
Believe it or not I have seen a similar case where it was ants in the switch
The problem is finding a good ground on mowers where everything is plastic .
If you mower has a carb solenoid and the solenoid works then you can ( for the purpose of testing only ) remove the kill wires , check that the solenoid shuts down the engine then mow like that for a few weeks to see if the problem goes away
Just be very careful because without the kill wires in place you can get off the mower with the blades running and new feet are a lot more expensive than new magnetos .

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Thanks Bertsmobile1, just wanted to give you an update. The carb solenoid was still working but I didn’t go that route. I just isolated the mag to an independent ground switch and it’s been working well, that is until I failed to turn off the ignition switch fully after the last cut. So the battery completely drained it self while powering the carb solenoid. Now the solenoid is burned out! A while back I read somewhere about replacing the carb bowl with one without a solenoid. Any idea what part that might be?
TIA.

Just remove your bad solenoid and cut off the pintle and re-install.

Watch a you tube video about such.

Forest#2 said:

Just remove your bad solenoid and cut off the pintle and re-install.

Watch a you tube video about such.

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Thanks Forest#2, I’ll do just that.
Cheers.

Done! I cut off the spindle, reinstalled and it fired right up.
Thanks a million!!

Also you might consider:
Leave the plug wiring to the Anti Backfire solenoid (ABS) disconnected because it's not needed. If you accidently leave the key on again the battery may not discharge plus the solenoid coil might be burned/shorted windings and drawing enough current to take out the charging system. Also letting the engine idle for awhile before using the kill switch may reduce the chance of a backfire.
If your neighbors are not neighborly and their dogs are gun shy just let it backfire.