Magnitos keep failing

[Skeefe146]

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 ?

 

[ILENGINE]

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.

 

[Skeefe146]

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.

I will try that. Thanks.

 

[StarTech]

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.

 

[ILENGINE]

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.

 

[StarTech]

IL do you remember the old Simpson 260 meters?

 

[ILENGINE]

Sorry Star I don't

 

[Forest#2]

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.

 

[Skeefe146]

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?

 

[ILENGINE]

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