Expert: Eric A. Jones - 9/28/2006

Question
Iam experiencing problem of a third magneto failure in two weeks on my 1997 Sears lawn tractor with a 19.5 HP twin B&S engine. Magneto failed for first time after 9 years of flawless operation of mower engine. My certified mechanic diagnosed problem as a failed magneto, replaced same and after four hours of cutting, it failed. Maybe, he thought, the new replacement was faulty, so another was installed. Again, after four hours, it too has failed i.e no spark.Everything else was checked out at time of first replacement. Could the problem be the lack of a ballast resistor in the supply line to the magneto, as was surmised in a discussion back on June 9, 2006 between your expert Allen Kirkland and a person who had a problem that sounds just like mine. I don't want to keep getting new magnetos which my mechanic has thus far replaced without additional charge (except for the first one) without some idea of what the underlying problem might really be. Any help would be appreciated. Jim MacRae

Answer
That new of a mower does not use an actual magneto.  Magnetos were actually old mechanical devices.

However, the ignition module/coil is a solid state electronic device as far as the spark triggering is concerned.

So, there is no ballast resistor used in the ignition system.  However, you key switch or another electric part that is allowing 12 volts DC to reach the coil.

There is a small wire connected to the coil. This wire is connected to the primary winding side of the coil.  If there is voltage present on this wire it could be frying the windings.

When you turn the key to the off position the switch grounds the primary side of the winding and stops spark for the engine to shut off.

You can use a voltmeter to test.  Dis-connect the small wire, turn the key to the run position and see if the is 12 volts DC on the wire.  

If there is this is your problem.  If you don't measure any voltage you can follow the small wire to engine block connection.  Assemble the engine re-connecting the small coil wire. Start the mower and then use a volt meter to see if there is any voltage at the engine block connection.

Let me know what you find.
Eric