How to test a magneto on a craftsman lawn mower to see if it is bad?

Here's what I've done with my Craftsman 16HP riding mower I bought in 1999.

Remove spark plug wire from plug. Insert phillips screwdriver into boot. Hold shaft of screwdriver close to a metal surface on the engine, no more than 1/8 inch. While being extremely careful not to touch any part of the mower, and holding the screwdriver by it's insulated handle, have an assistant run the starter motor. You should see a healthy spark between the screwdriver shaft and the engine. If not, locate the cutoff wire that runs from the mower's cutoff circuit to the magneto. For mine it was on the left side of the engine as looking at it from the driver's seat. Disconnect the wire (it was a barrel connector) and try the above operation, if there's a spark then the cutoff circuit is shorting to ground somewhere or the cutoff wire is shorting to ground and disconnecting it temporarily corrected that. There's a wiring diagram at sear.com that will show you all the places that cutoff wire connects through: ignition switch, blade switch, and seat switch. A VOM meter is handy here.

If no spark, the magneto is bad. They are not very expensive ($45 I think) and pretty easy to replace. There are a collection of bolts holding the engine cowl on that must be removed, and the plastic shield on top of the engine's fan must be removed also. I broke off two of those bolts because water collects in there and rusts them. It might be good to expose the magneto first before ordering/going and getting parts from sears. Be sure to check the gap of the old magneto before putting the new one on. The magneto I got from sears included a piece of cardboard that served as a gapping tool. I rotated the engine until the magnets were right over the magneto and bolted the new magneto on. The kit came with instructions.

Good Luck!

Go buy a inline spark tester .there only about 10 bucks.

If you don't get a spark with the above instructions make sure that the contacts are clean and rust free before you give up and order a new one. You can remove the magneto and sand it's contacts and sand the insert on the flywheel too. Reinstall the magneto with a regular business card between the magneto and the flywheel for a spacer. If that doesn't do it then it's time to order a new one.