The selonoid switch is probably bad or you might have a bad ground where the cable goes from the negative side of the battery to the body. Take it off and clean it. Also clean the battery terminals attached to your battery. If none of this works check the battery to make sure it is charged. You can bypass the selonoid just to make sure it still runs by hooking jumper cables to your mower battery and on the other end put the ground on the mower body and touch the hot to the red cable where it attaches to your starter. This should turn the starter over and start your mower.
bbaker, September 2009
