Seat switch connector has a built in "shorting bar" which will close the kill circuit when the connector is unplugged from the switch. Unplugging the connector from the switch gives the same results as if the seat is not occupied. Seat switch is just one switch in the safety interlock circuitry that can cause the engine to have no spark. The brake pedal interlock switch is actually 2 switches in one assembly. One of the brake switches is dedicated to starter inhibition, and the other switch is to kill spark and stop the engine. The PTO switch also has a circuit dedicated to the kill wire. You can disconnect the kill wire from the magneto to see if that restores spark. If doing that restores spark, something is grounding the kill wire somewhere. Could be a chafed wire.

The only way the battery can discharge with the key off is if the diode off the alternator is shorted. That's assuming the ignition switch is functioning properly. Try unplugging that connector after using the mower to see if the battery stays charged. IF you have a voltmeter, do the following- Test leads across the battery & engine at high speed. Measure DCV AND ACV. There should be 14ish DCV and negligible ACV. IF you are getting noticeable ACV, it would verify the diode is shorted. The diode could be marginally bad I guess???, but they tend to be good or bad. IF you don't have a meter, a 12V test light MIGHT show a bad diode. Unplug diode connector and insert leads in series with the diode. IF it lights, diode bad. IF you don't know what a diode is, it's basically an electric check valve. Current can only flow one direction.

It may be about time to pop the valve cover and look at the valve train for signs of unequal valve height (how far up the valve stem rises when the engine is turned by hand). If one of the valves does not rise as far as the other, a valve seat may have come out. Does the engine seem to spin faster than usual when cranking? Hear any kind of hissing or wheezing sound from the exhaust while cranking? Can you still "hear" the characteristic thump sound of the compression stroke while cranking or just the starter just seem to make a monotone, steady spinning sound like it isn't under any load?

A couple possibilities. That's a Briggs OHV engine. IF the valves get out of adjustment, the compression release doesn't function. OR, you simply have a bad connection in the cranking circuit. Clean the ends of all battery cables. make sure engine mounting bolts are tight. The ground side is just as important as the "hot" side. Then there's the possibility your battery has reached the end of its life. Charge & get it load tested to be sure. A bad solenoid is also a possibility. This post was edited by bill_kapaun on Sat, Nov 8, 14 at 22:34