First is to check the fuse. Your start circuit is relatively simple. The battery supplies 12V to the B terminal of the key switch through the fuse and ammeter (if equipped). When in the crank position, continuity is made in the key switch from the B terminal to the S terminal. That should be a WHITE wire from S. It then goes through the clutch/brake switch (pedal must be down) and through the attachment switch (must be off) to the starter solenoid small terminal. You can download an OM with the electrical schematic (page 29/56) at the link below, if you don't have one. Here is a link that might be useful: Owners manual

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.

I have the classic my tractor wont start problem but with a twist. I've replaced the solenoid, replaced the battery, and new starter. when i crank the key the motor turned about 1/2 a turn then nothing. I disconnect the battery and put jumpers direct to the starter and same thing turned the flywheel about 1/2 a rotation and nothing. I'm stumped its a 19.5 hp briggs and stratton sears craftsman. the fly wheel turns by hand a full rotation so its not seized up or anything but the starter doesn't seem to have the power to crank it around

Model 917.288563. 41 hours of use since new (on built-in hour meter). Mower runs normal for about 5 minutes from cold start at which time it starts to run rough, as if running on one cylinder. Ignition switch has 4 positions. OFF - REVERSE - RUN - START. When the problem started, I observed that the mower would operate in reverse even though the ignition switch was in the NORMAL (forward only) position. Normally the ignition switch has to be in the REVERSE position in order to operate in reverse. I removed the ignition switch a blew away grass clippings between the contacts as well as the connector thinking that there might be a current leakage path causing the problem Not so. On ONE occasion, I lifted myself briefly off the seat kill switch and the engine started to run normal once again but not for long. Since the schematic shows IGNITION SWITCH, REVERSE SWITCH, SEAT SWITCH, IGNITION UNIT and SHORTING CONNECTOR (whatever that is) as components that could possibly be the cause of the problem, I'd like to narrow it down to the culprit so as not to buy all of them to fix the problem. Any advice from someone who has had this experience is welcomed. Thanks for your response(s). Dean