I'm going to make a couple of assumptions here: you or someone replaced the original starter hoping to solve a problem starting, but it has not. Neither has installing a new battery. What hours on machine and how old is it?
Here's what I suggest in the mean time. I'm suspecting a bad cable, regardless of how clean the ends may be at all four contact points of both ground and positive cable. But first:check your fusebox for any burnt or corroded fuses, to rule them out. Inspect each one carefully. Make sure there is NO corrosion at any fuse connectors.
So, do this: Make sure tractor is in neutral, brake on, etc. Take your jumper cables and attach the ground cable ONLY from it to the negative battery post clamp, and to a known to be good chassis ground. Try starting. If the engine cranks faster or starts, your battery ground cable is toast.
If that doesn't change anything, try the positive jumper cable from the positive battery post,and have someone turn the key to ON, (not crank position), while you touch the other end of the cable to the starter solenoid small terminal, where it is fed by the ignition switch, just long enough to see if the starter spins the engine fast. If you get a fast turn over on the engine you may have a bad positive cable from starter to battery plus post.
Be careful- take someone with you to be on site just to help, and for safety.
Be extra careful when using the positive cable to activate the starter, do NOT let it ground out on the starter or any other metal. Wear glasses and gloves to cover all the bases.
Curious why you say the seat switch does not affect the starting sequence?
Post back results. One step at a time to figure this out.
