You probably can't do it by changing the single seat switch.

The seat switch is most likely closed when you're off the seat, and OPENS the "kill circuit" when you sit on it.

A separate switch CLOSES when you put the shift lever in reverse.

A third switch CLOSES if the mower deck is down (mower running).

If the seat switch is closed (you're off the mower) it's connected to the deck switch, so that the engine is killed if the mower deck is running. (The shift switch could also be in series with the seat switch to kill the engine if you get off with the gears engaged, but models that I've seen only sense REVERSE gear, so that's probably unlikely.)

The shift lever switch is in parallel with the seat switch so if the lever is in the reverse position OR the seat switch is on, the circuit to the deck switch is completed, so that lowering the deck closes the deck switch and completes the kill circuit.

For the engine to be killed, you must have:

Off the Seat (switch closed) AND deck down (switch closed). The shift lever can be anywhere.

OR

In Reverse (shift switch closed) AND deck down (deck switch closed). The seat switch can be either way.

You might be able to disable only the deck position switch, if you can find it. It's most likely a "push button" NO switch that closes when the deck lever is "anywhere but full up," probably actuated by a side link off the shaft that turns when you move the deck up/down lever, "inside" the body of the mower so that it works the same with any mower deck.

If you make the change by disabling the seat switch, you'll ALSO need to disable either the shift lever switch OR the deck switch (or both).

The interlock switches connect only to the magneto or distributor points, and grounding that point is what kills the engine by cutting off the spark. You have two parallel "safety circuits" - one through the seat switch AND deck switch and another through the shift switch AND deck switch, and completing either of the two circuits will kill the engine.

Disconnecting one wire at the switch you want to disable should be sufficient, and you shouldn't need to connect the wire back to anything in the usual circuits although you'll want to tape it or tie it so it can't short out on something. If you've connected the seat switch wires together, you've created an ON circuit, just as if you're off the seat. Unhook one (either) wire. You'll still need to attack the separate parallel shift/deck switches circuit to get what you want.

In the interlock wiring, generally:

Closed Switch = KILL.

Open Switch = RUN.

Unfortunately, MTD has never produced (released?) a legible wiring diagram. They draw it on a four foot long paper and then print it four inches long in the manuals, so even blowing it up in PhotoShop only gives you blurs. And the manuals don't mention much about the interlocks, except to say "they're there."

John