Where is the selenoid on a mtd 17.5 hp lawn tractor?

Most MTD mowers have the solenoid under the panel the is in front of the seat and between your legs when you are seated on the mower. There should be two Phillips headed screws holding the plate down. Remove them along with the knobs from any linkage that goes through the plate. Once the plate is off you will see the solenoid bolted the on right below the front edge of the seat.