Operator Presence System (OPS)
Sears Craftsman Riding Mowers conform to safety codes standardized for all American manufactured lawn tractors. The safety switch beneath the seat is only one part of the several interconnecting safety features of the modern riding mower. If the system works properly, the owner must be seated for the mower deck to operate. The tractor will continue running with the Power Take Off or PTO disengaged if the parking brake is set before the operator leaves the seat.
The seat switch consists of two electrical contacts normally separated by an air gap. A spring creates an open space between the two contacts. Mounted between two plates, the switch's spring compresses under the weight of the person sitting in the operator seat and completes an electrical circuit. With the switch closed the tractor's ignition system can be switched on with the ignition key. If the operator stands, the spring forces the gap open and interrupts the electrical ignition circuit.
Parking Brake Option
The safety circuit allows the riding mower motor to continue running if the PTO is not engaged, the parking brake is set, and the operator leaves the seat. When the parking brake is set a second switch in electrical parallel with the OPS closes. This bypasses the safety switch under the seat. If the operator stands, the parking brake switch still completes the ignition circuit.
Power Take Off Option
The mower blade turns only when the PTO engages. If the operator should leave the seat with the blade turning and under power, injuries could result. The mower blade would even continue spinning at full power if the machine overturned. To prevent this type of accident, engaging the PTO deactivates a connection in the original circuit and completes a second circuit in series with the OPS switch.
If the operator leaves the seat with the PTO engaged, the ignition interrupts and the machine stops. This often causes problems for new owners who forget to disengage the PTO when turning off the ignition. Until the PTO is off, no combination of settings will restart the mower.
Lawn tractor mowers will not automatically mow in reverse gear. The seat switch plays a failsafe role in this safety feature as well. In reverse gear the relay controlling the PTO switches off and the blade stops. To override that the operator must set the ignition switch to the manual override setting. Leaving the seat at any time during this procedure switches off the mower.