Abstract: An actuator assembly is connectable to a lift arm of an agricultural vehicle to sense whether the lift arm is in a raised or lowered position and thereby responsively disable or enable the flow of agricultural chemicals to a spray head associated with the agricultural vehicle, respectively. The preferred actuator assembly includes a cylindrically-shaped annular support housing adapted to being connected to the lift arm and positioned in a substantially vertical mounting plane. A switch housing disc is nested within said annular support housing and may be turned relative thereto about an axis which is substantially normal to the mounting plane.