Mayfair Replacement Automatic Bilge Pump Float Switch by Johnson Pump The Johnson Pump Automatic Bilge Pump Float Switch is constructed of ultrasonically sealed thermo-plastic housing. An anti-spike feature prevents rapid on-off cycling and turns on with an audible click before water can reach unacceptable levels. This bilge pump float switch turns on when the rolling steel ball changes the pressure on the micro-switch. This automatic bilge pump float switch is a direct replacement to older Mayf