Inertia Fuel Shut-off (IFS) Switch
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2009 Mercury Mountaineer, 2009 Mercury Milan, 2009 Mercury Mariner, 2009 Mercury Grand Marquis, and 2009 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.
The IFS switch is used in conjunction with the electric fuel pump. The purpose of the IFS switch is to shut off the fuel pump if a collision occurs. It consists of a steel cone held in place by a magnet. When a sharp impact occurs, the cone breaks loose from the magnet, rolls up a conical ramp and strikes a target plate which opens the electrical contacts of the switch and shuts off the electric fuel pump. Once the switch is open, it must be manually reset before restarting the vehicle. Refer to the appropriate Owner's Literature, Roadside Emergencies for the location of the IFS switch.