Fuel Select Switch (FSSW) Circuit
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2006 Mercury Mountaineer, 2006 Mercury Monterey, 2006 Mercury Montego, 2006 Mercury Milan, and 2006 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.
The FSSW circuit is used by the PCM to toggle between single-injection and dual-injection fuel modes. When torque demand reaches a calibrated limit the PCM requests the secondary injectors by toggling the FSSW circuit.