Fuel Select Switch Circuit
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2005 Mercury Sable, 2005 Mercury Mountaineer, 2005 Mercury Monterey, 2005 Mercury Montego, and 2005 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.
The fuel select switch (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.