Fuel Rail Pressure Sensor
On Crown Victoria 4.6L NGV, Fuel Rail Pressure (FRP) sensor is a diaphragm strain gauge device in which resistance changes with pressure. See Fig 1. Electrical resistance of a strain gauge increases as pressure increases, and decreases as pressure decreases. Varying resistance affects voltage drop across sensor terminals and provides electrical signals to PCM corresponding to pressure. Strain gauge type sensors are considered passive sensors. A passive sensor is connected to a voltage divider network so that varying resistance of passive sensor causes a variation in total current flow. Voltage that is dropped across a fixed resistor in series with sensor resistor determines voltage signal at PCM. This voltage signal is equal to reference voltage minus voltage drop across fixed resistor.
On all others, FRP sensor measures pressure of fuel near fuel injectors. See Fig 2. This signal is used by PCM to adjust fuel injector pulse width and meter fuel to each cylinder. FRP sensor senses pressure difference between fuel rail and intake manifold. Return fuel line to fuel tank has been deleted in this type of fuel system. Differential of fuel/intake manifold pressure together with measured fuel temperature provides an indication of fuel vapors in fuel rail. Both differential pressure and temperature feedback signals are used to control fuel pump speed. Fuel pump speed sustains fuel rail pressure which preserves fuel in its liquid state. Dynamic range of fuel injectors increases because of higher rail pressure, which allows injector pulse width to decrease.