[hotwheels]

The Barometric Pressure (BP) sensor operates as a piezoelectric (pressure-sensing) disc. However, rather than generating a voltage, its output is a frequency change. The sensor changes frequency relative to intake manifold vacuum. The sensor frequency increases as vacuum increases. The BP sensor is used to sense the changes in barometric pressure, allowing the PCM to sense the altitude at which the vehicle is operating. Its signal affects, spark advance for altitude compensation. The BP sensor was only used in early MAF systems. Starting in 1994 Ford learned that air pressure can be calculated once the MAF sensor tells the computer the amount of air ingested by the engine.