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.

Altitude	Barometric Pressure		BP Frequency
Feet	Hg	kPa	Hz
15000	16.9	57	120.4
14000	17.1	58	122.4
13000	18.3	62	125.5
12000	18.9	64	126.6
11000	19.5	66	128.7
10000	20.7	70	131.9
9000	21.0	72	138.1
8000	21.8	75	136.0
7000	23.0	78	138.3
6000	24.2	82	141.8
5000	25.0	84	144.0
4000	26.0	88	147.0
3000	26.6	91	150.0
2000	27.0	94	152.5
1000	28.9	98	156.1
0	29.92	100	159.0
-1000	31.0	103	162.4

29.92 inHg= Sea Level, 100 kPa = Sea Level, 5280 ft = 1 mile, 14.7 PSI= Sea Level

Article & images courtesy of fordfuelinjection.com.