Barometric Pressure Sensor: Description and Operation

Altitude Sensor:

At high altitudes the air is less dense than at near sea level. To correct the air fuel mixture during open loop operation for all elevations, an altitude sensor is used. The sensor, located behind the driver's side kick panel, consists of an element called an aneroid, and a potentiometer. The aneroid is a sealed, hollow capsule which contains a specific mass of a gas (usually nitrogen). When the pressure outside the aneroid drops (as with increasing elevation), the aneroid expands and moves the potentiometer, causing a change in the circuit resistance. The control units "read" the resistance and the "normal fixed current" to the differential pressure regulator is reduced proportionally for operation at high altitudes. This current value is then retained in the fuel control unit memory, so that the next time the engine is started the air/fuel ratio is correct for the elevation. The ignition timing is also adjusted for certain engine loads at different altitudes.