Barometric Pressure Sensor: Description and Operation

Fig. 3 Altitude Compensator:

The amount of fuel sent to the injectors is affected, in part, by the signals to the ECU from the altitude sensor, Fig. 3.
As altitude increases (decreasing air pressure) the mixture will be adjusted by a reduction of the differential pressure regulator current. Reducing the current lowers the lower chamber pressure in the fuel distributor and leans the fuel mixture. The altitude sensor continuously adapts the injected fuel quantity to the changing air pressure.