See Figures 1, 2 and 3Gasoline Engines Only
When the truck is operated at particularly high altitudes, where the air is thinner, the mixture ratio and intake manifold boost vary greatly; exhaust emissions also increase considerably. In order to decrease these emissions, an altitude compensation system is employed to correct the air/fuel ratio to that of sea level.
When the atmospheric pressure decreases (as altitude increases) to a certain value, an altitude switch is turned on, applying voltage to an altitude solenoid valve. When the solenoid valve turns on, additional air is let into the carburetor, allowing a leaner air/fuel ratio.