Pickup 2WD L4-155 2555cc 2.6L SOHC VIN E 2-bbl (1988)
Altitude Compensator: Description and Operation
Federal Vehicles
Fig. 103 High altitude compensation system (Federal)
To meet Federal requirements at all altitudes all carbureted vehicles are equipped with a high altitude compensation system, Fig. 103 in addition to the
feedback carburetor system. The high altitude compensation system consists of a high altitude compensator (HAC), a vacuum switching valve, and a
distributor equipped with a high altitude advance system. Fuel/air ratio at high altitude is modified by the HAC to be approximately the same value as sea
level. The HAC supplies additional bleed air into the carburetor secondary main fuel well through the HAC and and to the primary main fuel well
through a vacuum switching valve controlled by the HAC.
* At low altitude, the HAC opens to allow manifold vacuum to escape from the HAC to atmosphere. The vacuum switch and HAC's additional bleed air
passage remain closed and bleed air is not supplied to the carburetor.
* At high altitude, the HAC closes and manifold vacuum is applied to the vacuum switching valves and the distributor.
* The vacuum switching valves open and additional bleed air is supplied to the carburtor. At the same time, the distributor advances the ignition timing.
