High Altitude Compensation System: Description
NOTE:
The HAC system is optional on high altitude area Federal vehicles. Not all vehicles will be equipped with this system.
As altitude increases, air pressure decreases and air/fuel mixture becomes richer. The High Altitude Compensation (HAC) system supplies additional air to the primary low and high speed circuits on all models, and secondary high speed circuits. This helps to reduce emissions and improve driveability. Ignition timing is also advanced for better driveability. The systems include a high altitude compensation valve, a vacuum advance sub-diaphragm, and a check valve.