High Altitude Compensation (HAC) System: Operation

At altitudes over 4000 feet, the HAC valve opens and allows air to bleed through the valve into the carburetor. When the vehicle is at an altitude below 2600 feet, the valve is closed and no air can pass through. The valve may or may not be open between these altitudes. The BVSV on Tercel models is closed when coolant temperature is below 122° F (50° C). This cuts off vacuum from the HAC valve so air can't bleed into carburetor, but still allows vacuum advance.