High Altitude Compensation (HAC) System: Description

As altitude increases, air pressure decreases and the air/fuel mixture becomes richer. The HAC system supplies additional air to the primary low and high speed circuits on Corolla, Pickup and Land Cruiser models, intake manifold on Starlet models and high speed circuits only on Tercel models. This helps to reduce emissions and improve driveability. On all models except Corolla, the ignition timing is also advanced for better driveability.

The systems include a high altitude compensation valve and, on all models except Starlet, a check valve. Starlet models use a thermostatic vacuum switching valve (TVSV) and Tercel models use a bimetal vacuum switching valve (BVSV). Both serve to eliminate compensation while the engine is cold.