Nestled up against the firewall just to the right of the engine are two vacuum switches, in the same housing. The green one is normally closed, and opens when manifold vacuum reaches 10" of mercury (Hg). The other one is natural color, and is normally open, closing when ported vacuum reaches 4". The computer watches the states of these two vacuum-operated switches to decide how far your foot is into the carburetor. These serve the same purpose as the TPS (Throttle Position Sensor) on GM and other products.