Two vacuum-operated electrical switches (ported and manifold) are used to detect and send throttle position data to the MCU for idle (closed), partial and Wide Open Throttle (WOT). These switches are located together in a bracket attached to the dash panel in the engine compartment. The 4 in. Hg (27 kPa) vacuum switch can be identified by its natural (beige) color, while the 10 in. Hg (68 kPa) vacuum switch is green in color. The 4 in. Hg (27 kPa) switch is controlled by ported vacuum and its electrical contact is normally in the open position when the vacuum level is less than 4 in. Hg (27 kPa). When the vacuum exceeds 4 in. Hg (27 kPa), the switch closes. The 4 in. Hg (27 kPa) vacuum switch tells the MCU when either a closed or deep throttle condition exists.
The 10 in. Hg (68 kPa) vacuum switch is controlled by manifold vacuum. Its electrical contact is normally closed when the vacuum level is less than 10 in. Hg (68 kPa); if the vacuum level exceeds 10 in. Hg (68 kPa), the switch opens. This switch tells the MCU that either a partial or medium throttle condition exists.