Vacuum Switch: Description and Operation

4-and 10-Inch Vacuum Switch Operation:

These two vacuum switches are located on the firewall in the engine compartment just behind the engine. The Four-inch switch is controlled by ported vacuum and is a natural color. The Ten-inch switch is controlled by manifold vacuum and is green in color.

The Four-inch switches' electrical contact is normally in the open position when the ported vacuum is below 4 inches of Hg. If ported vacuum exceeds 4 inch Hg., then the switch closes sending a signal to the MCU. If the switch is open, less than 4 inches Hg., then the MCU doesn't get a voltage signal and it assumes the engine is experiencing an idle or deceleration condition. If the switch is closed, more than 4 inches Hg., then the MCU gets a voltage signal indicating the vehicle is under hard acceleration.

The electrical contact in the Ten-inch switch is normally closed when manifold vacuum is less than 10 inches Hg. If the vacuum is 10 inches Hg or more, then the switch opens. If manifold vacuum is less than 10 inches Hg., then the switch is closed and the MCU receives a voltage signal. If the MCU receives a voltage signal, then that indicates to the MCU that the vehicle is either lightly or moderately accelerating. If the MCU does not receive a voltage signal, then that indicates to the MCU that the vehicle is idling, cruising, or decelerating.

The MCU uses the information from these switches, along with other input signals, to determine what adjustments need to be made to the output devices to make the engine perform properly.