* One line for actuating the shift motor: This line controls the engagement and disengagement of the front axle. It supplies vacuum to either lock or unlock the axle, depending on the switch position.
* One line for a vacuum reservoir or vent: The second line could be connected to a vacuum reservoir or a vent to help with:
* Faster shifting: A reservoir helps to provide a quicker burst of vacuum to the actuator, leading to faster engagement/disengagement of the axle.
* Preventing leaks: A vent line can help release any trapped vacuum that might build up and hinder operation, ensuring smoother shifting. If there’s a slow leak in the system, this can be crucial.
* Vacuum signal feedback: In more sophisticated setups (though less common on a YJ), the second line might provide feedback to the system regarding the vacuum level or axle position.
Essentially, while one line *could* potentially work (especially if there's a significant vacuum reservoir already in place), the design with two lines aims for improved performance, reliability, and speed of the four-wheel-drive engagement.
Troubleshooting:
Before disconnecting anything, carefully examine the routing of both lines and where they terminate. This will tell you much more about their individual purposes. A vacuum diagram specific to your 1993 Jeep YJ would be extremely helpful. You can often find these online through Jeep forums or repair manuals. Only disconnect lines *after* you've identified their function. Improper disconnection can cause your 4WD system to malfunction.
