Typically 4WD Switches are located on the transfer case but in some cases may be found on the front axle. There are two types of designs--Vacuum or Electric powered.

4WD Switches typically send a signal to the 4WD actuator which engages the front axle. The Vacuum style fails when the unit's ability to regulate air pressure becomes compromised.

The Electric style fails when the electric signal becomes compromised due to corrosion. Both failures will not allow the front differential to engage, and the 4WD system becomes inoperable.