The first thing to check is the switch itself. Pull the switch and test it. If it is ok, you need to follow these steps:
1. Locate the 4WD and Two Wheel Drive (2WD) vacuum solenoids on the passenger side rear of the engine compartment, behind the battery.
2. Verify good manifold vacuum to both solenoids on the white vacuum line from the engine. Verify no leaks in the Light Blue vacuum line from the 4WD solenoid to the vacuum motor on the front axle, or on the Pink vacuum line from the 2WD solenoid to the vacuum motor. Verify battery voltage on the White/Light Blue wire at both solenoids from fuse 23 in the inside fuse box.
3. If all OK, put the vehicle in 4WD, and check for a ground on the Red wire at the 4WD solenoid from the Generic Electronic Module (GEM). If this wire is grounded, but there is no vacuum from the solenoid on the Light Blue vacuum line, replace the solenoid.
4. If there is no ground on the Red wire at the 4WD solenoid when the transfer case engages, check for an open in this wire between the solenoid and the Generic Electronic Module (GEM). The GEM is mounted to the back side of the inside fuse box. If this wire is OK, further diagnosis of the GEM will be necessary
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