The 4x4 system on an 88-98 full size gmtruck is controlled electronically except the shifting of thetransfer case which is done with a lever. When the transfer case isshifted into a 4x4 gear a switch in the upper driver side of thetransfer case is closed this sends power to the front axle actuatorwhich engages the front wheels to the front axle. A sensor is alsolocated on the front axle which is closed when the actuator shiftsthe axle into gear.
The first thing to check is the 4x4fuse if it is bad replace it.
Next check the switch on the transfercase connect a continuity tester to the switch and shift the transfercase into 4x4 the stitch should close, if it dose not replace it ifthe switch is functioning properly replace actuator on front axle
If you are you talking about the front axle actuator, apply vacum to the front axle actuator with a hand pump or other source with both front wheels off ground, engine not running. As vacuum is applied to diaphram axle should engage and front wheels will turn opisite directions when turned by hand. If vaccuum does not build and hold then the diapram is leaking. Disconnect actuator cable and pull by hand and you should be able to engage by hand. If it don't engage by hand then mechanical problem in front axle, if it engages properly when vacuum is applied you need to verify vaccum present at servo with vehicle running and 4wd engaged. If vaccum is present when running replace servo. If no vacuum check vacuum switch on transfer case has vacuum present from engine to it, common for these to go out. Good luck.
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