That truck did not use a vacuum actuator, it used an electric motor motor. If your transfer case is engaging and you can see the front drive line spinning, then you either have a bad actuator, switch, or the wiring is bad. I would recommend unplugging your actuator and checking for power at the wire while someone puts the truck in four wheel drive. If there is power going to it, then replace your actuator. if not, follow your wiring back and check all components. They also sell an aftermarket cable actuator for those trucks which are much more reliable.
SOURCE: four wheel won't engage
Theres a vacume dash pot under the battery box. It pulls a cable that engages the front diff. Put the vacume line and you'll have lights and four wheel drive. (it happened to my blazzer)
SOURCE: 1994 gmc 4x4 non engaging high range four wheel
I would check to see if it is the Axle Actuator. On that model truck there is an upgrade available that is electrical upgrade. Instead of using a vacuum to engage the axle, it uses a small electric motor. The cost for this part isn't much more than the older vacuum model. Usually the electrical one runs around $100. This is a relatively easy change that most any person can do with just about any experience.
SOURCE: I have a 1994 gmc 4x4 and the front axles wont engage
If Your Front CV shaft is broken it will not engage...Shaft can be in visibly good shape, but you might hear a knocking sound or a shutter when turning sharply. I had this problem this last winter. Get under the truck and try and turn the CV by hand. if it can be rotated, it is bad.
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