I have a 94 gmc cierra z71 off road 4x4. the 4 wheel drive does not work but the actuator and transfer case are both ok. can someone plz help me?

Are the vacuum lines properly connected and have vacuum available to the switch?

jack up front wheels turn a front wheel it should turn front drive shaft with 4wd engaged and the other wheel should turn backwards if it does that part is good if not possible broken axel

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Do you know which of these your truck has ?
Transfer Case - BW 4401 (Two Speed Manual)
Transfer Case - BW 4470 (Two Speed Manual)
Transfer Case - NVG 241-NP2 (Two Speed Manual)
Transfer Case - NVG 243-NP1 (Two Speed Selectable)
Transfer Case - NVG 246-NP8 (Two Speed Automatic)

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.

check the vacuum lines to the actuator in the front wheels . check the actuator operation in the transfer case.

there are 2 sensors 1 on transfer case and one on front axle if the light is lighting up than its most likley the front axle sensor if the light isnt lighting up than its the transfer case sensor. hope this helps

Check the 4WD fuse and the illumination fuse in the I/P fuse block. Also check the Transfer Case Switch and the Front Axle Actuator for a fault. I would say that the Front Axle Actuator is the most common failure. I think there is even an updated actuator asy available. Check with you local chevy dealer for price and availability.

Thats a fairly common problem with that model. Typically it involves a 75.00 part called a transfer case actuator. I personally think the price is right if the vehicle is in good condition other than the actuator. Be advised the Jimmy's normally came with a 700R4 automatic transmission that were prone to failure. Very expensive to repair 2200 to 2600. Mine failed twice before I sold it. Hope this helps. Marvin

Sounds like the chain in the transfer case has broke or the linkage is out of adjustment

On the full size, the front actuator is electric, not vacuum.

When in 4wd mode, a switch on top of the transfer case conducts 12 volts out to the actuator. Wire #50 (brown) is the 12v feed, and it conducts to the light blue wire (#900) to the actuator.
When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.

Check voltage at the connector for the front actuator (at the front differential - it looks like a large bullet that threads into the front diff.) If it has 12 volts, the actuator is the problem. If it does not have 12 volts, check the transfer case switch and the power feed to the transfer case.

I'll assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.
The way this actuator works, is it is a sealed chamber that gets heated by voltage. When it heats, the pin in the end swells and pushes a shift fork in the front differential. The shift fork engages a spline on the passenger side of the differential, and boom! you have 4WD.

GM offered an upgrade to this actuator that is a motor instead of a heated device. However, it requires a small harness addition, a spacer, and a new actuator. It is about a $150 to $200 option.

If I were you, I would buy a new heated actuator. They are available at most auto parts stores, and are about $90. They are also on eBay for about $55 plus shipping. These are very easy to change - just thread out the old one, and thread in a new one.

Good luck, That actuator is usually the problem 90% of time.