4 Wheel Drive/SUVs: 4WD will not engage, chevy 1500, drive shaft

Question
QUESTION: I have a 1992 Chevy 1500 4wd with manual transfer case on an automatic transmission.When I put it in 4WD the transfer case engages but the front axle does not. when the problem first started it would function when I first started the truck but it would not after the vehicle ran for a while.what tells the front axle to operate?I see two plugs on the front differential. It almost seem vacuum related.

ANSWER: There is an actuator on the front differential, it might be steel or it might be plastic , depending on if it has been replaced before, thats whats wrong with your 4x4 if it lights up but dont engage . work on them all the time. Go to Auto Zone and buy an accuator. Thanks ADAM

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QUESTION: I see the actuator . It has two wires running to it. Took it out and there is a cone shaped cylinder with a rubber boot covering it. Is this what you are discribing? Didn't make it clear that the light on the console DOES NOT LIGHT UP when the front diferential does not engage. Do you still think it is the actuator?

ANSWER: The light might be burnt out, but I would put it in 4x4 and have some one watch and see if the front drive shaft turns than you will know for sure ..Thanks ADAM

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QUESTION: Adam, bench tested the actuator and it works fine. Glad I didn't spend $100 at auto zone on a non-return item. There is a black ( I assume ground) and a dark green wire feeding the actuator .I would guess they come from the transfer case. Is there some sort of switch on the top of the transfer case that sends it a single? If so what makes it function? Don't mean to be a bother but this is something that I have never dealt with. Put a volt meter on the plug and sometimes it would read my voltage and sometimes not.

Answer
Michael, Yes there is a switch on top of the trnsfercase that sometimes goes bad, and I dont think that one costs too much. Maybe you should try that . And fyi you never told me initally that the light didnt light up, and when you did I told you to put it in 4x4 and have some one see if the driveshaft turns. Thanks ADAM