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My 4wd is not turning on in 4 wheel high or 4 low.

(1999 Chevrolet S10)
in Des Moines, IA on January 29, 2016
zr2 truck not blazer. when i push the button the 4wd actuator clicks, the lights flash but but then goes back to 2wd. the vacuum part is working but the electrical box isn't. how can i test the parts i need and then how do we fix it?
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Popular Answer
on January 29, 2016
When you say "the vacuum part is working".. You mean the front axle servo (under the battery tray) is engaging the front differential??... Explain a bit more. The "vacuum part" is only supposed to send vacuum to the front axle servo AFTER transfer case has actually shifted into 4wd.. You using the PROPER shift sequence??..Got to have more Input! .. Keep in mind I use a high dollar professional scanner to diagnose these types of problems but I am 'very' familiar with how this system works! .. I have a Blazer myself...exact same setup! .. This post will disappear in 1 - one hour if no reply!
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on January 29, 2016
To explain what is happening here goes. You push the button for 4wd high and you hear a click by your feet on the passenger floorboard, the lights flash on 4 high then go back to 2wd. I took the motor out of the back and watched when my wife clicked the button and it didn't engage. But if I manually turn the back motor with a crescent wrench it will go into 4wd.
on January 29, 2016
Lightly tap the T-Case shift motor with a small hammer or the like while helper pushes 4x4 high button to see if motor armature is stuck.... Can test for voltage to the motor with volt meter or test light while helper is still handy to push button(s)... Let me know.
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