Remove the transfer case and check it.internally.
The procedure to remove the transfer case is as follows:--
crawl under the truck you will see the bolt pattern facing the front of the truck should be 15mm and they will be in a circle pattern between the transmission and transfer case .that is where you remove it.You will have to take the things off as to remove the transfer case.
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In some cases its noticed that transfer case motor gets faulty.
In this system the transfer case motor must engage first, then a signal is sent to engage the front differential motor to engage the differential. I think the problem is one of two items, either the front differential engagement motor is in-op or the transfer case motor is malfunctioning and sending false signals to the VCM (vehicle control module).
If the front axle for any reason does not allow the front propeller shaft to spin while in the 2WD setting, the clutch pack in the transfer case will overheat because of the speed difference (slip) across the clutch pack between the stationary front driveshaft and the rotating rear driveshaft.
the front axle, the transfer case should also be inspected for overheating by inspecting for burned fluid or a free rotating front driveshaft. In addition to correcting front axle concerns, necessary repairs should be made to the transfer case.
The auto locking hubs are not engaging properly and will need to be removed and spline sets checked for broken teeth on locking hub and locking shaft.
Also check that the engaging spring inside the hub has enough tension to engage the hubs they may be broken. If the hub are all ok then you have a problem in the transfer box, where the 4WD lever is not meshing the transfer box properly.
POSSIBLE THE ACTUATOR ON THE FRONT AXLE IS BAD ? YOU NEED TO SEE IF THE FRONT DRIVE SHAFT IS ROTATING WHEN THE 4 HI IS ENGAGED WITH OUT RUNNING YOUR SELF OVER IN THE PROCESS.
THERE IS AN ACTUATOR ON THE FRONT AXLE AN ONE ON THE TRANSFER CASE, THE ONE ON THE TRANSFER CASE LOCKS THE DRIVE SHAFT IN AND OUT OF 4 HI & 4 LOW. THE ACTUATOR ON THE FRONT AXLE LOCKS THE FRONT DRIVE SHAFT IN.
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OK - Thanks for the ideas -
New info -
4 low does not engage at all
When I have all 4 wheels off the ground all the all axles appear to function.
I lower the front axle and the shaft from the transfer case stops moving.
I then selected "auto 4wd" and put the truck in drive with the rear wheels off the ground.
This caused the "service 4wd" light to come on.
The fluid level in the transfer case is normal, but the fluid is black. (possibly burned?)
thanks
Replace that burned fluid.and then check.
Thanks for keeping updated with your query.
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Finally got to the GM dealer for the transfer case fluid - i was surprised, it helped.
So 1 problem down - now it engages 4 low, 4 high -
but front axle isn't getting the power to the wheels -
so i'm thinking something is up with the front diff?
1999 GMC Sierra 4WD - 5.3V8 Auto
The 4 wheel drive is not working - (Auto push button type)
I raised the vehicle on jackstands to see if the 4wd would engage.
When all 4 wheels are off the ground the 4wd engages and all the wheels spin.
But when I lowered the front wheels back to the ground, just the back tires spin and the driveline from the transfer case does not spin.
Finally got to the GM dealer for the transfer case fluid - i was surprised, it helped. So 1 problem down - now it engages 4 low, 4 high - but front axle isn't getting the power to the wheels - so i'm thinking something is up with the front diff?
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