Front axle problem

Musselmark · April 7, 2020

2002 chev silverado 4.8 auto, manual 4x4. First off my shift on the fly doesnt work properly, if moving at all it grinds trying to put it in 4x4. Will engage fine if stopped. Second when in 4 low hauling heavy boat up a slip there is sometimes a loud grinding/gear teeth skipping sound from front axle, only under very heavy pull. Third if going from 4x4 high to 2x4 while driving there is a loud clunk from front axle when it disengages. Are these three issues all related, where do I start looking for a problem. Thanks for any help.

dtnel78 · April 7, 2020

When was the last time you checked the front diff oil level? Anyways you may want to pop the drain plug just long enough to let some oil out and see if there's any shavings, loose pieces, etc. IF so it may be a good time to drop a lighted mini cam about $35 on Amazon down in there and take a peek at the gears.

When going in and out of low the few times I have to verified it's worked I have the vehicle in neutral but I have the auto transmission.

Might want to check the oil and if not good then open it up as I would.

Of course that would be me with my own labor and factory level repair information as it can be had online for a year for less than $30 at Mitchelldiy.com or eaautorepair.net which both have wiring diagrams, factory repair information, torque values which are handy when reassembling stuff during a repair as things still need to be torqued.

You can also get a multiyear subscription to a vehicle if you plan on keeping it any amount of time. I've had mine since it was new in 2005. It's a 04 Tahoe with a 4.8L engine.

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Musselmark · April 7, 2020

I think all my problems have something to do with the front axle disconnect system. How does it actually work, cant find anything online about it. When shifting on the fly 2hi to 4 hi I would think the actuator would have to do its thing before shifting transfer case so both driveshaft are going the same speed. I cant understand how it works. Again this is manual transfer case. Is there a switch wired into manual lever that couples the front axle disconnect milliseconds before transfer case gears are engaged?

newdude · April 7, 2020

Actuator pushes a plunger that moves the shift fork that then engages the front axle. The shift fork slides the drive sleeve onto the right side stub shaft that comes out the right side of the diff.

I'd be suspecting an issue in that extension housing. Probably going to find bad springs on the fork, or the shift sleeve failed, or a gear stripped out. If the actuator is weak, that is a possibility as well.

Musselmark · April 7, 2020

Thanks, could you plase explain it a bit further if you can. So driving down the road at 40mph I assume the front driveshaft is not turning on 2 hi. When I pull the manual lever to 4 hi how does the driveshaft get up to speed, is there a synchro in transfer case that does this or does the front axle shaft disconnect sleeve somehow engage first? OR is the front driveshaft always turning in 2 hi? I am familiar with part time systems with lock out hubs such as on my old CJ7 jeep but never understood these systems. Thanks again

davester · April 7, 2020

The two cv axles spin with the wheels, along with both stub axles going into the front diff on each side. But, because the passenger side axle is disconnected from the diff in 2wd mode, it just spins, and the driver's side axle spins the spider gears, when then spins the inner stub axle on the passenger side (it's a lot less resistance to spin that stub axle, than for the ring gear to turn and then drive the front driveshaft).

There is a synchro mechanism in the 4wd engagement mechanism in the transfer case, possibly the friction disc in it is worn out, or maybe glazed from running without fluid sometime in the past?

There isn't a synchro mechanism for the range mechanism (between hi/lo/neutral), which is why GM says to only shift between 2hi, N and 4lo is when stopped or at low speed (and you may hear some grinding, as shifting through neutral results in the gears going different speeds when then going into 4lo).

I've rebuild my NP261HD transfer case when I upgraded my truck to 4wd (mostly just so it had new bearings/bushings and a pump rub kit installed), and when I did it, I also replaced that friction disc at it looked a little worn to me. Since doing that, I haven't had any grinding noises shifting between 2hi and 4hi, either stopped or at speed.

As for the front axle, I would suggest first removing the actuator from the front axle and then check that it moves in/out far enough (about an inch or so), when 4wd is engaged. If that checks out, then remove the passenger side where the actuator screws in and then examine the parts there to see what's up.

Musselmark · April 7, 2020

Thanks very much for the explanation. I had the pump rub through transfer case which leaked fluid so that is likely what caused the problem with friction disc. As for the horrible grinding /gear teeth skipping noise when hauling the 10,000lb boat up the slip in 4 low, could that be the axle disconnect sleeve allowing the passenger axle and stub to spin independent (not locked together enough) of each other. Again this only occurs under this very heavy pull.