midwest_guy515

Alright folks. Here's one that's got me stumped. I have a 99 Silverado 1500 Z71 with 125,000 miles on it, a 4" lift and 33.5" tires, limited slip rear diff, and some minor engine mods (including correcting the computer for tire size). I have blown through a couple front diffs so far. Cracked 1 on a rock, snapped an axle in another, had another gernade for a reason I still have to determine.
The other day I discovered that the front diff I am currently running is not driving the tires when engaged in 4 wheel drive. I was being dumb climbing a big mulch pile and left black patches from my rear tires on the driveway but a front tire never even started to spin in the loose mulch. The front driveshaft wasn't even turning. I tried putting the vehicle in park and turning it by hand with the 4x4 engaged and it was locked up as it should be (it spins free in 2wd). I tried in both 4 High and 4 Low. There isthat lovelydistinct clunk when I drop into and out of 4 Low and I hear the shift motor operate when I shift from 4wd to 2wd and back. I checked the axles and they both seem good. The CVs are still good too.

Does anyone have any idea on what this could be or where I should start? I'm thinking the linkage in the front diff between the shift motor and whatever it moves to engage is damaged or malfunctioning. Please help me out on this one. Is it possible I got a front diff with a 4.10 instead of a 3.73 that's in the rear? Anyone know where I can be an exploded view or assembly drawing of the guts inside the front diff and transfer case? It's getting to a point where I'm actually considering trading the ol girl in and that would just break my heart.

John86

well first of all in a 4x4 u want ur gear ratios to match if not it will lead to other problems like transfer case failure. but ur front axle actuator maybe not but working. is ur service 4x4 light on? do u have push button 4x4 on ur dash or manual shift in the floor board?

midwest_guy515

Gear ratios match at 3.73. Tires are all same size. Found that when I had a local mechanic change the fluid he put 80W-90 Gear Oil in it instead of GM Autotrack fluid. Replaced the gear oil with the autotrack but still have the same problem. I have the pushbutton shift. T-case is a NP-246 autotrack. I can hear the shift motor actuate and the 4x4 light does illuminate. No error/troublecodes pop-up.
I was thinking I might have a snapped chain but there were no signs of metal in the fluid when I changed it. I would guess I would have seen at least a little bit on the magnetic drain plug if a loose chain was flopping around in there.
I'm pretty certain the problem is with the transfer case and not the front diff. Just not sure what it is. Is there a vacuum actuator or shift motor on the transfer case itself that I don't know about that could be the problem?