what makes a transmission CRACK at tail housing??

I have a 1997 Tahoe 2door. The trans cracked so I had it replaced with a "new" rebuilt one from the dealership. They did not know the
cause of the crack...just said it happens sometimes. 50 miles down the road it cracked again! still no cause found so they replaced it once more and then conveniently voided my warranty by blaming the crack on the suspension lift. I bought the truck with the lift and drove it a year before the crack so its not due to the lift...something else is cracking it (what could it be). The only thing the dealer could come up with is to get my driveshaft lengthened so I did that but i'm still afraid that is not the answer (because it was like that for a year before)
Does anyone have a real answer????????????
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snipped-for-privacy@gmail.com wrote:

Sorry, but they probably gave you the "real" answer, although they didn't offer to fix it (why the heck are you at a dealer for a modified vehicle, anyway???) Lift kits are hell on tail-bushings in transmissions because setting the driveshaft angle gets very critical. If the down angle of the transmission isn't exactly the same as the up-tilt of the rear axle housing, you get a twice-per-revolution impulse load on the driveshaft that really tears things up. The temptation when doing a lift is to rotate the snout of the rear axle upward so that it "points toward" the transmission above it, but that's the worst thing possible because the front U-joint (transmission end) is now at a different angle than the rear u-joint and the speed variations don't cancel out correctly.
I would recommend that you look at your driveline *very* carefully checking for matched angles, any damage to U-joints, a bent driveshaft, or an out-of-balance condition.
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snipped-for-privacy@gmail.com wrote:

I would suspect you have wear or 'slop' in a bushing or maybe the spring u-bolts came loose a bit which put the driveshaft over critical for either length or u-joint angles.
My Jeep CJ7 has only 1" extended shackles on a 2.5" spring lift and this is 'just' enough to set up a harmonic in the u-joints if I am running empty with the top and doors off and low on fuel.
If I have a load, full gas tank or the hard top and doors on, I have no issues.
The u-joint angles are critical. They must be parallel with each other and 'in phase' with each other.
If your tranny tail has a u-joint at say | this angle, then the diff has to have its u-joint at | angle. If the angle at the tranny is say / , then the diff has to be / angle too.
So it looks like |----------| or /----------/
The other thing is phase. The wishbone or ears that holds the front driveshaft u-joint needs to be on the same plane as the one that hold the rear u-joint. So if you put line the front wishbone shape vertical, the one at the rear must be vertical, not off a quarter turn so it's on a horizontal plane.
There is also a slip joint in some driveshafts. If this goes dry of grease, you can get impacts.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile...
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if it is raised more than 3 inches then you need to take it to a frame shop that does driveshafts and also have the pinion angle corrected
snipped-for-privacy@gmail.com wrote:

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ok everyone, thanks for the information.... I just got it back from another mechanic and he says nothing wrong. Where would this tail bushing be? Is it in the transfer case?
The angles appear correct (and so says 4 wheel parts) and the lift has been on the truck two years with no trouble. U joints are fine. mounts are ok. driveshaft is strait---no vibrations or out-of-balance symptoms... the dealership says they tested that also.
could the driveshaft bind up and cause enough pressure to crack the entire housing? If the shacks are bad could it cause enough impact?
It has to be something that just broke (that no one can see) like this bushing maybe...because I broke two transmissions in the same place (by the trans mount holes) back to back!!
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snipped-for-privacy@gmail.com wrote:

Is there a spline slip joint in the driveshaft? If they dry out or get filled with mud they can cause an impact.
Most can be greased.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile...
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...If I knew what that was I could tell you...I know there is grease splatter on the underside of the truck where the driveshaft meets the t-case...there is a rubber boot there and now that the driveshaft is longer it pushes into that boot a little bit when I put it in drive but i guess thats ok??
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snipped-for-privacy@gmail.com wrote in wrote:

either get your pinion angles checked or enjoy your next broken trany. So it has been lifted for 2 years. you drive it differently than the first owner. Check for the known problems with a lift kit first. KB
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snipped-for-privacy@gmail.com wrote:

The driveshaft VISIBLY pushes deeper into the T-case when you drop it in gear? That doesn't sound right- that sounds like the rear end is torquing nose-up and pushing the shaft forward. The rear end shouldn't rotate AT ALL simply when dropping into gear, and should rotate only minimally under maximum acceleration.
This is why lift kits are such a pain in the ass. Lifting blocks on leaf-spring suspension can greatly increase the rotation of the rear end under acceleration, which screws up U-joint angles and also hammers the driveshaft into the transmission.
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...so did something come loose or would the lift cause that all the time? The shaft does not move when in reverse and n. only in drive and pushes a bit further after the wheels start moving.
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snipped-for-privacy@gmail.com wrote:

Lifts tend to cause this all the time, but for modest lifts its doesn't usually result in visible movement. Pushing further "in" in drive is the expected result. The blocks increase the rear-end's twisting leverage on the leaf springs, and in forward gears the torque direction tends to rotate the snout of the differential upward, pushing the driveshaft into the transmission further. You can check the lift kit for tightness, you might be lucky in that its just something coming loose and it gave you fair warning before falling completely apart....
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thanks Mike and Steve, you have been a great help...I will be back for more input if the dang thing breaks again (after I get it looked at once more with my new info)
Thanks again,
Joe
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snipped-for-privacy@gmail.com wrote:

OK.....
Make sure any grease nipples on the driveshaft or u-joints get used regularly.
The fact that your fix appears to be a longer driveshaft implies you hit a bump just wrong with the old one with that 2 extra years of wear on all your suspension bushings allowing it to pass the movement limit.
You could have been running just at the cusp of trouble for angles like I am when I run empty and that little extra wear and.... I carry extra u-joints....
Mike
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yeah the only greese nipples are on the u-joints...guess my next step is to check the angles again
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snipped-for-privacy@gmail.com wrote:

Well, slip yokes with lifts like you describe are known for having the axle pull out too far so they bind and snap the tail piece. They even make slip yoke eliminators for Jeeps because of this trouble.
You should be ok now I would think with the longer shaft.
Mike
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snipped-for-privacy@gmail.com wrote:

Its in the tailshaft itself, it would have been swapped. I don't think the bushing is the *problem*, but its the point where excessive side-loads and vibrations generated by the driveline get transferred to the tail housing.

What do you mean by "bind-up?" in this context? Is the shaft too long such that when the suspension compresses it jams hard against the stops in the tail housing? That can certainly break things. Also if its too short so that it almost disengages, the yoke might "wobble" enough on the tailshaft splines to cause havoc without actually disengaging the splines.
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