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blown np231 t-case....


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what could cause this?

 

4wd on highway? no lube? bad/bent 4wd linkage causing it to be semi-in gear?

 

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this happened to my buddy's (now mine) 95 xj the night before he left for his 15 months tour of the sandbox with his primo-marine pals. something tells me it's bad karma, but I sure hope not.

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He was flat towing a YJ with the tcase in 4wd and the tranny in neutral. I have no idea why he felt that was the best way to flat tow it... Anyways, it somehow got into gear and carnaged the tcase entirely. It is possible that the tranny locked up due to lubrication issues. I don't recall if he threw out the tranny after that or if it was fuct too (he threw out the entire drivetrain within short order, a 2.5L doesn't do much for a guy who loves speed).

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I have seen 4 or 5 transfercases break like this and all were caused by running in 4wd part-time on pavement. None of them broke the chain. The case is the weak link when the driveline binds.

 

My vote is it was in 4wd on pavement.

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...this is a t-case not an axle.

 

what happens is on dry pavement, there is no slip for the tires. the np231 is not a full time case, it's only part time. so it is designed to have some slip. no slip between the front and rear axle, the stress point between them is what brakes...and that point is the t-case.

 

if you're hauling with it, in 4wd, on dry pavement, it may not blow because that extra weight would potentially make at least one tire slip a bit.

 

but there is no reason for 4wd on dry pavement.

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I can tell you with reasonable certainty that your tires will always slip enough to take up the difference in wheel speed while moving at a decent pace. To think that your tires always have 100% traction on the road surface is simply wrong. They always slip a little, and the side walls flex to make up for slight inconsistancies in drivetran speed. Not to mention that you do not make sudden changes in relative wheel speed while travelling fast - it simply isn't possible. If you've driven anything with a spooled axle at high speed and really thought about exactly what is going on - you'd understand. And on a related note, the stress is highest is actually when you're going slow and making tight turns.

 

My NP231 logged far more time on the street in 4hi than you'd think. Sure, a fair amount of it was in low traction (slow, ice), but almost always the case was left in 4wd for the entire duration of the winter. Both I did that and the PO (original owner). Once it gets cold enough the roads clear up again and you again have a high traction situation.

 

Why didn't it break?

 

Simple. There's not enough torque load on it from the marginal amount of binding it sees on the street to actually produce a failure.

 

Cases that have failed on the street are most likely already damaged. Bad bearings, stress cracks in the housing, worn chains, etc will often rear their head while seeing the cyclic stresses of being in 4wd on a higher traction medium.

 

But, I'll not disagree that street use will prematurely wear out a tcase.

 

 

 

And Jeepco... Extra weight is ALWAYS extra stress. If it causes you to lose traction more than otherwise, the actual torque loading on the parts is still MUCH higher than had it not been in that situation.

 

 

I know of many other part time tcases that get left in 4hi for the winter. Clear roads or not.

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found reason

 

 

replaced t-case. it's a 23 spline short input t-case. the only thing that blew was the case itself, all the bearings are good, and the internals look like new.

 

BUT the front driveshaft universal intermediate assembly blew the f*k up.

 

so I'm thinking that that's been broke a while. then the wobble (there was wobble...I didn't know what it was) kept going until the case cracked, and fell off.

 

oh, and it was in 4wd as well. the shift fork in there tells me that in fact it was...

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found reason

 

 

replaced t-case. it's a 23 spline short input t-case. the only thing that blew was the case itself, all the bearings are good, and the internals look like new.

 

BUT the front driveshaft universal intermediate assembly blew the f*k up.

 

so I'm thinking that that's been broke a while. then the wobble (there was wobble...I didn't know what it was) kept going until the case cracked, and fell off.

 

oh, and it was in 4wd as well. the shift fork in there tells me that in fact it was...

 

My bet is this was the cause "oh, and it was in 4wd as well" and this was after the fact "the front driveshaft universal intermediate assembly blew the f*k up".

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Transfer case have a bias built into them.. just cause you are in 4wd doesnt mean both axles are seeing the same bias. What it appears to have happened is some kinda of torsional stress.... more then likely cause by fatigue. I think lack of trany fluid or.. popped into gear with too much right foot.

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