Stoopid Chevy Transfer Cases!

[snowblind]

Well, I'm glad you got your truck fixed. I'm going to second what Underdrive said, and add a bit of engineering insight... Well, I'm an engineer, so it'll probably be more information than you wanted... This insight is based on my experience from 20 years in the auto industry, mostly working with 4wd systems. I do not wish to offend or start a pissing contest, only to provide useful information.

Happy to have your insights.

First, and very importantly, cast magnesium is NOT soft. At it's worst, it is very comparable to cast aluminum, but usually it's slightly harder and a little stronger.

"Soft" was a relative term and I used it incorrectly in that statement. My apologies. Many times "stronger" materials like magnesium are made thinner so the "strength" of the parts they replace is the same but the design and performance is different. Aluminum tail-piece is the "cure" for pump-rub issues because it doesn't wear thru like the stocker. Is this because it's thicker or stronger or both? Yes that lightness was the primary reason for using magnesium in the T-case.

It certainly didn't mask any failure by "absorbing vibrations"... Not anymore than an aluminum or iron case would have, anyway.

Really? Mag and Iron sound the same when you hit them with a hammer? I always thought the Iron goes PING when you hit it and the mag is "deader."

The t-case being of magnesium instead of aluminum or even iron was NOT your problem, and the material of the housing likely didn't contribute AT ALL to the failure you experienced.

The REAL problem is/was not perceiving that that a failure was happening. I am 100% ok with people thinking it was obvious and I just didn't notice... but in my opinion you do so at your own risk.

I've seen similar failures in Jeeps (with aluminum t-case housings!) so I'd bet my dog (and I do love her dearly!) that your failure was due to one of two things... Either a really bad u-joint let go at road speed, causing much carnage, or more likely, you had a bad bearing/seal on the front bearing of the t-case,

There was no catastrophic failure of the U-joint. Just a lot of wear/play in the U-joint.

which would have allowed all the fluid to leak out (The seals won't last at all if the bearing is even just a little bad...) and thus your t-case was eventually (and likely fairly quickly) REALLY low on fluid.

Never leaked fluid.

Definitely sucks for you, and anyone else it might happen to, but it's REALLY uncommon, and piling on to GM for using magnesium is pretty far off base. Your failure was, one way or another, crappy timing, or a missed maintenance issue.

The discussion had moved from "cause of failure" to "perceiving drive shaft vibration" before the topic of magnesium cases came up.

All signs point to initial cause of failure being a prematurely failing U-joint. Could have been the output bearing but that never leaked. Perhaps the bearing was rotating it's race in the case... I didn't get a good look at the bearing. Just the U-joint.

SNIP CAD and Transfer Case operation description.

As for reliability, it is my experience that the CAD actuator on the ~'98+ 400 and all 800/900 series trucks is hardly EVER the issue. The old style (~'97 down) heated gas chamber unit on 400 trucks was known to have issues though, particularly in cold climates, or on older trucks. There is a retrofit kit to use the motor driven unit on older trucks that I HIGHLY recommend if you have an older 400 series truck. There is also a solid billet unit that will just keep it engaged all the time. Good for ranch trucks, snow plowing, etc, but probably not for full time daily driver use.

Installing a Posi-Lock in place of the electric CAD does make it a manual process, and allows you to just leave the front engaged, or leave it dis-engaged for 2wd low if you need low speed to maneuver, but don't want the binding that happens in 4wd. (Useful for backing trailers) In 2wd, the Posi-lock can ONLY be engaged if you are stopped. You can engage on the fly by shifting to 4wd first, then engaging the Posi-lock.

Same basic idea as mechanical unlocking "hubs" but a single lock that connects the driveshaft to the diff? Very good info that it can be used for SOTF if the transfer is in 4WD.

As for the driveline spinning, in 2wd, the driveshaft is spun only by the friction/oil drag that's in the front differential and t-case, not by any mechanical means. (Just like on any older 4wd truck, even with true lockout hubs...) The ones I've watched on chassis rolls start spinning at about 25mph, and spin fairly slowly at pretty much all speeds.

Great info. So that's on a rolling-road dyno kind of thing from below or design/test with no body on the vehicle? In hindsight my drive shaft was impacting the trans case at a frequency of once or twice per second. Probably below the speed of the rear driveshaft at the same time. During my testing I also found that the shift to Auto happens much faster if you are on the throttle versus coasting.

It may spin a bit more with the "auto" t-case, as the clutches also add some to the rotation, particularly on a new t-case, which is what you have now... it'll slow as you get some miles on the t-case and the clutches loosen up. Regardless of the speed, since there's no load, there's relatively little wear anywhere, and on the plus side, it serves to keep u-joints, seals, and differential parts oiled up and moving freely for people who seldom engage 4wd.

You engineers... Always looking on the bright side. For a lot of people it might be VERY surprising to learn that 4wd components are wearing at all and can even WEAR OUT AND FAIL even though they rarely use 4wd. My damage all happened under "no" load, and at 70mph, but as you said, new/worn parts could effect how fast the shaft was spinning. I actually do use my 4WD often. I work in the mountains and regular S-O-T-F.

Whew. My fingers hurt... I hope everyone finds this information to be helpful, and not hurtful, as that's how I meant it... And hopefully my facts (and experience based opinions) are not too offensive.
And as usual, Your Mileage May Vary...
Chris

Thank you for the info. Very informative and well presented. Not sure who is getting offended here. Was there some argument I missed?

I have one question though. All the mechanics and engineers I spoke to were SURE that front drive shaft wasn't spinning when in 2WD... until I showed them the damage. You are SURE that the driveshaft is moving above 25mph because you witnessed it. If you had not witnessed this personally, what would your opinion be?

I totally get it. Time marches on and vehicles evolve. I want smooth highway manners, fuel economy, shift-on-the-fly, heated seats, A/C, etc, etc and the auto engineers come up with ways to provide that. But sometimes there are unintended consequences from trying to have your cake and eat it to.

The more I learn the more I realize that the "modern" 4WD systems require as much (or maybe more) maintenance/inspection as the "modern" AWD systems. If you get a defective part in either system it can cause very expen$ive damage. As a whole, from engineers down to mechanic, I don't think the auto industry understands this. If they do understand they do a very poor job of spreading that info.

Thanks again for the comments.


Matt

 

[legendaryandrew]

I know for an absolute fact that my gmt800 autotrac front drive shaft spins even in 2wd. Accidentally tested it.

 

[1stDeuce]

Thank you for the info. Very informative and well presented. Not sure who is getting offended here. Was there some argument I missed?

I have one question though. All the mechanics and engineers I spoke to were SURE that front drive shaft wasn't spinning when in 2WD... until I showed them the damage. You are SURE that the driveshaft is moving above 25mph because you witnessed it. If you had not witnessed this personally, what would your opinion be?

Matt

Haha, well, it wasn't this thread, but I've been beat down for supplying insight... People believe what they believe, and facts sometimes tick them off...

I would not have thought the driveshaft would spin at all in 2wd were it not for seeing it on older trucks. (Ones where you could see the driveshaft while the truck was moving down the highway.) BUT I will say that the oil drag spin that I'm talking about is NOT enough to do any damage AT ALL. It's VERY slow relative to road speed. You can easily watch the driveshaft spin and count revolutions.

In the case of your truck, which I understand has a CAD unit, the driveshaft should NOT be spinning much in 2wd. (Assuming you were in 2wd, NOT in Auto, right?) It certainly shouldn't spin fast enough to cause damage... So it sounds to me like you may have had more going on than we thought... Possibly a CAD unit that wasn't able to disengage, or a t-case that was somehow engaging when it shouldn't... You are SURE you were in 2wd, and not Auto, right?

Assuming the CAD and front axle haven't been touched in the repair, you might want to crawl under the truck (in 2wd) and make sure the driveshaft can be spun by hand. If it can't, then your CAD unit is likely locked up, and you should check it out. If you unplug the electrical connector, it will just unscrew from the axle, and once removed, should unlock the axle. If it still stays locked, then the splines inside are likely damaged, and that's what's holding it engaged...

Chris

 

[Buliwyf]

GM and Dodge really need to get with it and put manual hubs on their trucks. If I want to shift in and out of 4 wheel drive on the fly, I just leave the front hubs locked in. All summer I'm unlocked, all winter I'm locked.

 

[PlethoraOfGuns]

GM and Dodge really need to get with it and put manual hubs on their trucks. If I want to shift in and out of 4 wheel drive on the fly, I just leave the front hubs locked in. All summer I'm unlocked, all winter I'm locked.

Today's men are too lazy to get out and lock their hubs. Or that's what the automotive engineers believe...

 

[1stDeuce]

Today's men are too lazy to get out and lock their hubs. Or that's what the automotive engineers believe...

While this is true, the even bigger truth is that they're also too lazy to pull a dang lever inside... And trust me, it's NOT driven by the engineers. Pretty much NOTHING is driven by engineering anymore. Bean counters and program (mis)management make all the decisions that tick us off. Engineering just gets told what to do.

And it's not just today's "men", which I'd argue aren't any better than today's "women" in many respects related to "trucks". My wife can and does drive a stick. And she can put all of our vehicles in 4-low, and knows when to do that. That puts her well in the minority of both women AND men these days...

Buliwyf, I hear ya. BUT I do like the way my '06 has held up, and I REALLY appreciate not repacking hub bearings every year like I had to on my old solid axle Chevy. Haven't replaced a single U-joint either. (215k now, and all original u-joints and half shafts...) I don't think things are bad for how 90% of the population uses their trucks (you know, like a car...) but the 99%-ers here don't really fit well in that population.

I had considered putting the CAD motor on an override switch so I could unlock it or lock it at will. Unlock for 2-low, lock for easier SOTF... But it works fine like it is, and I haven't needed 2-low since I had the '98 Super-Sonoma, which had too high a reverse in the manual trans. No such issue with the Allison.
Chris

 

[snowblind]

I would not have thought the driveshaft would spin at all in 2wd were it not for seeing it on older trucks. (Ones where you could see the driveshaft while the truck was moving down the highway.) BUT I will say that the oil drag spin that I'm talking about is NOT enough to do any damage AT ALL. It's VERY slow relative to road speed. You can easily watch the driveshaft spin and count revolutions.

In the case of your truck, which I understand has a CAD unit, the driveshaft should NOT be spinning much in 2wd. (Assuming you were in 2wd, NOT in Auto, right?) It certainly shouldn't spin fast enough to cause damage... So it sounds to me like you may have had more going on than we thought... Possibly a CAD unit that wasn't able to disengage, or a t-case that was somehow engaging when it shouldn't... You are SURE you were in 2wd, and not Auto, right?

Assuming the CAD and front axle haven't been touched in the repair, you might want to crawl under the truck (in 2wd) and make sure the driveshaft can be spun by hand. If it can't, then your CAD unit is likely locked up, and you should check it out.

Definitely not in Auto.

I checked the front driveshaft today. Free-spinning in the driveway. And not very much resistance either... Makes sense to me that it could spin sympathetically at a pretty high speed.

It also makes sense to me that the engineers WANT it to spin sympathetically. Sympathetic rotation makes Auto4WD engage faster and (as you mentioned) keeps front end parts lubricated. There really is no down side... unless a bearing fails. Can I go back to saying those engineers are "stoopid" now ;-)



Matt