Stoopid Chevy Transfer Cases!

[snowblind]

Turns out that the rear u-joint on the front driveshaft of my 2001 Suburban 2500 was a piece of crap. Unfortunately the crappiness manifested as a slow disintegration that created play in the transfer case output bearing. The play in the output bearing then destroyed the transfer case housing. At 110,000 miles it developed enough slop that the driveshaft yoke suddenly contacted the corner of the the 4WD actuator while rotating at 70 mph. BANG!

End result = $1300 for a rebuilt transfer case + $400 for 4wd actuator + $100 for rebuilt driveshaft + labor and fluids.

The real pisser here is that my transfer case failed not because of use/abuse but because of a cheap u-joint and weak transfer case housing. Failure was caused by rotating in 2WD with NO LOAD on the front driveshaft. Almost seems like they added weakness in areas that were never a problem before.


Matt

 

[DiploStrat]

Wait, you got over 100,000 miles on a U-Joint and you're complaining? And that on a non-greasable factory joint? That's amazing.

 

[snowblind]

Wait, you got over 100,000 miles on a U-Joint and you're complaining? And that on a non-greasable factory joint? That's amazing.

Got that and more out of many u-joints in the past. The two on my rear drive shaft that get power 100% of the time are still good. Should I rebuild the rear driveshaft right away? The other front u-joint was solid also. And when the old u-joints went "bad" they didn't take the transfer case with them. Maybe if they did I would be in the habit of proactively changing them.


Matt

 

[DiploStrat]

Who knows? Back in the '70's I could only get a year or two out of U-Joints on the rear, less than that on the front. To be fair, it was a lifted truck with full time 4x4. Still, I think I might be redoing the U-Joints at 50,000 miles on principle.

I have a diesel 3500, part time 4x4. May be that I will get a much longer life than I expect. Sounds like the quality has really gone up.

I am intrigued that a U-Joint not under load could fail.

 

[Rockhounder]

Same exact friggin thing happened on a 1997 1500 4wd suburban we owned. Was driving on the 210 fwy out by upland, in 2wd, no load, just cruising, and suddenly BAM! from under our feet. We pull over, see the stupid ALUMINUM housing cracked wide open, pouring out tranny fluid...... No lead-up noise, no warning......

We had about 200k miles on it then.

 

[snowblind]

I am intrigued that a U-Joint not under load could fail.

That is the $2500 question now isn't it? ;-)

Here's the deal. ANY vehicle that has "shift-on-the-fly" four-wheel-drive MUST keep the entire 4WD system engaged at ALL times. Doesn't matter if it's a 72 Blazer or a 2001 Suburban. If I want to shift from 2WD into 4WD at 30mph my front drive shaft and front axles MUST be spinning at 30mph to engage. "Part-time 4x4" on your 3500 means that the front half of the 4WD system is not being driven by the transfer case. It is however still connected to the transfer case and is being "dragged" up to speed by the front wheels/axles/diff/driveshaft. So 30K on a 4WD truck essentially is 30K IN 4WD.

Back to all my other u-joints being fine but the one that failed. U-joint is "Powertrain" right? As in 100k Powertrain Warranty. One of the more durable areas usually... In this instance I would say the one U-joint was a "bad-egg" but it sure is an expensive one. It would probably have cost less than $500 to proactively rebuild both driveshafts on my truck. :-(



Matt

 

[snowblind]

Same exact friggin thing happened on a 1997 1500 4wd suburban we owned. Was driving on the 210 fwy out by upland, in 2wd, no load, just cruising, and suddenly BAM! from under our feet. We pull over, see the stupid ALUMINUM housing cracked wide open, pouring out tranny fluid...... No lead-up noise, no warning......

We had about 200k miles on it then.

From what I can tell the "modern" transfer cases use strong chain-driven mechanisms similar to the old cases but the CASES themselves are much lighter but much less durable.


Matt

 

[82fb]

$1800 for tcase, actuator and driveshaft? Do they not have junkyards near you? Seems like $250 would have had you back on the road.

 

[underdrive]

Here's the deal. ANY vehicle that has "shift-on-the-fly" four-wheel-drive MUST keep the entire 4WD system engaged at ALL times. Doesn't matter if it's a 72 Blazer or a 2001 Suburban. If I want to shift from 2WD into 4WD at 30mph my front drive shaft and front axles MUST be spinning at 30mph to engage. "Part-time 4x4" on your 3500 means that the front half of the 4WD system is not being driven by the transfer case. It is however still connected to the transfer case and is being "dragged" up to speed by the front wheels/axles/diff/driveshaft. So 30K on a 4WD truck essentially is 30K IN 4WD.

No, a SOTF vehicle must have the front driveline at vehicle speed before it starts transmitting torque thru it. Doesn't mean it needs to be up to speed ALL the time, just that in needs to be BROUGHT up to speed before torque flow happens.

The way GM used to accomplish this was via a rather complicated setup. First when you shift the thing into 4x4 an electromagnetic clutch spins up the gears and chain inside the t-case and syncs their speed to the rear output shaft before the fork slides and couples them mechanically, this also brings the front ring and pinion up to road speed. Then some trucks (halfton and smaller IIRC) had a vacuum signal sent to a servo motor up front to pull on a cable to engage the center axle disconnect (CAD) mechanism and lock the passenger side axle shaft (well, the intermediate shaft inside the axle housing) to the stub shaft coming out of the differential, while larger trucks IIRC used an electric actuator for the CAD. So what happens in 2wd is that yes your wheels are spinning the axle shafts, but the passenger-side shaft only spins the intermediate shaft which is just sitting there NOT connected to the differential, the driver-side axle shaft is in fact spinning its respective gear inside the diff but because the other side (stub shaft for the CAD mechanism) is free to do as it pleases it ends up spinning at the same speed as the driver-side gear but in the opposite direction. Overall the differential's body, and respectively the ring, pinion, and front driveshaft, all remain still and thus incurs no wear.

The exception to that were AWD vehicles - those had a special transfer case, the front axle didn't have the CAD mechanism, and the intermediate shaft was longer and now connected the passenger-side axle shaft to the diff itself. Those trucks do in fact turn the front shaft at full speed all the time regardless of how much actual torque flows thru it, and thus are subjected to the wears you describe.

AWD is obviously not part-time, however part-time can and often is shift-on-the-fly. Don't think I've ever seen an older AWD 3/4-ton, but most of the part-timers were SOTF-capable...

What I'm getting at is that one needs to know what kind of SOTF system their vehicle has, and if it has a CAD and the front driveline is still turning full speed even when in 2wd mode then that would be an indication of a problem with the CAD.

 

[2025 deleted member]

That t-case was on borrowed time anyways. Its a magnesium case. Look up "gm pump rub" real common, happened to my 2002 2500hd.

 

[snowblind]

$1800 for tcase, actuator and driveshaft? Do they not have junkyards near you? Seems like $250 would have had you back on the road.

$250 eh? The core-charge for the rebuilt T-Case alone was $465.


Matt

 

[snowblind]

No, a SOTF vehicle must have the front driveline at vehicle speed before it starts transmitting torque thru it. Doesn't mean it needs to be up to speed ALL the time, just that in needs to be BROUGHT up to speed before torque flow happens.

On my truck it was at speed all the time. Maybe not the exact same speed but very close to it. When I shift into 4WD at 40mph it happens with no drive line noise and in less than a second. Very hard to do that if everything isn't close to speed already. Overall design isn't that different than used in my 1972 Blazer were if the hubs were locked you were spinning the whole front driveline. Big difference is how the front hubs/axles are locked/unlocked to the front driveshaft.

The way GM used to accomplish this was via a rather complicated setup. First when you shift the thing into 4x4 an electromagnetic clutch spins up the gears and chain inside the t-case and syncs their speed to the rear output shaft before the fork slides and couples them mechanically, this also brings the front ring and pinion up to road speed. Then some trucks (halfton and smaller IIRC) had a vacuum signal sent to a servo motor up front to pull on a cable to engage the center axle disconnect (CAD) mechanism and lock the passenger side axle shaft (well, the intermediate shaft inside the axle housing) to the stub shaft coming out of the differential, while larger trucks IIRC used an electric actuator for the CAD. So what happens in 2wd is that yes your wheels are spinning the axle shafts, but the passenger-side shaft only spins the intermediate shaft which is just sitting there NOT connected to the differential, the driver-side axle shaft is in fact spinning its respective gear inside the diff but because the other side (stub shaft for the CAD mechanism) is free to do as it pleases it ends up spinning at the same speed as the driver-side gear but in the opposite direction. Overall the differential's body, and respectively the ring, pinion, and front driveshaft, all remain still and thus incurs no wear.

Your description of the system sounds right but I think the un-driven components are spinning pretty fast. My FRONT drive shaft was in 2WD @ 70mph when it rotated with enough force to physically tear a bolt out of the transfer case. And the u-joint and output bearing on the FRONT of the transfer case are what wore out. Surprising in a system were the "front driveshaft, all remain still and thus incurs no wear." My 3/4 ton truck also supports "Auto" 4WD where I believe the transfer case is fully engaged and the then the CAD is engaged/disengaged with input from the ABS system.

What I'm getting at is that one needs to know what kind of SOTF system their vehicle has, and if it has a CAD and the front driveline is still turning full speed even when in 2wd mode then that would be an indication of a problem with the CAD.

I will be able to check tonight when I pick up my truck. I will have my lady drive slowly while I see if the front driveshaft is moving. At slow speeds I would expect zero movement if disconnected via the CAD or a slightly delayed movement if responding to indirect rotational forces.


Matt

 

[snowblind]

That t-case was on borrowed time anyways. Its a magnesium case. Look up "gm pump rub" real common, happened to my 2002 2500hd.

I know. I checked for leaks regularly. Still... a small hole/leak would have been preferable to a dangerous failure at speed. And it wouldn't have taken the 4WD controller with it also. The rebuilt cases fix the pump rub issue with a reinforcement or by using aluminum in place of mag for the tailpiece.

 

[underdrive]

Your description of the system sounds right but I think the un-driven components are spinning pretty fast. My FRONT drive shaft was in 2WD @ 70mph when it rotated with enough force to physically tear a bolt out of the transfer case. And the u-joint and output bearing on the FRONT of the transfer case are what wore out. Surprising in a system were the "front driveshaft, all remain still and thus incurs no wear." My 3/4 ton truck also supports "Auto" 4WD where I believe the transfer case is fully engaged and the then the CAD is engaged/disengaged with input from the ABS system.

Yeah I guess I should have said that they should remain fairly still. To be honest I never liked that system, it relies on torque generated by the wheels taking the path of least resistance, which as you found out isn't always the one the engineers intended. Still tho, it takes quite a bit of power to rotate the driveshaft via the ring and pinion, which is why I find it concerning that your front driveline was spinning at such high speed. It still sucks that you had to deal with all that carnage, but I'm thinking it's not normal and there is something else that could be the cause of it. Just trying to save you some future aggravation is all...

I will be able to check tonight when I pick up my truck. I will have my lady drive slowly while I see if the front driveshaft is moving. At slow speeds I would expect zero movement if disconnected via the CAD or a slightly delayed movement if responding to indirect rotational forces.

This may be a dumb idea, but how about taping a phone to the frame rail nice and good, then going for a faster drive? Then you can play with the different modes of the 4x4 (2wd, Auto, and 4hi) and see exactly what the driveshaft does in each of them? I'd imagine this will be more productive as the GM frames tend to hide the front driveshaft quite well, so I'm not sure how much you'll be able to see of it while just walking next to the truck with someone else driving it.

 

[Watt maker]

I'm with underdrive on this one, that front shaft shouldn't be spinning enough to do that much damage. You could also just jack the front of the truck up until the wheels are off the ground and then try to turn the front shaft by hand. I would try it both running and with the engine off. In 2wd, you should be able to freely spin the shaft without the tires turning. If the tires turn in 2wd, the CAD may be jammed up or getting a signal from somewhere to engage. If you can't turn the shaft at all, it is engaged in the transfercase.