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Gear Ratios and Transfer case failures - related/unrelated


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So, working on my problem child again..  2008 MC 3500 SRW 6.7L 6MT 4WD..
Original tcase took a dump, due to a randomly-engaging 4wd motor.  Truck has 150k / 6400hrs on it currently.
Rebuilt the 273D tcase, replaced all gears, seals, bearings, chain, bushings, etc.  shafts were fine.   7 moths later, it **** the bed, with the motor trying to engage 4WD randomly, regardless of speed; switch in 2WD position.

Tried sourcing a local 271D and convert to manual shift.  Couldn't find a 271D but found a 271F.   VERY minimal differences in the two, with one being shift pattern and the other being the output flange/shaft.
rebuilt 271F using the D's shaft.  Installed manual shifter, boot, etc.  Worked like a champ, using all new parts..   that was 4/19.   Welp, here we are again..  4WD mode, apply  engine load/torque, and it sounds like the chain/sprockets are jumping teeth..  violently..  It doesn't take much pedal pressure, either..  If steady moving, 2nd gear, couple miles/hr, no pops.  let off the throttle to prepare to stop, and pop-pop-pop from the tcase..   Trying to figure out why this truck keeps knocking tcases out, I decided to jack a wheel up, turn/count revs to quickly determine axle ratios..

 

 - Back Story - My buddy bought this truck used about 5 yrs ago.  At that time it had about 75k miles on it.  Wanted to do exh, DPF/EGR delete, tuner, etc..  I opened the hood, and I find every wiring harness retainer (from radiator to the transfer case) not fastened..  My first thought was someone's had this drivetrain out before..  The Tcase was also repainted silver, so I assumed it was rebuilt.. 
  Since then, it's had a clutch replacement, leaks fixed, transfer case rebuilt 2x, replaced once, injector set replaced (noticed a non-stock bolt on the #3 rocker section), radiator, AC condensor, rear door jam harnesses repaired, blower motor, resistor block, main window switch, and various other things fixed on this POS.  I told him to sell it, but he's in too deep on it.

 

Back on topic,  jacking the LF wheel up, trans in neutral, I rotate the tire one rev, and the Fr DS rotates about 1.6 times.   Same for rear.  Obviously this is an incorrect measurement.  Do BOTH wheels need to be rotated for a proper count?  I'm trying to avoid knocking off the diff covers if at all possible.

Also, the driveshaft's PN (why is this piece a sticker??) is illegible to read.  I'm assuming this truck has 3.55s  I wanted to check them because with everything else that was shady AF with this truck, I wonder if it's got 2 different ratios in it.   In 4HI, on gravel, it seemed to track out fine, with no weird hopping/marking of tires in gravel, so I think they're the same ratios.

 

The driveshaft, from the rear-most portion of the output shaft seal, to the lip on the driveshaft is about 2-2.5 in.  My 07 QCLB DRW has same looking shaft, but much longer space between same two points.  I know, different trucks, etc..  But I'm starting to wonder if the driveshaft in this truck could be too long, knocking the *** out of the transfer case, causing the death to the TCase..  I'm grasping at straws, at this point..

Thoughts?   Suggestions? 

Edited by Rogan
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9 hours ago, Rogan said:

Do BOTH wheels need to be rotated for a proper count?  I'm trying to avoid knocking off the diff covers if at all possible.

 

To answer your question - yes, both wheels need to be rotated or one wheel needs to be rotated two revolutions.  You can also do it another way.  Jack up one front wheel just off the floor.  Using a marker of your choice, mark the bottom of the tire near the tread and place another mark on the floor.  Have someone hold the tire in that position while you take up the slack in the front driveshaft.  After taking up the slack, mark the end yoke and the  pinion case for a reference point.  Now rotate the drive shaft 3.55 turns and check front wheel position.  If the front wheel has turned two full revolutions, then you have 3.55 gears.  If not, continue rotating the driveshaft until 3.73 turn have been completed and check again.  Or, 4.10 turns - you get the picture.

 

- John

Edited by Tractorman
Can't ever get it right the first time.
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Second pulling the covers, especially if you have never seen inside before. I'm unsure on the AAM axles, but the Dana's have the ratio on a stamped metal tag on the outside. It is possible that someone swapped the gears or the axles. 

 

Can always put it in 4lo and have someone idle it straight in gravel and look for any slipping. Depending on if the rear is limited slip or open/locker will determine whether both wheels need to spin. Spinning the driveshaft is an option.

 

Has the output shaft on the trans been checked? Also, a little basic trigonometry will tell you if the driveshaft is too long.

 

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16 hours ago, Rogan said:

So, working on my problem child again..  2008 MC 3500 SRW 6.7L 6MT 4WD..

  I'm assuming this truck has 3.55s  I wanted to check them because with everything else that was shady AF with this truck

 

 

 

 

Third gen AAM axles didn't use 3.55's, only 3.73 or 4.10. With the 6spd manual, my guess is 3.73's, 4.10's are really slow with one overdrive. I think the 3.42's started with the 4th gen. I would also drain your t-case and look for metal contaminated oil.

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2 hours ago, NIsaacs said:

 

 

Third gen AAM axles didn't use 3.55's, only 3.73 or 4.10. With the 6spd manual, my guess is 3.73's, 4.10's are really slow with one overdrive. I think the 3.42's started with the 4th gen. I would also drain your t-case and look for metal contaminated oil.

3.55s was a guess..  I found out after the fact that it's 3.73/4.10..    It is a 6MT so it's listed as 3.73.
I'll drain the t-case soon, but wanted to test everything else I could before doing that,
It's really weird that this same 'symptom' keeps coming back, even with a replacement t-case. 
I'll drop the front diff cover, maybe today, to see what that looks like..  I don't think it will be a diff, but you never know.  It did this same noise before I did the t-case conversion, and didn't do it after I swapped them..  Until now.
I also found out that the MCSB SRW is 160.3" wb, and the QCLB DRW is a 160.5" wb.  so I have a struggle thinking that even a DRW driveshaft ( 2/10ths inch longer) would be long enough to cause damage, if it has been swapped.

 

I feel sorry for my buddy, because every time he turns around, he's running into some issue with this truck..  So I'm trying to help him out as much as I can.

9 hours ago, That Guy said:

Second pulling the covers, especially if you have never seen inside before. I'm unsure on the AAM axles, but the Dana's have the ratio on a stamped metal tag on the outside. It is possible that someone swapped the gears or the axles. 

Will do.

 

Quote

Can always put it in 4lo and have someone idle it straight in gravel and look for any slipping. Depending on if the rear is limited slip or open/locker will determine whether both wheels need to spin. Spinning the driveshaft is an option.

Did this last weekend.  Saw no slipping/chirping.

 

Quote

Has the output shaft on the trans been checked? Also, a little basic trigonometry will tell you if the driveshaft is too long.

I plan to check it for excessive movement, once I pull the t-case.  I don't think the DS is of issue, but I will examine this further, once I tear into the t-case removal.

Thanks for your info, guys!

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Sleeping on it, I remember reading somewhere about that weird double ujoint front driveshaft killing xfer cases somehow when it starts going out. I have tried to find it again to no avail, you may have better luck. 

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  • 2 weeks later...

UPDATES:

  1. Front diff assembly looked fine, no play.  fluid was super clean, no metal anything came out.
  2. Does not 'pop' when no front driveshaft is in (duh...)
  3. Front driveshaft is timed, u joints and cardan joint is fine..  *one* u-joint cap is spinnable by hand, which is odd, but not issue-causing
  4. Rear driveshaft is fine, true, not bent, no bad joints (it's one-piece DS)
  5. Rear output shaft seal leaks a tad; it's just over a year old, since t-case rebuild.
  6. Removed both shafts, removed tailshaft housing and snapring
  7. Can't get the case to separate; Permatex "Right Stuff" is refusing to let go.  Tried driving a putty knife in the seam, pry bars, mallets, etc.    Not gonna come apart.. 
    **Ordered a couple GM tools designed to separate case-halves , gonna try to get them to 'work' when they get here..

Tools should be here about Tuesday or Wednesday.  They're regularly about $300ea, but bought them (2) for about $13ea from a GM dealership closed down and liquidating tools/parts.

Truck showed up with a laundry list..

  1. Low on full-throttle power
    Truck runs  RaceMe tuner, EGR/DPF delete, stock airbox, 5" TBE
  2. Rear Door locks, rear power windows, and one rear speaker not working
  3. Transfer Case banging/popping
  4. Rear parking brake inop
  5. Random door lock/unlock, window clicking, etc when it rains.

Fixes:

  1. air filter was beyond nasty.
     - Replaced filter, test drive showed crazy power has returned!
  2. LR door harness had 7/8 wires broken, RR door harness 5/8 wires broken
     - Rebuild door harnesses; all doors function as intended
  3.  Still troubleshooting
  4. LR park cable sheathing was busted, equalizer adjuster under left rocker was max-adjusted
     - replaced LR park cable, adjusted brake shoes, re-adjusted equalizer.
  5. Apparently driver's window rolled down a tad while driving, and rain comes in, drips on master switch, viola!  Causes issue!
     - roll window up when it rains ;)

 

So t-case is all that's left..  again, just waiting on tools to see if they'll split the tcase.

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