[jor]

09 Sierra LMM. Bone stock. 200k miles. 2 months ago I was on a 900 mile road trip about 300 miles in my truck felt like it started skipping or slipping. Started doing every 20-30 miles then progressively got worse. If feathered the throttle constantly it seemed to delay it happening. About 400 miles later after it started the engine light came on (P0729-00 P2771-00) and it would come out of gear and almost redline. It did that twice and the truck quit. I rolled off the off ramp and the truck wouldnt move at all. I would put it in gear and it would whine and not move. Leaking fluid from transfer case. I got it towed to a shop. Next day I showed up to get some of my stuff out and I started it for the hell of it. Put it in gear and it moved. They took a look said transfer case was shot so I had it replaced. Drove it around while I was at my destination. Drove it home 900 miles with no issues. About 2300 miles into the new transfer case it started skipping again. It only does it at cruising speeds and only after it is warmed up. Any help is appreciated.

 

[jor]

I understand you just got a new transfer case but what has your transmission maintenance been throughout the years? When your symptoms first where noticed, were you hauling anything or just at highway speed? Now ruling out the transfer case, which is definitely plausible initially, my next guess would be an issue with your torque converter

Thanks for the reply Bsimster. I've drained and refilled tranny fluid twice since I have owned it. At 100k and again at about 190k. It has 207k on it now. I was just cruising along about 75 not hauling anything when it happened. Was sudden out of no where. Thought I hit a bump the first time it happened. Almost feels like the truck just shuts off for a split second. It doesn't actually shut off just trying to explain what it feels like.

 

[jor]

First thing I thought was torque converter or really low on trans fluid, perhaps a leak from the trans or even a clogged trans filter. I wouldn't think it would be a transfer case, being that when in 2WD hi, it is connected directly to driveshaft via gears (or is it a chain, don't know which transfer case your model has). The surging is definitely the torque converter coupling/decoupling the trans with the engine, can't imagine why the transfer case was suspect and replaced for this issue.

Thanks for the reply. Anything you can think of that would make these symptoms and ruin the transfer case? Just curious because after the transfer case was replaced it ran flawlessly for 2500 miles.

 

[jor]

And to add to my last post, the "whine" gives it away! If it were the transfer case, that noise would be much more horrific than a whine!

It was a pretty bad whine/scratching sound 😆

 

[jor]

The noise you would hear from a broken transfer case would be more like stripping gears, with lots of loud bangs and crunching/grinding. A whine, it would not have at all, unless all the teeth were stripped off a few gears, but once that happened, you would not be able to ever move again. This is why I believe its a trans/torque converter issue. More than likely, fried the clutch pack in the trans and clogged up filter, or the torque converter is damaged. And, in either case, torque converter is probably toast from the clutch material. You should be able to see it in the fluid though, pull a sample, put it on a white paper towel, see if its pink or dark brown. If it smells burnt, its probably done. And if you see any, ANY(!!!!) metal particles on the paper towel (will look like the metallic flakes in a paint job), chances are its gonna need a full rebuild.

most times, this is all caused by lack of changing the trans fluid and not changing the trans filter on a regular basis. in your 200k, how many times has this maintenance been done?

Thanks, I'll definitely pull some and check it out this afternoon and post results. I said previously I did it twice but i forgot it was actually 3 times, once when my wife took it for an oil change at midas and well you know the rest of that story lol. Once at 100k (bought it with 60k) second time (midas) was prolly around 140-150 and 3rd was at 190k. Any idea why it wouldn't move and inch when I broke down but the next day it would move? I didn't drive it just put it in gear and it moved forward to the next parking spot. I'm just confused why once the transfer case was replaced it ran fine. Drove it from southern illinois to Canada with no issues straight through 14 hours and drove it for around a month before it started acting up. I don't doubt you 1 bit like you said it isn't the transfer case right now but i feel like whatever it is caused the transfer case to fail the first time. Could the torque converter cause that?

 

[jor]

Seems really strange to me too! What you initially stated sure sounded exactly like what I had with a Durango we bought brand new. The whine, the slipping in and out of gear feeling, and ultimately for us, the complete loss of movement forward or reverse. That trans cost us 3500 bucks to get going again, only to go out again 5 weeks later! More parts and another 2400 bucks later (long story on this one, I won't bore ya), it was back up and running again and we sold it as soon as we got it back!

I now keep an eye on trans fluid color and smell, those are the two biggest signs of a trans headin' south. There's a ton of images and videos online showing the discoloration of trans fluids, they can give you a much better idea on whether there's something going on with the trans.

As for replacing fluids, do you know if they did a "flush" or just a pan drop and replace removed fluid? I no longer do flushes at all, as its been documented as nauseum on how they destroy transmissions. Just did one on my 4runner last year that had a weird vibration-like slip into 2nd when warmed up. Dropped pan, replaced filter, cleaned magnets, replaced about 5 quarts of fluid, trans has been running perfect again with no slip at all for the last 7,000 miles.

Can't imagine your issue being electronically-related, as there wouldn't have been a whine, it would have either been in gear or out of gear. And to add to that, perhaps the shop you took it to when the case got replaced threw a quart or two into the trans to get it to move?

Have you seen any trans leaks at all in the last few thousand miles?

Sorry for the late reply got caught up. Yea they were flushes just filter changes and change fluid. As for them adding any tranny fluid they hadn't even looked at it I just showed up on my own to get my stuff. No leaks at all. I checked fluid and it doesn't smell and isn't off color at all.

 

[jor]

If your fluid looks good and doesn't smell burnt I would lean towards an electrical issue. Whenever a customer with an Allison comes in with transmission complaints I rarely suspect a transmission failure, most of the time it's either wiring related or speed sensor related. I can often find it by graphing data and finding the glitch. Your codes would point towards something going on with the speed sensor mounted on the transfer case. I doubt the shop replaced it when they installed the new transfer case. With the Allison, a complete failure is last on my list.

Thanks for the advice. Something to check out. Anything specific to look at on graphs? I have an autel scanner that gives me real time data. It isn't throwing codes at the moment. It only threw codes when it shit the bed before the transfer case was replaced. Has ran fine for 2000+ miles since then with no codes.