2015 Suburban Lost 4th Gear

SmokeShow

Well-known member
Nov 30, 2006
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Accelerated hard to make a turn at a light before it turned yellow/red and then immediately let out. When I let out it had just shifted 3-4. It tied up or something and shifted so hard it chirped the tires. I didn’t realize it then as I slowed to make another turn almost immediately after that light. Once I was done in a parking lot and attempted to accelerate once back on the road, shifts 1-3 were a little funky but then when it shifted to 4th there was nothing - free revved. Tried to manually shift it past 4th. Nada all the way thru 6th. Pull over. Try a restart in case it was in some sort of limp mode that needed a “key cycle” (although it’s push button so idk). No dice. Still free revs immediately upon shifting to 4th. Limp it home manually shifting up to 3rd. Once home I try 5 more start & shut off cycles, again, just in case it takes a few “key cycles” to clear it if it’s in limp mode like our trucks. Nope, still free revs when it shifts to 4th. No lights have shown up and trans isn’t getting hot.

Broke a shaft is my assumption. Thoughts?
 

NC-smokinlmm

<<<Future tuna killer
May 29, 2011
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Get the codes, they are in the TCM or bcm. My 6l80 did some weird shit once and I cleared the codes, it was a generic p700 and another for clutch engagement or some thing like that. I'm willing to bet once cleared your good.
 

SmokeShow

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Nov 30, 2006
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Took it in last weekend. Warranty Co. required them to do a fluid and function test. Fluid came back "burnt" and had "excessive metals" in it and technician confirmed no 4-5-6 gears. I don't think they tore it down any further than that. A brand new GM transmission with 3yr-100k mile warranty was swapped in. It was done yesterday morning, picked it up today and it shifts and acts fine. :thumb:
 

NC-smokinlmm

<<<Future tuna killer
May 29, 2011
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Crazy, I run 850 crank thru mine stock, only tuned spinning 22's and 33"s and pound on it daily in awd and your dies a stock death...:spit:
 

KyleC4

Tech
Dec 30, 2016
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Bay Area
Probably smoked the 456 clutch due to converter clutch worn out and scored the pump housing. ‘16 or 17 model years had an issue with snapping the 456 hub clean off the shaft to give similar symptoms but no burnt clutch smell. Good thing they got you a SRTA now. Hopefully tech flushed and flow tested cooler while he was there
 

SmokeShow

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Nov 30, 2006
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They mentioned knowing full well that they needed to thoroughly flush the cooler. :thumb:

Think I should collect and send a sample to a lab? maybe a few hundred miles apart so that any "break-in" material isn't skewing the results?

The tech was not in when I picked it up so I wasn't able to ask them, but what is a proper "break-in procedure" on a brand new transmission? I test drove it before I paid in which I went through stop-n-go traffic as well as about 75% throttle acceleration through about 5th gear. Speeds and traffic didn't allow WFO through all gears on the test drive. AND, IDK if that was even recommended yet? :confused:
 

Mike L.

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Aug 12, 2006
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Dealer does the reprogram and sets adapts. Normal driving for a day or so will be fine.
456 clutch pack has an uneven apply piston. It is not flat and will cause uneven wear on the first 3 clutches above the piston.
 

Mike L.

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I read that from one of your guys on here Mike. I forgot about the issue with the newer 6l80s. Is it all of them?

I built a 6L90 for a 750 hp CTSV using an Alto clutch pack and told them it would never work. They said they never had a problem with the setup. Well, the 456 clutches failed. They came down to the shop to verify. They agreed. Then they fixed it with a thick flat steel at the bottom. No problem since.
 

JoshH

Daggum farm truck
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I built a 6L90 for a 750 hp CTSV using an Alto clutch pack and told them it would never work. They said they never had a problem with the setup. Well, the 456 clutches failed. They came down to the shop to verify. They agreed. Then they fixed it with a thick flat steel at the bottom. No problem since.
Mike, have you ever tried the Sonnax 4-5-6 apply piston kit? I'm getting ready to build a 6L80 in a few weeks, and I plan on using that kit.
 

Mike L.

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It should work fine. That was not available when I did mine. I would also do the aluminum valve body plugs as they are known for leaking like a sive.
 

JoshH

Daggum farm truck
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It should work fine. That was not available when I did mine. I would also do the aluminum valve body plugs as they are known for leaking like a sive.
I also have a zip kit that comes with o-ringed end plugs.