6L80e Breaking Necks

[ckruzer]

Hey fellas. Hope some of you Ls gurus can help. The 2012 Tahoe (5.3 6l80 LTZ 4wd) was dropped off a few weeks ago and got the transmission rebuilt. It was refusing to go in 1st or Reverse, unless you let the RPMs climb high enough and then it would slam into gear.

The trans shop rebuilt it and replaced the clutches and a shift fork. They replaced the torque converter as well. Supposedly some kind of solenoid as well, and also had it "programmed".

On pickup, when it was above 150ish degrees, it would do the same thing but not so drastically. Only 3 times or so that day amongst many city stops. Almost like it did not have enough pressure for first gear until the rpms would build then it would lurch forward, is the symptom. We were told to drive it for the weekend to see if the computer relearned itself. Towards the end of the weekend, if the trans was hot (>150), it lurched into first every single stop. And when coming to a hard stop there was a delay in the final downshift and you would feel a shift lerch after the vehicle was stopped.

Dropped off and they rebuilt again and supposedly replaced the first reverse clutch gear cage thing which supposedly is associated with like symptoms and they found a micro crack in the pump and replaced the pump. We picked up and no symptoms. The next day we started getting the same symptoms again. Not as often, but still occasionally happening. Yesterday, after a few days of having it, its doing it even more often now. Lurching downshift at hard stops (post vehicle stop) and Lurching shift into first from a stopped position. This time in city traffic, the trans also climbed the highest ive ever seen at 219 degrees when it was 87 degrees outside. Typically Im used to seeing about 80-90 degrees above ambient before these problems arose.

They are willing to try again, and suggestion maybe the case is bad? Doesnt make sense, but im no expert. The way this "growing" post rebuild suggests something is being done wrong that is getting worse with driving? Just dont know what to do, and I dont know how many times they are going to try before they say "our warranty doesnt cover this anymore, it cant be the transmission" etc.

I appreciate any help you guys can lend. Thank you

 

[a_kelley]

No shift forks in an automatic.

Not sure on 6l80e construction, but I don't think it has any bands, making it entirely clutch driven. Leaking/nicked seals, leaking/cracked drum clutch drums/shafts (or drum to shaft interface) , leaking/cracked apply pistons, and/or cross leaks in the valve body can all cause exactly what you describe. Some cracks are so tiny hair line when not under pressure that they are almost invisible. Basically it's losing pressure to apply the clutches, until it's revved (higher line pressure) and then quickly engages. There are so many little details that can cause it, and common failure points vary to each particular transmission model.

I will say that if said shop has already pulled it twice and been in it twice - they missed/overlooked an element that's leaking line apply pressure. I do believe that 1/R are related, and if you look at an apply chart, it would highlight what elements are in use and then I would look at the other gears and narrow it down to the elements only applies in 1/R. Of course, whenever I go into a transmission, I try to figure out what's wrong with it before I take it apart - if I don't find what I expected, I know I need to look further. Also - clutch burn up is a result of low apply pressure... Just installing new clutches without fixing the cause is no more than a temporary kludge.. kinda like changing a fouled spark plug when the compression is low on that cylinder.. just a temporary patch.

Hope that helps. I put you in touch with my local trans shop... Brian can probably tell you right off what's likely wrong. Or a list of things anyhow.