4th Gear issue after rebuild

[DemonicDonut]

Hello all, I'm hoping one of you wizards can save my bacon.

2010 Yukon SLT 5.3 LMG

I recently did a rebuild on my 6l80. New torque converter (local TC shop, billet and high carbon clutch), sonnax zip kit, transgo shift kit, transgo unbreakable ring kit, sonnax slide pivot pin, new bell housing, pump stator, tehcm seals and diaphragms, pioneer rebuild kit (all but steels)

I didn't disassemble clutch drums as I don't have a good press to reassemble, so I didn't replace the seals on them. I air tested all pistons but maybe I didn't don't right?

I'm having an issue intermittently with 4th gear.

When shifting into 4th under low load (~30pct throttle) the shift will attempt but flare and go back down and repeat until the CEL comes on and P1751 - shift valve performance code is in the trans. I have what seems to be called "busy shifting"

I have a HP tuner MPVI3 and have some logs I can attach soon that I recorded when the issue was happening.

When it does the flare, I can let off of it and sometimes it will shift but the shift time is very high (1 second or more)

It will also do this when in 5th and downshifting into 4th with light throttle.

I've played with TCC lockup and shift pressures, which seems to have helped but it's still happening at times.

I've pulled the tehcm and valve body off. Opened it up, cleaned everything really well and made sure everything was assembled correctly, check balls in place etc. I used the correct gasket for my valve body year.

I didn't ohm the solenoids though, which I'm kicking myself for now.

The main weird thing is the intermittent aspect. It was bad and I adjusted pressures and it seemed to be resolved for over a week. We towed our TT to the beach and it did the busy shift 1 time out of 4 hours of driving on the way there. I adjusted oncoming and off going pressures slightly as well as shift pressures (slightly increased low load section of tables) and on the way home it never had a problem.

The next day it did it to my wife and I played with programming again, it drove fine for me afterwards and now it happened to her again.

I'm at a loss for what it could be, unless the adaptives are taking too much pressure out of the 3-4 and 5-4 shifts over time. But towing was perfect?

Is there something in the programming I need to change with the new TC or parts I changed out?

Any help would be great. Feel free to ask questions. I'm working but have a slow day and will check here throughout the day. I'll be adding the logs as soon as I can.

Thanks for the help.

 

[LsHart]

Man, that sucks. Hope someone here can help.

 

[donjetman]

@NickTransmissions

 

[DemonicDonut]

I'd like to note as well that this is a new issue, and wasn't present before the rebuild. I wanted to rebuild due to mileage (192k now) and some occasional harsh shifts and shuddering TC. The shifting has been very smooth other than these flares and the TC shudder is gone.

I'm trying to upload the files but they aren't supported. Do I need to upload to dropbox or Google Drive etc and share the link?

 

[NickTransmissions]

I'd like to note as well that this is a new issue, and wasn't present before the rebuild. I wanted to rebuild due to mileage (192k now) and some occasional harsh shifts and shuddering TC. The shifting has been very smooth other than these flares and the TC shudder is gone.

I'm trying to upload the files but they aren't supported. Do I need to upload to dropbox or Google Drive etc and share the link?

I have a feeling you're going to be going back into the transmission to replace all of your piston seals/bonded pistons as that should have been done to begin with. You'll need to purchase the tools or bring your drums and new parts to a shop who will do it for you. Did you replace the turbine shaft seals?

Also, is the TEHCM brand new? If not, I'd replace it with an OEM GM TEHCM as well as IMS and speed sensor harness assembly for your vintage. Call the the dealer, speak to 'Parts' and provide the last eight digits of your VIN. They will sell you the TEHCM you need.

I usually don't mix Sonnax and Transgo components in these transmissions. Typically, I install the Zip kit but occasionally will use the Tow-N-Pro. Not saying this is the problem but I'd pour over the instructions for both kits again (if you haven't already) and see if there are any potential conflicts w/combining anything.

One other diag step, before taking the pan off, is to thread in a line pressure gauge, take readings at idle then drive it with the gauge installed (some folks will tape the gauge face to the wind shield so they can monitor pressures while still keeping their eyes on the road). If you see a pressure drop when shifting into 4th in normal 'D', no Tow Mode', you have a leak in the 4th gear apply circuit somewhere.

The only other thing I'd try is a full adaptive reset / programmatic reset to bring back the factory tune. You should have zero flare shifts at any throttle angle, road speed or driving situation if everything is mechanically sound in the transmission with a stock engine and factory program.