2007 with 6L80 and 6.2L code U0073-00 check engine and stabiltrak light

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West 1

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2007 with 6L80 and 6.2L code U0073-00 check engine and stabiltrak light

Hoping to find a quick solution to a ongoing problem. My 2007 Escalade has been a challenge this year. It dropped a valve seat, broke a piston and ruined a cylinder head. Got all that fixed. It was running amazingly well. I had to tow with this car, 6,000 pound trailer, it did the job very well but coming off a street light with this load when it went to shift from 2nd to 3rd and it went into neutral, check engine light came on and I coasted to a stop. I had my scan tool with me so I checked the code. U0073-00. Says lost communication with can bus A. I cleared it and drove away again. Same thing 1/4 mile later. I limped it home and called a tranny guy local and he said very common issue the drum inside the tranny has cracked and is expanding when it tries to pull the load which dumps all my line pressure. Sounded right to me based on the failure so a New Tranny installed, I did the install, he rebuilt the 6L80 with all brand new parts from GM or Transtar. Drum was new GM, he said it is improved so this won't happen any longer. The Module was not replaced but it was refreshed with the (Sonax) kit that has all new seals for the solenoids. After install it shifts great. After reset of the TCM you are supposed to drive easy and let it slowly reprogram. Did this with zero issues and it is shifting really well. After this initial break in, I try to accelerate harder, maybe 30% throttle, not hot rodding. It did the exact same thing it did before the new tranny install. Damn, out $2500 and a ton of labor installing for no change? I will also share the problem originated with towing the heavy trailer, coming off a light under lots of torque. I imagine the engine was twisting more than normal on the engine mounts, maybe pulling on some wires or grounds? The failure only happens when under load accelerating. Not race car stuff, just a little harder than normal stop and go driving. Never triggers going downhill. Always triggers on the 2-3 shift if pulling any harder than gentle.
The code clearly states loss of communication. Tranny guy now says it is the module. Another $1,000. Have not done that yet. I have a good scan tool with 2 way communication. I am trying to learn how best to test the Can Bus system. I am not a good electronics guy but trying to learn.

Side Note: Since both heads, the oil pan and starter and all been removed for the recent engine repairs I am wondering if a ground or other wire may have been pinched or left loose causing these CAN system error codes?
I sure wish I had walked this route before installing a new tranny. I think my old tranny was fine, I even studied the repalced tranny parts and they all looked good, even the old drum looked perfect, I think the Can Bus is the real issue, might be my module but could be many other issues.

So far I have tested the Can Bus wires for continuity and did a 5 amp load test on them. There are 4 Can Bus wires to the Transmission TCM. Two for high speed data and two for low speed data. One of the high speed wires shows 2.8V constant, I do not have a osciliscope to test with. The low speed Can one wire shows a 2.2V constant. Both of the other two Can wires are dead, no voltage, I don't know if this is normal or not. The ground at the TCM and the 12V+ are good, the 12V Acc Power is also good with the key on. Not much else I can share at this point.

I have seen others on this forum with the exact and similar issues. Hoping someone can share a common solution. Thanks
West
 

NickTransmissions

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2007 with 6L80 and 6.2L code U0073-00 check engine and stabiltrak light
Does this happen only on the 2-3 shift or do you have issues when in reverse or fifth gear?

Background
The 3-5-R drum is one of the two most common pattern failures in units produced prior to 2010, when they updated the design but these still leak just not as frequently (the other being the 1-2-3-4 piston cracking - also redesigned in mid-2010) but seeing UU0073 would have prompted me to bench test the TEHCM (I do that anyway with all 6L80Es I rebuild) to confirm/deny it was the source of the problem. I also test every single 3-5-R drum I install to ensure it's not leaking after I have it re-welded (i never buy a new drum, just have the existing one welded unless there's something else wrong with it). DTC P0776, a gear ratio error code, is most commonly associated with the 3-5-R drum leaking

Next steps
If your scan tool provides live data feeds from the ECM and transmission's TEHCM, monitor the feeds as you drive it and see if a shift is being commanded by the ECM/TEHCM and confirm/deny the shift is mechanically occurring. If there's any breakdown in communication between the ECM and TEHCM, your transmission would go into limp mode and the trans will default to either third gear if the failure happens when you're in 1st, 2nd or 3rd gear and 5th gear if you're in 4th, 5th or 6th gear then downshift as you come to a stop. If your speed sensors fail (input or output/vehicle speed sensor or "VSS" for short) you'd have speed sensor codes and harsh, late shifts or only first gear and reverse if the VSS totally fails.

So my questions for your transmission builder - did he
> Replace the belleville style return springs in the 3-5-R drum?
> Replace the snap rings for those belleville springs (as well as the eye-loop snap ring for the 4-5-6 clutch)?
> Leak test the drum once it was fully assembled?
> Retro an updated stator-pump cover assembly with the re-designed sealing rings w/support o-rings or use the updated GM sealing rings for the first design stator to correct inadequate sealing between drum and stator?
> see TSB here for more info
> Install either the Sonnax Zip kit or Transgo Reprogramming kit - both contain numerous updated/redesigned valves and springs to correct common problems with various valve trains in the valve body (upper and lower)?

I'd also want to know why he didn't test the TEHCM upon initial teardown and inspection as it sounds like he didn't otherwise he wouldn't have told you it's the TEHCM when the same symptoms returned.

Those questions would at least give a better idea as to what was done so we narrow down (or eliminate) mechanical problems w/the transmission as the cause of your problem.

Check out my 6L80 thread for more info and a complete parts list for overhaul when rebuilding these transmissions, if you haven't already.

Here's some additional context for anyone else following the thread:

GEARS Mag: Diagnosing CAN communication issues (no scan tool)
6L80/90 TSB - UU0073
6L80 Wiring Diagrams (attached)
 

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Geotrash

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You’re welcome, Joseph.

Wonder if any of that helped @West 1 at all.
I wonder that too. Hopefully he'll come back and honor you with a thoughtful response. But that's the great thing about forums like this. That kind of information isn't easy to come by because it's borne of a particular set of skills and experiences that few people have. But once it's shared as you so generously have, it's out there to help all future comers who are wrestling with the same symptoms and challenges. So I too, thank you. It's a gift to have your participation here.
 

NickTransmissions

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I wonder that too. Hopefully he'll come back and honor you with a thoughtful response. But that's the great thing about forums like this. That kind of information isn't easy to come by because it's borne of a particular set of skills and experiences that few people have. But once it's shared as you so generously have, it's out there to help all future comers who are wrestling with the same symptoms and challenges. So I too, thank you. It's a gift to have your participation here.
Would be nice, and even better if he ended up fixing it and shares what the problem(s) and solution(s) was/were. This way, he gives back to the forum and everyone can benefit...I'd even add a blurb to my 6L80 thread about it.
 
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