6L80 issue

[Iflyfish]

I tried to post this in the 6L80 thread but could not.
my son has an 06 Yukon Denali AWD with the 6L80 transmission (200,000+ miles) I’ll try to keep this brief. His issues started with the transmission “going into neutral“ while he was driving. Not literally but it would just go out of gear. No sounds, bumps etc. Just out of gear and coasting. After a few minutes beside the rd with the engine off it would start and go a few miles and do it again.
We did some searches and saw several possible issues. He replaced several vacuum hoses and checked connections. No change.
we replaced the TCM and fluid. the magnet had the normal tiny flakes of metal but nothing of any size and to was very little. We had it programmed at GMC dealer. It started but would not go in gear at all. Towed it home and dropped the pan and checked connections and linkages. Couldn’t find any issues but it still won’t go in gear. It will go into park and neutral. Dropped the TCM again and followed the troubleshooting guide in the 6L80 thread. It appears to be good.
Every shop we call says replace the transmission. One trouble shot on the phone and said the torque converter has come apart, change the transmission. It may very well need a new transmission but we want to make sure it’s not something external to the transmission or something we can fix without spending the $$ of a complete replacement.
Any advice/recommendations/suggestions would be greatly appreciated.Thanks.

 

[j91z28d1]

I'm no tranny guy, but from what I've seen on here they throw error codes. do you have a scanner that will read the tcm trouble codes? might lead you to something

 

[Doubeleive]

I tried to post this in the 6L80 thread but could not.
my son has an 06 Yukon Denali AWD with the 6L80 transmission (200,000+ miles) I’ll try to keep this brief. His issues started with the transmission “going into neutral“ while he was driving. Not literally but it would just go out of gear. No sounds, bumps etc. Just out of gear and coasting. After a few minutes beside the rd with the engine off it would start and go a few miles and do it again.
We did some searches and saw several possible issues. He replaced several vacuum hoses and checked connections. No change.
we replaced the TCM and fluid. the magnet had the normal tiny flakes of metal but nothing of any size and to was very little. We had it programmed at GMC dealer. It started but would not go in gear at all. Towed it home and dropped the pan and checked connections and linkages. Couldn’t find any issues but it still won’t go in gear. It will go into park and neutral. Dropped the TCM again and followed the troubleshooting guide in the 6L80 thread. It appears to be good.
Every shop we call says replace the transmission. One trouble shot on the phone and said the torque converter has come apart, change the transmission. It may very well need a new transmission but we want to make sure it’s not something external to the transmission or something we can fix without spending the $$ of a complete replacement.
Any advice/recommendations/suggestions would be greatly appreciated.Thanks.

must mean 08
having it rebuilt properly with all the updates and fixes is a much better option and usually slightly cheaper than a already rebuilt one that was done just to oe spec only.

 

[Iflyfish]

I'm no tranny guy, but from what I've seen on here they throw error codes. do you have a scanner that will read the tcm trouble codes? might lead you to something

P0700 transmission control system (MIL request) and P0777 Solenoid B Stuck on.

 

[Iflyfish]

must mean 08
having it rebuilt properly with all the updates and fixes is a much better option and usually slightly cheaper than a already rebuilt one that was done just to oe spec only.

I think that would be a good option. But we want to make sure it’s the transmission first (if we can).

 

[Doubeleive]

I think that would be a good option. But we want to make sure it’s the transmission first (if we can).

you can jack it up so all wheels are off the ground get under it and click it into gears manuallly if it doesn't go then there are internal issue's going on
which could range from tcm not relearned properly to anything from a-z
a scaner like a tech2 may reveal something, maybe depending on the issue
I assume you checked already but be sure the wheels turn and you don't have a blown transfer case or differential locking it up

 

[j91z28d1]

p0777 is definitely a fault, but I don't know forsure from reading it what it would be doing.

all I see is. the TCM detects an incorrect off-going clutch gear ratio, or tie-up, when the 3-5-R clutch is commanded off for 1.2 seconds and the transaction input shaft speed is greater than 40 RPM from the anticipated input shaft speed.




sounds to me like it should still move. I think you're right about the tcm program. especially since it moved before they changed it and programed it.

do you happen to have the old tcm still? if you go back in to replace solenoid 2. maybe put the old tcm back in?

 

[Iflyfish]

p0777 is definitely a fault, but I don't know forsure from reading it what it would be doing.

all I see is. the TCM detects an incorrect off-going clutch gear ratio, or tie-up, when the 3-5-R clutch is commanded off for 1.2 seconds and the transaction input shaft speed is greater than 40 RPM from the anticipated input shaft speed.

View attachment 419396


sounds to me like it should still move. I think you're right about the tcm program. especially since it moved before they changed it and programed it.

do you happen to have the old tcm still? if you go back in to replace solenoid 2. maybe put the old tcm back in?

Does the TCM need a “relearn” after programming? The dealer only did a “programing”. I didn’t get a real warm feeling after dealing with the GMC dealership but I don’t know enough to say they did not program it correctly. Had to give the old TCM up as a “core”.

 

[donjetman]

Does the TCM need a “relearn” after programming? The dealer only did a “programing”. I didn’t get a real warm feeling after dealing with the GMC dealership but I don’t know enough to say they did not program it correctly. Had to give the old TCM up as a “core”.

Always include the resident expert @NickTransmissions when asking question(s) about transmissions.

 

[NickTransmissions]

His issues started with the transmission “going into neutral“ while he was driving. Not literally but it would just go out of gear. No sounds, bumps etc. Just out of gear and coasting. After a few minutes beside the rd with the engine off it would start and go a few miles and do it again.

What happened when you tried to post in the 6L80 thread? There isn't any restrictions beyond simply having to be a member to post in there...maybe the site or underlying host cloud platform glitched out...

Re your son's transmission: P0777 is clutch pressure control solenoid #2 stuck on which is off in 3rd, 5th and Reverse so if this persists through the drive cycle, you'd have no 2-3 upshift 4-5 upshift and no reverse, unless the fault is intermittent. False neutrals while in a forward range setting at speed are the result of a complete loss of line pressure - question is why? read on...

we replaced the TCM

What TEHCM did you purchase and from whom - was it the GMC Dealership that did the installation and programming work?

Does the TCM need a “relearn” after programming? The dealer only did a “programing”. I didn’t get a real warm feeling after dealing with the GMC dealership but I don’t know enough to say they did not program it correctly. Had to give the old TCM up as a “core”.

Good question - yes, the TEHCM must have a fast-adapt relearn regardless of whether any reprogramming was done...Reprogramming overwrites the pre-existing OS software version with the version that corresponds to your vehicle so the new TEHCM communicates correctly with the rest of the vehicle's CANBUS system and ECM, otherwise they won't play nice with each other...Once the programming is done (usually off-vehicle), the TEHCM has to have a fast relearn done to reset it's shift adapts and strategies to factory default settings.

The unit has over 200,000 miles on it so, regardless of the outcome of your diagnosis, I would start planning for an overhaul or replacement as it's on borrowed time...That said, replacing the transmission right this minute may premature, given the info you provided...It needs a full diagnostic work up to know if we should condemn it or not...But first, perform a fast adapt relearn; clear any codes before doing the fast adapt...If the fast adapt completes successfully, take the vehicle on a shake-down cruise with the scan tool plugged in, paying very close attention to the transmission data streams...Specifically, you want to look at the following:

1. Make sure all displayed upshifts match reality and that the gear the computer thinks the transmission is in is actually the gear it is in
2. Observe the converter efficiency, slip speed (RPM) at all duty cycles, especially when the TCC is applied at highway speeds - if the TCC slip speed is 5RPM or less at 90%+ duty cycle, the converter is fine (though the JMBX factory converters are pieces of crap and love to fail, as I'm sure you know)
3. Observe shift speed for each gear - all should be completed within 1 sec or less - anything more should be flagged for further investigation
4. Observe operation of the pressure switches and solenoids - should be all normal; no DTCs should set
5. Pay attention for any drivability symptoms and see if you can freeze-frame and post images of the live data feeds when a symptom presents

If while driving, you experience false neutrals again, especially if in 3rd or 5th, and P0777 sets again, pan must be dropped and TEHCM removed for bench testing. If you see a whole bunch of black clutch material at the bottom, fluid smells burnt, etc simply put the pan back on w/a few bolts and yank it out for overhaul or replacement.

Let us know what happens or if you would like any more info...