When DTC P0777 exists for a 6L80 - especially after a refurb or upgrade

iSky172 · April 6, 2019

Possible cause of DTC P0777 for a 6L80 transmission -

especially after a re-build, upgrade, or maintenance of the valve body

(a chapter from my upcoming book, "Learn from my mistakes!")

Here's the context:

2007 GMC Yukon Denali
6L80 Transmission (Early, 1st Design, original equipment, no prior work or refurbishment actions taken on this transmission/valve body/TEHCM (TCM) - just trans fluid change and filter change)
195,000 miles (not bad, eh?)
6.2 Liter L92 engine (Original Equipment, no serious work has ever been needed, never rebuilt - just well maintained)
Vehicle is - from an interior perspective - 95% 'new'. I've spent the last couple of years restoring it. I've spent time and money on everything from new carpets to a new dashboard, HVAC assembly (in-dash), new seats, new seat motors, new windshield, new everything, for the most part
I've done all this work myself. I've built up quite a tool collection and quite a bit of knowledge
I work in the world of IT (Digital Experience Strategy for large and medium-sized companies)
Being in this field of IT means I work all day in a world of total ambiguity and chaos where nothing is ever cut and dry and the options, ideas, and risks are endless...as are the personalities I deal with. Working on my vehicle - along with vehicle's owned by friends and family aware of my growing skill-set - provides me a level of calm and relaxation I can't find elsewhere

How This Story Originates:

In the last 4 to 6 months, I've noticed the transmission exhibiting a moderate 'clunk' when - IMO - it is shifting down from 3rd gear to 2nd gear, or during any low-load, low-RPM upshift from 2nd to 3rd gear. This happens only when there is low RPM / low 'Load' on the engine...followed by a quick increase in requested 'load'. For context, a situation wherein it commonly happens is during a slow-down at a stop light and my intentions are to turn right. I.e., 'right on red' with a slow, cautious crawl for safety. Assume no cars / traffic to negotiate (for simplicity). I'd say I'd be going at about 10 to 15 miles per hour...perhaps as much as 20 to 25 miles per hour...from a pre-braking speed of ~30 to 40 mph. I would take my foot off the accelerator about 100 to 150 feet before the red light and apply light braking. Once I'm in a position to begin my right turn safely, I would take my foot from the brake and apply ~30% to 50% throttle on the accelerator. At this point wherein I engage the throttle, it seems as if the transmission doesn't know which gear to be in. While it only takes about a second, it eventually chooses 2nd gear but in the process it makes a noticeable 'clunk'. To mitigate this 'clunk' I usually try to feather the throttle up a little bit before I'm about to release the brake (i.e., 2 - footed driving). Slowly getting the RPM's up above 1,000 before starting my right turn and releasing the brake seems to negate the clunk.

I've also noticed this clunk never really occurs until the transmission fluid temperature reaches about 110 (or above) Fahrenheit ( + /- 10 degrees).

All other gear shifts in all other situations are rather comfortable and exhibit no observable issues.

The transmission (nor the engine) never produces any DTC codes. Using a capable scan tool to reset the Trans Adapts and manually doing the Garage Shift procedure doesn't result in a 'fix'. Neither has anything else I've tried. I have not - to this day - ever tried doing a "Service Fast Learn". This is a procedure only incorporated into a few of the highly expensive scan tools such as the OE Tech2 and the Snap-On. My Autel Maxisys Elite definitely doesn't have this special function. One of these day's I'll have this procedure done by the Dealer or someone with a capable tool.

Last weekend, I decided to install a Sonnax Zip Kit for the 6L80. My thought process was that the temperature increase of the trans fluid was a key indicator and gave me good direction. I was thinking pressure was being lost when the internal components of the valve body heated up to a certain point and expanded ever-so-slightly...we're talking a thousandth or so of a millimeter in various places throughout the valve body veins / assembly...but enough where some trans fluid pressure was being lost in key areas and causing the resulting clunk. Because the valves themselves were a different metal (and some of them a type of polymer / plastic) and probably not expanding like the valve body casing, I figured this was a good enough hypothesis to start working against.

The Troubles Begin....

To make a long story shorter for you TLDR-er's, the Zip Kit is now installed and the shifting concerns seem alleviated. I'll give it another couple weeks to make a full conclusion, however. But the big story I wanted to share was about a mistake I made during the installation of the Zip Kit...but I imagine it could happen during any valve body refurbishment, upgrade, or maintenance. It has nothing to do with the Zip Kit itself. The Zip Kit was well manufactured and has fantastic instructions...with pictures too! The Zip Kit installation - plus a desire to inspect and clean all valves - simply gave me the opportunity to make the massive screw-up which occurred. So to help out fellow techs or DIY'ers who may experience this in the future, I have attached a summary pictorial explaining what I did and what it caused...along with the now-obvious solution.

Initial Start-up and Test Post-Cleaning, Assembly, Refurbishment - aka, "The Crucible":

What should have taken just about 3 hours to complete ended up taking 4 days. After putting everything back together the first time, I started the vehicle. I left it in Park for about five minutes. No unusual noises, no "Check Engine" lights. Normal oil pressure, normal slow rise in trans fluid temperature. Normal idle RPM's. I shifted from Park to Reverse and back to Park - allowing about 3 seconds between each move. This was an effort to flush air out of the veins/pump which may have accumulated during the overnight hours (I started this job in the evening but didn't re-install the valve body and other components until the next morning.) I then shifted from Park to Drive. I expected - as one usually does - to feel the torque and the car's desire to move forward. But no, there was no forward engagement whatsoever. I then shifted back to Park and let it sit for a minute. I shifted to Reverse - no reverse engagement. I left it in Reverse for 5 to 10 seconds thinking it will engage once the 'air is pushed out from the pump and valve body veins'...and that's when the Check Engine light came on. Silly me. I put it back in Park. I figured, let's try this again...maybe it was a solar flare or perhaps CNN is talking about Mueller again... Nope - same bad result - no forward, no reverse, not even the sound of it trying to engage. From Drive I was moving the shift lever back towards Park...and during that time I stopped briefly at Neutral. A faint squeal noise was observed from the transmission. Knowing that a squeal noise coming from a transmission is not SKOOKUM (h/t Ave on YouTube - one of my all-time favorites), I left it in Park and shut the engine off.

Exhausted, bewildered, and full of self-doubt, I removed the valve body and TEHCM two more times. For anyone familiar with accessing the transmission valve body on a 2007 - 2013 Yukon (and perhaps 2014 and beyond), you know it's not easy. Each time, the catalytic converters need to be moved / dropped. This means the O2 sensors need to be removed, the front drive shaft needs to be undone and moved off to the side a bit, the front right wheel and wheel well cover need to be removed, etc. etc. ....in addition to draining the trans fluid oil and making a mess of your garage floor and yourself. I did this without a lift...only a jack and jack stands...making me ache and strain in ways I haven't felt in years. Note to DIY's...it takes the catalytic converters a bit more time than you think in order to cool down to a manageable level...ask me how I know.

Each time I went over the instructions for the Zip Kit step-by-step to make sure I put the pieces where they were supposed to go. Each time I questioned most everything about how I put things back together - were the seals matching up correctly? Were the electronics / wires completely connected? Were there any green fuzzies on the main connector going from the outside of the trans and into the transmission case? Were there any cut or broken wires I didn't notice or failed to inspect? Each time I examined every piece that I had touched and questioned everything!...or so I thought. The struggle was REAL! Approximately 8 liters of new Dexron VI ATF - along with both hands, the right side of my jaw, and my lower back - were abused in the process of this event.

The Cause:

In short, during the re-assembly of valves which I had removed for cleaning, I put two of them in backwards. When they were first re-installed, they seemed to fit and it appeared how I re-installed them was exactly how I took them out. But I was incredibly wrong on both points. It wasn't until I noticed the ATSG manual's depiction of the valve body assembly diagrams - and comparing them to how I had re-installed my valves - that I finally realized the reasons for DTC's p0777 and p0700 suddenly appearing.

For SEO, (but explained in the pictorial), I'll simply say that DTC p0777 is "Clutch Pressure Control (CPC) Solenoid 2, Stuck ON" and was the only result post-assembly and start-up. However in the end there was nothing wrong with solenoid 2 whatsoever. The TCM/ECU only threw that code because the situation matched the DTC p0777 trigger conditions. At that point I was really close to spending ~ $600 bucks on a new TEHCM...and that's when I noticed I had installed Clutch Select Valve 2 and Clutch Select Valve 3 backwards. THANK THE GOOD PEOPLE AT ATSG WHO MAKE GOOD TRANSMISSION MANUALS..SPECIFICALLY THE DIAGRAMS (if only they were in color, btw.) Now getting people to read carefully...a different story , of course.

I couldn't find too much on DTC P0777, so I suppose it's rare. But if you ever come across this DTC or other "solenoid stuck", "solenoid failed" types of codes - and especially if you've just completed a refurbishing / cleaning / Zip-Kit installation on your valve body - take triple notice of how you reassembled and re-installed those valves....ALL OF THEM!