6L80 Mechanical and Powerflow

This video covers the mechanical operation and the powerflow of the GM 6L80/90 RWD transmission used in many Chevrolet and GMC RWD and 4WD vehicles. These transmissions are still offered in 2020 models and arrived on the scene in about 2007. In this video, I cover the clutches and gearset, then I go through the powerflow gear by gear. This video is intended to teach a student or a technician that's new to automatic transmissions the mechanical operation of this transmission. This is part of the SIU Automatic Transmissions Curriculum, which can be found at:
www.automotivetextbook.com/au...
Go to that site to view other videos on this and other transmissions, such as the FCA 41TE, Ford 6F50, GM 6T70, GM 4L60e, ZF 9HP48, GM 6L80, and Jatco CVT.

[@richardlewis7471]

I’m a retired aircraft mechanic who loves your videos. I really just want to learn the workings of the a/t. Like you say rewatch them to better understand. I do, over and over again and yes the principles do in fact become clear. One of the best things that’s only you make a point in your basic principle videos is the fact that the planet carrier is the largest gear. That to me was never self evident until you said so. That made the planetary gear set premise easy to understand. I don’t know why other trans video instructors never explicitly point that out. Anyway thanks and keep up your great work!.

[@saagiebawlz]

This is incredible

[@Username64870]

Hi Nice Explanation..Keep it up...

[@rafaelceja81]

Very informative sir, wealth of knowledge for my resto-mod c3! 🤩👏🏽

[@johnmwangi6291]

God bless you sir...

[@32vnorthstar]

Thanks

[@Roscotech]

Now if could better understand the hydraulic portion. From the regulation of the initial pump pressure to the way that pressure is then again regulated in the clutch apply circuits, then finally applied to the clutch pistons.

[@thomasjohnson1142]

I have a few questions? From a performance stand point I have built 6l80 I built it but when I putting down anything over 700hp going through the gear I get quick slip but it’s almost like a missfire but I am the tuner as well so I can see it in my input shaft speed to my output SS . The trans has all raybestos clutched , steels and 35r zpack and all the sonnax and transgo valves I could put in it , pump and valve body . Would taking out my wave played in the 1234 drum help with clamping force and added another friction I ask because my zpack deleted the wave plate . Yes it’s a harsh shift but I was able to tune it out. This happens in 1st and second sometimes 3 if there’s not enough heat in the trans .

[@cmitchell17a1]

My 6L80 decided to neutral itself after coming to a stop sign for a second then grabbed back into first. It then started a whirrling/straight cut/hydrostatic transmission noise in first gear and reverse (it also seems to make noise in 6th maybe).
Logic would tell me there is something wrong with the lo/reverse sprag/roller clutch/overrunning clutch. I know the lo/reverse clutch comes on in first but in normal gear selector in drive operation I think the lo/reverse clutch is only applied up until about 2-3 mph, it then releases while you are still accelerating in first. However, the sprag will still hold the carrier stationary and maintain first gear. For one I guess this helps spread the load (especially when the torque converter is at high stall torque multiplication) through not only the sprag but also the lo/reverse clutch. I think this also allows for the output shaft to overdrive the input shaft in case you were still in first gear and let off the gas while accelerating and prevent the dreaded throwing your head forward in stop and go traffic.
I think you said it but it is amazing that the sprag with very 2 highly smooth hard metal surfaces can jam between them and hold power. I am imagining someone in first gear trying to abuse it by slamming on the gas and letting off the gas. However, I think the TCM would notice that and apply the lo/reverse clutch so the sprag doesn't take all that shock loading.
Anyway, since my noise is clearly happening in first and reverse, it doesn't seem like it could be the sprag, since the sprag should have no relative motion, unless you let off the gas of course?
I can put it in manual 1st mode (I believe this should hold the lo/reverse clutch on the entire way through 1st gear) and it still makes the exact same noise. So maybe its not the sprag?

[@siuautomotive]

Good diagnostics! The rear gearset is known to fail (especially on the 6L90s). A few things come to mind since I cannot hear this in person. The converters on these transmissions fail often and take out the pump. Is it pump noise you hear? Does it only make noise when moving? Or can you hear it in park or neutral. If you only hear it in 1st and Reverse, it sure sounds like the rear gearset. You might pull the pan to see if there's a bunch of metal on the magnet.

[@cmitchell17a1]

@@siuautomotive Thanks, so I pulled pan, few pieces of metal in pan. Pulled transmission out and found the rear gearset gears chewed up, but interestingly the sprag seems ok. It does have a little resistance on it. I somehow got ahold of a new rear gearset for $200 (it is a 650-700 dollar part new) and I am currently swapping it in. What is amazing to me is how good everything else looks like the clutches, bushings, bearings, no wear on shafts etc., however I am not a experienced mechanic but it seems to me like all the stuff in there has a bunch of life left. I am going to replace the gearset and put a Sonnax Zip Kit in and leave everything else and see how many miles I can get out of it, its got 250k now.
So the noise was only hearable in first and reverse and no other gears except maybe 5th or 6th it was just too hard to tell with other road noises. This is weird because some gears are moving around all the time in the rear gearset, and the noise completely goes away upon the shift to second and both the front and rear planataries on the gearset have chipped teeth. Rear sun had one little mark on it, and front sun had a few teeth broken off. Rear ring was fine but front ring had a few teeth broken as well. There was noticeably more play with the assembled rear gearset than with the new one, I would be really curious to swap it back in and just see how long the transmission lasts with the few teeth broken.
Besides a few scratch marks on the outside of the rear gearset carrier, it appears all damage was limited to it and the pieces just fell out in the pan. So looks like it was a fatigue failure I guess and I guess it started in the front sun and the pieces fell out before too much damage. I did pull the pan off about a month before it failed and didn't take the exhaust out so I had to kind of jam the pan back in, but theres no way debis could have fought gravity and gotten up in the gears I don't think.
The other weird part is that the day it happen I noticed I went to back down my driveway in reverse and it wouldn't move backwards and I thought something was blocking the tire, so I shifted back to drive and then back to reverse and it backed out fine, then a few miles down the road stopped to turn right and the low reverse clutch held on until its 2-4 mph release point and then went into neutral. I feel like the only way for that to happen is the sprag had to of failed, but like I said it feels fine. I had ordered a new one and already sent it back which I probably should have just replaced it but I am trying to save money.
I am about to open up the pump to put in the Sonnax, but so far I don't see any signs of converter failure. I am pretty OCD about noises and stuff and I think I would notice if the TC had been slipping.
Also thanks for the help and being the most honest and educating person on youtube, when its filled with click-bait, useless videos that ask you to like/share/subscribe every 30 seconds haha.

[@Diazlsx07]

Which one is the forward clutch on the range reference

[@mastertechnicianmindset2181]

This and a couple others of yours are the most informed and most helpful 6L80 videos that I used to rebuild mine my 2011 suburban back in February. My rebuild went great! But now, 11,000 later, it developed a rolling noise in Park and Neutral, that immediately stops when its put into either reverse or drive, and immediate returns when put back into neutral or park. Whenever it has the powerflow held by the brakes, no sound. Its a nasty sound and as i think about it more and more, and I feel that it could be the front gear set being spun in the 1234/35R clutch assembly, which i do not think is a normal occurrence.
It also seems have a slip in first gear and reverse at start of launch. However, the transmission shifts fine on a test drive that i made around the block.
I was wondering if you might have a thought on this.
I'm going to get it in the air and start disconnecting it today, any thought from you would be very insightful and appreciated.
Thanks

[@raviolisauce103]

Hey! Did you ever happen to rip it apart? Just curious lol

[@mastertechnicianmindset2181]

@@raviolisauce103 yes I did. I think this time around my rebuilt a lot better. The front sun gear and input shaft housing were severely chewed up. I think on the last build I put the wrong bearing up there and it came apart inside that front drum. It's all good now, it's been running fine since this last rebuild. I picked up a new input shaft assembly in Gears that were all damaged, and it's going on almost a year now with no problems. Thanks for checking in

[@raviolisauce103]

@@mastertechnicianmindset2181 Oh nice, sounds like you did a good job. I think I have a sprag acting up. Causing my 1-2 flare.

[@ToDaMn]

I’ve been trying to apply some logic to my 6l80 it will drive in first and Shift to second but will not exceed about 15 MPH. Reverse is fine and strong. Seems to appear fault free with the trans fluid cold. Em I correct it thinking the fault is most likely the 1234 drum or clutch? and that as the trans fluid warms up the thins out some and escapes through a cracked drum

[@siuautomotive]

The 1234 piston is known to crack. The 35r drum cracks at the weld. But if you have a good first and second and reverse, it sounds like those are okay. Do they start to slip once warm? Any dtcs? If you have a scan tool, can you command other gears or watch the data and see if other gears are being commanded on?

[@ToDaMn]

@@siuautomotive Stall test in reverse RPM~2100-2400. In Drive ~3300RPM, When cold the vehicle will move in drive and shift from first to second but not exceed 15MPH. Reverse is strong as if there is no issue with the transmission. I'am curious as to what the issue would be. All this happened within 5k miles of getting a trans flush. No Codes. Can It be a clogged filter? The trans was well taken care of never towed flushed at 45k and 90K.. I really expected more miles out of her. I'am prior Infiniti Tech and I would see there vehicles go way over 150k easy with good maintenance. Are you guys having any EV specific course's really interesting but also scary time for the industry

[@cuthburtyes9529]

I would like to enrole into your school sir but im in southAfrica how do i do it thanks inadvance .

[@siuautomotive]

You can find program information at automotive.siu.edu. We actually have quite a few international students at times.

[@cuthburtyes9529]

@@siuautomotive Thank you god bless.