6L80 questions

builds4christ · Apr 28, 2026

Hey all, just a quick few questions on the 6L80.

Has anyone had hard shifts into drive? That's the only time it's hard.

So far, all mounts except the front diff bushings are new. (Doing that this Thursday).

Also, sometimes when I go from slowing down to acceleration, I get a knocking near my feet. I suspect that's the diff mounts though. Unless there could be another reason, of course!

I've recently dug into the TCM and valve body and swapped out the laminates (two were broken). It shifts really well, but on my way home the other day going about 70, and dang it happened so fast I almost feel like I imagined it, but it felt almost as if it very quickly fell out of gear and re-engaged. It was so fast that it seemed as if I hit something on the road, but there was nothing there.

Has anyone experienced theae things with theirs? Mine has 193k at this point. When I picked it up, it did not look as if they kept up with the servicing. I changed it once, and then again about 2 weeks later when I discovered it needed laminates. So 8 clean quarts, haha.

Joseph Garcia · Apr 29, 2026

@NickTransmissions is our resident transmission expert, and when he views your post, he will reply.

West 1 · Apr 29, 2026

Any chance it is the torque converter failing? When the lock up fails it puts a mess into your oil and ruins a good tranny. Hope that is not your issue. I replaced my torque converter at 145000 miles as preventative maintenance.

NickTransmissions · Apr 29, 2026

builds4christ said:
Hey all, just a quick few questions on the 6L80.

Has anyone had hard shifts into drive? That's the only time it's hard.

So far, all mounts except the front diff bushings are new. (Doing that this Thursday).

Also, sometimes when I go from slowing down to acceleration, I get a knocking near my feet. I suspect that's the diff mounts though. Unless there could be another reason, of course!

I've recently dug into the TCM and valve body and swapped out the laminates (two were broken). It shifts really well, but on my way home the other day going about 70, and dang it happened so fast I almost feel like I imagined it, but it felt almost as if it very quickly fell out of gear and re-engaged. It was so fast that it seemed as if I hit something on the road, but there was nothing there.

Has anyone experienced theae things with theirs? Mine has 193k at this point. When I picked it up, it did not look as if they kept up with the servicing. I changed it once, and then again about 2 weeks later when I discovered it needed laminates. So 8 clean quarts, haha.

At 193k, it’s on borrowed time so I’d start planning for an overhaul if you haven’t already started along those lines.

False neutrals at highway speed can be a few different things but 456 clutch wearing and in the process of failing has to be a suspect at 193,000 on the clock.

builds4christ · Apr 30, 2026

NickTransmissions said:
At 193k, it’s on borrowed time so I’d start planning for an overhaul if you haven’t already started along those lines.

False neutrals at highway speed can be a few different things but 456 clutch wearing and in the process of failing has to be a suspect at 193,000 on the clock.

Hey Nick, good to see you again brother.

It's Astro from the OBS forums!

Man that's a tough thing to hear since I just bought it. I knew to some degree going into it with so many miles, but I was hoping to get a few years out of it before major work.

Would there be anything else to watch out for? What other signs would show the wear you're talking about? Do you think the decel/accel clunk could be a symptom of the tranny versus the front diff mounts? It seems to otherwise be shifting pretty well. IIRC, you don't doing shipping, so was wondering if you had any recommendations for buying a tranny and what to look for.

I really wanted to get into learning the transmission better so that I could rebuild it myself, but having a new little one really cut my "me" time to nearly zero.

Not complaining at all, totally worth it!

@West 1 I'm not sure about the converter. This is my first foray into the 6 speeds and I haven't learned their quirks yet. As Nick will attest, I'm used to the 4L's

@Joseph Garcia thanks brother. Looking at your build sheet, I think I'm going to have some questions for you! Sounds like a badass ride!

builds4christ · May 1, 2026

Well, changed out the two diff brackets and that wasn't the source of the clunking when transitioning from decel to accel. If I slowly get into the throttle from decel, it's fine. I'm not sure what it could be at this point, but the reason I suspect the tranny is because my 4L60 in my 96 Sierra did the same thing. It blew up and I did a 4L80 swap. The clunk went away when I swapped trannys.

Kinda annoying. Idk guys, I guess it is what it is. Whenever it goes, I'm going to go pick one up from pick N pull for a few hundred bucks with decent miles and go from there.

At least then I'll have two of them, one of which I could possibly dig into or save up for overhauling.

If y'all still have any tips or advice, I'm all ears. Thanks again!

In case I didn't mention it, it's got new cvs, all new mounts, and all new u-joints.

Hobert · May 1, 2026

Have you checked engine mounts? Left usually fails first "they say".

donjetman · May 1, 2026

Sloppy loose chain in the transfer case ?

builds4christ · May 4, 2026

Hobert said:
Have you checked engine mounts? Left usually fails first "they say".

Yessir, every single mount is new; 1) engine, 2) Front diff lh/rh, 3) transmission.

@donjetman hey Don, is there a way to check that without taking it apart? Seems like a good possibility.

wjburken · May 4, 2026

builds4christ said:
Yessir, every single mount is new; 1) engine, 2) Front diff lh/rh, 3) transmission.

@donjetman hey Don, is there a way to check that without taking it apart? Seems like a good possibility.

On my AWD Denali, I check the slop by getting it all four wheels up on jack stands and turn the rear drive shaft and see how much it turns before the front drive shaft starts to turn.

While you're there, I'd check to see how much play you have at your rear differential and the front axle shafts. I've had bad CV joints on my front axle shafts give me a clunk as well. Also, check the condition of your rear transmission mount.

One last thing to check would be your engine mounts if you're looking for clunks.

builds4christ · May 4, 2026

wjburken said:
On my AWD Denali, I check the slop by getting it all four wheels up on jack stands and turn the rear drive shaft and see how much it turns before the front drive shaft starts to turn.

While you're there, I'd check to see how much play you have at your rear differential and the front axle shafts. I've had bad CV joints on my front axle shafts give me a clunk as well. Also, check the condition of your rear transmission mount.

One last thing to check would be your engine mounts if you're looking for clunks.

Gotcha, that's a good test! But how much slop is acceptable?

All the mounts, including transmission, are new (engine mounts were busted for sure) and the cv axles are new as well. The axles were bad, but the boots were torn up and I figured I'd just get it over with haha!

When I got this truck, I really went through it to make it safe and reliable for my wife to drive around.

Really the last thing is the clunk. I've told her to just ease into the throttle when transitioning

wjburken · May 4, 2026

builds4christ said:
Gotcha, that's a good test! But how much slop is acceptable?

All the mounts, including transmission, are new (engine mounts were busted for sure) and the cv axles are new as well. The axles were bad, but the boots were torn up and I figured I'd just get it over with haha!

When I got this truck, I really went through it to make it safe and reliable for my wife to drive around.

Really the last thing is the clunk. I've told her to just ease into the throttle when transitioning

I figure if I can make it clunk with just my hand, then its getting sloppy. Others may have differing opinions.

builds4christ · May 6, 2026

Gotcha, I guess that makes sense.

I wonder if there's supposed to be any slop at all. I can't imagine it being perfectly tight, but I'm not an expert with this system.

Any insight from others would be greatly appreciated.

@NickTransmissions what do you think Nick?

NickTransmissions · May 6, 2026

builds4christ said:
Gotcha, I guess that makes sense.

I wonder if there's supposed to be any slop at all. I can't imagine it being perfectly tight, but I'm not expert with this system.

Any insight from others would be greatly appreciated.

@NickTransmissions what do you think Nick?

Pull the back cover off and check it. Also check for wear in the cover itself. You may end up replacing both chain and cover.

builds4christ · May 6, 2026

NickTransmissions said:
Pull the back cover off and check it. Also check for wear in the cover itself. You may end up replacing both chain and cover.

Thanks man, is this doable with it on the vehicle, I wonder?

Probably not, but maybe. I gotta ask.

SpareParts · May 6, 2026

Pull it. You could pull it, rebuild it and get it back in, in less than a day.

NickTransmissions · May 6, 2026

builds4christ said:
Thanks man, is this doable with it on the vehicle, I wonder?

Probably not, but maybe. I gotta ask.

You should be able to do so but I'd just pull it out all together...Drain the fluid first, note anything unusual in the fluid. If a bunch of metal, debris, etc I'd definitely yank it off the vehicle, throw it on the bench and tear it down. Or you can see if you can find a good used replacement in the meantime, if you don't have the bandwidth to tackle it now but want to overhaul it later.

builds4christ · May 7, 2026

NickTransmissions said:
You should be able to do so but I'd just pull it out all together...Drain the fluid first, note anything unusual in the fluid. If a bunch of metal, debris, etc I'd definitely yank it off the vehicle, throw it on the bench and tear it down. Or you can see if you can find a good used replacement in the meantime, if you don't have the bandwidth to tackle it now but want to overhaul it later.

Yeah all good points.

I changed the fluid when I picked up the truck and it seemed very clean. The tranny is another story on the fluid lol!

I almost feel another 4L80 swap thread coming up for this forum!

But yeah man, inlay just get a newer unit for now.

I definitely don't have much bandwidth. Just a little baby girl and have been having a fun time hanging out with her.

Do any of you know which transfer case is under there? The AWD one is special right?

SpareParts · May 7, 2026

Probably a BW44-85

donjetman · May 8, 2026

builds4christ said:
Do any of you know which transfer case is under there? The AWD one is special right?

Borg Warner BW4485 if your glove box rpo code is NR3?

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