AT4 Locking Rear Differential

killramos · July 28, 2019

Hey guys done a bunch of searching around and couldn’t find a definitive answer so here it goes.

I have my 19 AT4 still on order and hasn’t arrived yet. But I am trying to figure out how the locking rear differential on this car works? From what I understand there is no button to engage it? I’m not used to having a locking diff that you don’t activate.

To confirm it’s not an LSD? But a true 100% locking electronic differential? Just the truck activates it when it feels like it?

When does it engage? need to be in 4Lo? Off road mode?

Thanks guys!

econometrics · July 28, 2019

Watch...

davester · July 28, 2019

From googling, I found this: https://www.cnet.com/roadshow/reviews/2019-gmc-sierra-1500-4wd-crew-cab-157-at4-review/

It says the AT4 has a G80 (standard locking diff that GM has been using forever). It's not electronic, but rather depends on specific physical conditions to lock and unlock on it's own. You do not want to abuse it, as it's knickname is the Gov-Bomb. Abuse it and you can expect to need to repair and/or replace it.

FDHog · July 28, 2019

It's kinda like a Detroit True-Trac by Eaton. If mine goes, that's what's going in to replace it.

I had one in my Off-road Wrangler and it worked great.

killramos · July 28, 2019

Detroit True Trac is definitely an LSD not a true locker. It has its benefits but is generally considered inferior to true locking for offroading.

Based on that video it doesn't seem like its an LSD? lots and lots of spin before finally locking up where an LSD would react quicker. Seems like the truck is essentially using electronics to make the full locker into an LSD by only engaging when it senses excessive slip. But there is ALOT of spinning ( aka getting stuck ) before the thing engages which is pretty frustrating.

As for the "gov-bomb" thing, if they sell me a locking diff on an off road vehicle that destroys itself readily when used (i disagree with this being abuse) then they can feel free to replace it all life long as part of powertrain warranty lol.

I just wish there was a button to engage, as driver I will typically know when its needed well before the computer does, at least based on how long it takes to react in that video.

edit: Actually the more i read about this G80, mechanically its a limited slip differential. Doesnt sound any different than any LSD I have had before. That's quite dissapointing on an "Off-Road Truck"

Edited July 28, 2019 by killramos

econometrics · July 28, 2019

5 hours ago, killramos said:

edit: Actually the more i read about this G80, mechanically its a limited slip differential. Doesnt sound any different than any LSD I have had before. That's quite dissapointing on an "Off-Road Truck"

I have no idea where you are reading, but the G80 is most certainly a full locker.

https://www.autoevolution.com/news/a-simple-guide-to-the-g80-locking-differential-94874.html

The reason it seems to spin a lot in that video is because they are babying it. When you’re in real mud or loose traction, it locks pretty quickly, as most anyone on this forum could attest to from real world experience.

killramos · July 28, 2019

I dunno.

A differential that mechanically senses a difference in rotational speed between 2 axles, that actuates a clutch pack to lock up the two axles and subsequently send torque to the wheel with less traction sounds a lot like a limited slip differential to me lol

But still lots of good info, no electronics, diff is entirely mechanical.

Edited July 28, 2019 by killramos

Z45 · July 28, 2019

Z45 · July 28, 2019

killramos · July 28, 2019

39 minutes ago, elcamino said:

Good video! Not sure I agree 100% with Chevy here though on definition of LSD. Perhaps that’s true of their applications.

What they say is true of a Torsen/gear style lsd, but a clutch pack LSD does not need both wheels to have grip to function.

Z45 · July 29, 2019

¨ Optional Equipment (G80 Order Code) on all GM two-wheel and four-wheel drive light trucks and sport utilities.

Benefits

¨ Improved traction differentiates the vehicle relative to other rear wheel and four-wheel drive vehicles on the market.

¨ Maintenance free; requires no lube additives.

¨ Enhanced towing and off-road performance.

¨ Patented carbon friction disc technology enhances performance and durability while reducing warranty claims.

Principles of Operation

¨ Wheel speed difference (left to right) in excess of 100 RPM will cause a flyweight mechanism to open and catch a latching bracket.

¨ The stopped flyweight will trigger a self-energizing clutch system, which results in the cam plate ramping against a side gear.

¨ Ramping increases until both axles turn at the same speed (full lock), which prevents further wheel slip.

¨ At speeds above 20MPH, the latching bracket swings away from the governor and prevents lockup from occurring.

Technical Specifications

¨ Automatic locking takes place within a fraction of a second, so smoothly that it is unnoticed by the average driver.

¨ Unlocking occurs automatically, once the need for improved traction is gone, and is unnoticeable to the average driver.

¨ Compatible with anti-lock brake systems.

AJMBLAZER · July 29, 2019

It’s a locker with clutch packs instead of ratcheting teeth that locks it up. Allows it to not engage/operate with abrupt clunks like a Detroit Locker or Lock Right.

Problem is it basically locks against it’s own carrier which in the past meant the 8.5” 10 bolt axle the half tons used was prone to breakage. Just not enough carrier. Beating on it or sustained use (work truck off road or a truck that was offroaded a lot) would eventually cause it to break.

Good news is the 9.5” and 10.5” 14 bolt axles the 3/4 and 1 tons used had beefier parts and were less prone to breakage. The 9.5” axle these have is a redesigned 14 bolt.

Only issue is the locker’s clutch packs do wear out over time and it will act funny when they do. Nature of all clutches.

Z45 · July 29, 2019

I have had at least 5 trucks with the G80 over the last 20 or so years. I do not abuse the trucks, run oversize tires or suspension mods but I do live in a climate that is winter for 6-months and I never had one issue with the G80 on my trucks or our GPS Survey team work trucks.

PS - I never leave the oem (low bidder) fluid in my personal trucks, I R & R it around 5,000 miles with a 100% synthetic

Edited July 29, 2019 by elcamino

JR09country · July 30, 2019

I went through 2 g80s on my 99 with the 10 bolt. Had a 13 with the 6.2 and 14 bolt and never had one isuue with the g80. The 14s and newer use a 12 bolt design thats a little weaker than the 14 bolt but i have not heard of any issues with the g80 in it. Should be good to go.

Its nice not having to worry about pressing a button to get traction, it just works when you need it and then goes away. I wouldn't have one rock crawling but for every day use it is perfect.

Sent from my ONEPLUS A6013 using Tapatalk

AJMBLAZER · July 30, 2019

5 hours ago, JR09country said:

I went through 2 g80s on my 99 with the 10 bolt. Had a 13 with the 6.2 and 14 bolt and never had one isuue with the g80. The 14s and newer use a 12 bolt design thats a little weaker than the 14 bolt but i have not heard of any issues with the g80 in it. Should be good to go.

Its nice not having to worry about pressing a button to get traction, it just works when you need it and then goes away. I wouldn't have one rock crawling but for every day use it is perfect.

Sent from my ONEPLUS A6013 using Tapatalk

FYI - only the housing of the new 12 bolt is different than the 07-13 14 bolt semi floater axle. Internals are the same.