2003 GMC Sierra rear diff posi

JerryCanada · May 12, 2016

Hi all,

Did a search in this forum and can't find any thread with this issue.

Just purchased a 2003 Sierra 1500 4x4 with the 4.8L. Before purchase I got the build sheet printed and it mentioned it had posi. Made it spin on dirt to check it the other evening and only 1 mark. The rear end doesn't appear to lock.

Has anyone has had this problem before and would know the cause and what it needs to be fixed?

Thanks,

JerryCanada.

00Silverado4x4 · May 12, 2016

Have fast were you spinning the tires? I only ask because the GM Factory G80 locking rear will only locking while wheels are spinning around 20mph or less.

txab · May 12, 2016

Correct over 20 MPH no engagement.

Needs to be 100 rpm differential speed between the 2 axles

https://www.youtube.com/watch?v=tTGZOJQQBeE

JerryCanada · May 12, 2016

Thanks for the replies. I started from a dead stop and didn't over do it. Just wanted to verify if both would leave a mark.

muddkatt · May 13, 2016

Look in the glove box and see if it has G80 on the tag.

Nanotech Environmental · May 13, 2016

If you need to replace the posi, or put a new one in, just put a Truetrac in it. Way better and cheaper to buy than a G80, which is a POS....

intheburbs · May 14, 2016

The G80 is perfectly fine. It doesn't take abuse well, so if you feel compelled to do frequent burnouts/powerslides and other silly stuff, then yes, an upgrade is probably in order. All three of my trucks have had the G80, and I've never had one fail. I drive them like I stole them, do a lot of heavy towing, and also some fourwheeling in Colorado, Utah, etc.

To the OP, the G80 is a locking rear diff, not a limited slip. So if you have the rear jacked up, it'll behave just like an open diff - spin one rear wheel and the other rear wheel will spin in the opposite direction. Short of waiting until winter to put one tire on ice/snow and the other on dry pavement, I can't really think of an easy way to test it.

Nanotech Environmental · May 15, 2016

The G80 is perfectly fine. It doesn't take abuse well, so if you feel compelled to do frequent burnouts/powerslides and other silly stuff, then yes, an upgrade is probably in order. All three of my trucks have had the G80, and I've never had one fail. I drive them like I stole them, do a lot of heavy towing, and also some fourwheeling in Colorado, Utah, etc.

To the OP, the G80 is a locking rear diff, not a limited slip. So if you have the rear jacked up, it'll behave just like an open diff - spin one rear wheel and the other rear wheel will spin in the opposite direction. Short of waiting until winter to put one tire on ice/snow and the other on dry pavement, I can't really think of an easy way to test it.

Unfortunately, myself and most G80 users wouldn't agree with your statement. For the record, I never abused mine; no burnouts etc. Mine never worked properly from new. I could never count on it to engage in snowy, icy, or muddy conditions. Sometimes it would, sometimes it wouldn't. The same sentiments have been echoed on the various GM boards. The issue is that the G80 is an inherently bad design and should have never went into production. It appears to possibly be a clever design on paper, BUT - It not only doesn't work reliably, the basic design is a time bomb, pure and simple. Any mechanical design that requires small components to slam into each other at high speeds, is ultimately designed to fail; with or without abuse.

After many thousands of km of putting up with a G80 that never worked properly, I finally gave up & put a Truetrac in mine. It actually works the way a posi unit should; opens when it's supposed to, engages when it's supposed to every - single - time AND I don't have to worry about it grenading on me.

(I use the term 'posi' as a generic term applying to all forms of locking or limited slip differentials, so anyone reading this - please avoid the the urge to get into sematics)

Besides- why use an unreliable, failure prone posi unit that costs much more that a truly reliable unit?

JerryCanada · May 15, 2016

I can see there's quite different opinions on the G80. I purchased this truck to use in the woods and to do mud runs and other stuff I wouldn't want my 2015 exposed to. I might take it apart see if I can find if it can be cheaply fixed with few parts. If not, chances are I might just get it welded.

muddkatt · May 15, 2016

Unfortunately, myself and most G80 users wouldn't agree with your statement. For the record, I never abused mine; no burnouts etc. Mine never worked properly from new. I could never count on it to engage in snowy, icy, or muddy conditions. Sometimes it would, sometimes it wouldn't. The same sentiments have been echoed on the various GM boards. The issue is that the G80 is an inherently bad design and should have never went into production. It appears to possibly be a clever design on paper, BUT - It not only doesn't work reliably, the basic design is a time bomb, pure and simple. Any mechanical design that requires small components to slam into each other at high speeds, is ultimately designed to fail; with or without abuse.

After many thousands of km of putting up with a G80 that never worked properly, I finally gave up & put a Truetrac in mine. It actually works the way a posi unit should; opens when it's supposed to, engages when it's supposed to every - single - time AND I don't have to worry about it grenading on me.

(I use the term 'posi' as a generic term applying to all forms of locking or limited slip differentials, so anyone reading this - please avoid the the urge to get into sematics)

Besides- why use an unreliable, failure prone posi unit that costs much more that a truly reliable unit?

Count me as one that disagrees with your statement. Having owned 9 trucks and Suburbans through the years that had the G80, I never had one that did not work as designed, or failed. I did have a 1983 Silverado that I had to add the additive to the gear oil to stop the jerking it made when turning sharp. Some of the vehicles that I mention were used in excess of 400,000 miles before being traded in or sold. I will say that I have had rear ends fail, but it was never the G80. It was a rear seal and axle bearing once, and the pinion bearing on more times than I care to mention.

I can say that I have put over 2 million miles on the G80, with no problems, so in my book, they are a damn fine unit.

intheburbs · May 15, 2016

Unfortunately, myself and most G80 users wouldn't agree with your statement. For the record, I never abused mine; no burnouts etc. Mine never worked properly from new. I could never count on it to engage in snowy, icy, or muddy conditions. Sometimes it would, sometimes it wouldn't. The same sentiments have been echoed on the various GM boards. The issue is that the G80 is an inherently bad design and should have never went into production. It appears to possibly be a clever design on paper, BUT - It not only doesn't work reliably, the basic design is a time bomb, pure and simple. Any mechanical design that requires small components to slam into each other at high speeds, is ultimately designed to fail; with or without abuse.

After many thousands of km of putting up with a G80 that never worked properly, I finally gave up & put a Truetrac in mine. It actually works the way a posi unit should; opens when it's supposed to, engages when it's supposed to every - single - time AND I don't have to worry about it grenading on me.

(I use the term 'posi' as a generic term applying to all forms of locking or limited slip differentials, so anyone reading this - please avoid the the urge to get into sematics)

Besides- why use an unreliable, failure prone posi unit that costs much more that a truly reliable unit?

Because the TrueTrac is a limited slip, not a locking differential. Put one of your drive wheels up in the air, and see how well that TrueTrac works for you. I've seen guys with 2wd G80 locker-equipped trucks go into some pretty crazy stuff and not get stuck.

And if I ever had to replace a G80, I'd go with a locker, not a TrueTrac. IMO, the TrueTrac is more of a performance rear, better-suited to street cars than trucks. Pound-for-pound, it can handle a lot more power than standard clutch-pack posi units.

kstruckcountry · May 15, 2016

I can see there's quite different opinions on the G80. I purchased this truck to use in the woods and to do mud runs and other stuff I wouldn't want my 2015 exposed to. I might take it apart see if I can find if it can be cheaply fixed with few parts. If not, chances are I might just get it welded.

A welded diff can put you in the ditch fast in the wrong situation. I would check the RPO codes and make sure the G80 is what you have. The TruTrac is a very viable replacement, despite not being a locker. It's a decent design that's pretty tough.

Sent from my SM-G920V using Tapatalk

Nanotech Environmental · May 15, 2016

Because the TrueTrac is a limited slip, not a locking differential. Put one of your drive wheels up in the air, and see how well that TrueTrac works for you. I've seen guys with 2wd G80 locker-equipped trucks go into some pretty crazy stuff and not get stuck.

And if I ever had to replace a G80, I'd go with a locker, not a TrueTrac. IMO, the TrueTrac is more of a performance rear, better-suited to street cars than trucks. Pound-for-pound, it can handle a lot more power than standard clutch-pack posi units.

If one wheel gets in the air, all you do is touch the brakes lightly while applying power. It then pushes power to the wheel with traction. The nice thing about the Truetrac is that it starts grabbing within 1/8 to 1/4 turn of the slipping tire, something the G80 can never hope to do. AND it does it without the constant shock loads.....

Oddly enough, the TrueTrac being a Torsen differential, was actually designed to get thru the worst offroad conditions, not as performance differential on pavement. That's why they came on the H1 Hummers.

The Torsen was designed by someone very clever, the G80 was likely designed by a bureaucratic committee.......

Nanotech Environmental · May 15, 2016

Count me as one that disagrees with your statement. Having owned 9 trucks and Suburbans through the years that had the G80, I never had one that did not work as designed, or failed. I did have a 1983 Silverado that I had to add the additive to the gear oil to stop the jerking it made when turning sharp. Some of the vehicles that I mention were used in excess of 400,000 miles before being traded in or sold. I will say that I have had rear ends fail, but it was never the G80. It was a rear seal and axle bearing once, and the pinion bearing on more times than I care to mention.

I can say that I have put over 2 million miles on the G80, with no problems, so in my book, they are a damn fine unit.

In all fairness- If most or all of those miles were done in the south, you really haven't experienced the conditions that expose many of the weaknesses of the G80- snow, ice and cold conditions.

muddkatt · May 15, 2016

In all fairness- If most or all of those miles were done in the south, you really haven't experienced the conditions that expose many of the weaknesses of the G80- snow, ice and cold conditions.

I will say that I don't travel up north, so you have a point on the cold, snow and ice. In northern Louisiana and south Arkansas, we got some ice and snow a couple of times a year at the most. Our low temps were nothing compared to yours up in Canada.

My trucks were and still are used to work. Probably 35 to 50% of the miles are pulling trailers in pastures, red mud roads and wet highways Now that I am in Florida, its wet boat launches and sand roads. I have just had good experiences with them doing what I need them to do.