rear chattering

mattchrz · June 25, 2019

i was late, very late changing the fluid in the rear of my 2005 silverado hd 2500 6.0, now its beginning to chatter inadvertently (i could kick myself). i talked to a mechanic, he said it will have to be rebuilt or replaced. can anyone direct me to a reputable company where a can purchase a rebuilt G80 4:10 auto locking rear differential 14 bolt? i only have 80,000 miles on the truck ( bought new) and its in very good condition and worth the money. keep up with everything else stainless steel brake lines, trans, power steering ,oil cooler lines, knock sensors, steering shaft and all the other maintenance that goes along with, just missed the rear. duh

Jglew82 · June 25, 2019

Are you dead set on a rebuilt G80? You might consider upgrading to a Detroit Truetrac or Yukon since you need to need to replace it anyhow. That's what I personally would do (as well as many others).

sdeeter19555 · June 25, 2019

Before you get into rebuilding, dump a bottle of LSD modifier in and see if the issue goes away or decreases...you have nothing to lose at this point. Fluid type/brand can make a difference...

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Racindave · June 25, 2019

I wouldn’t think it would be that bad off at 80,000....?

sdeeter19555 · June 25, 2019

I wouldn’t think it would be that bad off at 80,000....?

That's my thinking as well and the fluid was changed which lead to the condition...

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MaverickZ71 · June 25, 2019

Did you use the correct viscosity of GM fluid with the friction modifier additive for the posi? (whatever is listed in your Owner's Manual?)

Asking because sometimes folks use another brand or viscosity that is supposedly "just as good as or better than factory" and then the rear dif chatters or slips or such.

If not, I'd try that before I shelled out $ for a mechanical fix.

Formulabruce · June 25, 2019

This rear is NOT a Positraction, it's Not even a Limited slip, It's a LOCKER.

Correct fluid can help, but these low speed lockers work on centrifugal force. This G80 only locks up with a wheel slip at initial movement, and will "open" at around 20 mph.. This Greatly improves tire wear and fuel economy.

These lockers are typically not rebuilt, and are not worth spending the money on as many many used ones are out there.

If it is really as bad as it sounds, I would just put another GM locker in your rear, and while its apart, change BOTH rear axle seals.

This Rear has a DRAIN on the bottom just forward of the cover, on a slant with the case and the plug is FLUSH with the case, and can be masked by grease, dirt, etc.

Good luck ! Here is all it does..

Formulabruce · June 25, 2019

This is what it looks like, Not your typical friction disc "posi"

sdeeter19555 · June 25, 2019

Still has clutches...

We had several discussions on this over the years...it is a LSD that overcomes the slip of the clutches using the flyweights.

YMMV

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mattchrz · June 25, 2019

thanks for the input. it started to chatter before i changed the fluid. probably put like 5k on the new fluid with no chatter, than it started up again after about 60 mile drive down the turnpike to delaware at about 70 to 85 miles an hour. i used lucas synthetic 75W-90. unfortunately i didn't do the first change till 75k, even had the trans fluid changed when i replaced the cooler lines .should have paid attention to my manual better. an expensive mistake! again the chatter is not consistent but annoying when it happens. there's a picture of the fluid i used. again thanks for the info!!

jake111 · June 25, 2019

As others have said, the G80 is sensitive to having some friction modifiers, not too much and not too little. It does have clutches that engage the locker. Use either genuine GM grape juice, or I've been using Mobil 1 synthetic gear oil, 75-90 that says "LS" on it, for a long time without any issues.

Unless the Lucas specifically says it has friction modifiers for limited slip, then the G80 is definitely going to chatter. Or they might be the wrong modifiers. Stick with what's known to work well!!

mattchrz · June 26, 2019

ok thanks, ill try it! im 63 years old and i don't remember ever changing the fluid in the rear on any of my vehicles and i had my share! cars and trucks! so clutches in the rear is new to me.