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Limited Slip Gear Fluid What Type To Use


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I know this post is very old, but there is a lot of mis-information in it. For instance, "GMC has never put a limited slip diff in a vehicle" wrong, correct answer, almost all GMC trucks have a limited slip (posi) rear end, YES they are the same thing. Most chevy trucks come stock with an open differential. At least older models I'm not sure about newer so don't quote me on that, that is why people go to the parts store or junk yard and pick up a GMC rear end for there chevy. The diffrent types of rear ends are open, limited slip, locker, and spool. Spool is mostly for drag racing or trucks used strictly for off road. Open is the most common oem diff, there are a few diffrent types of lockers, automatic lockers, electrical or pneumatic selected, and mechanically actuated. Now a positraction differential is pretty much a slang word used to describe limited slip when GMC first came out with it and it has stuck and been used since. A lot of people think posi means both tires spin all the time but it does not. that is what a spool does and it is pretty much useless in a daily driver. think if you are turning a 90 degree corner the outside wheel has to travel more distance than the inside so it would not work, Either you would go thru a lot of tires or a lot of rear ends. an open differential solved this problem. But then you had the problem of getting stuck because all of the power goes to the wheel that spins more. So they made limited slip (posi), this lets both wheels get power when traveling in a straight line but when turning transfers most of it to the inside wheel, or when stuck it wont just let the free wheel spin it will transfer some of the power to the other wheel.

As far as fluid, read the manual it will tell you the correct part number and weight so you can either buy it from a store, or just go to a part store tell them the make, model, year, and engine size of your truck and they will give you an oem equivalent.

 

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I know this post is very old, but there is a lot of mis-information in it. For instance, "GMC has never put a limited slip diff in a vehicle" wrong, correct answer, almost all GMC trucks have a limited slip (posi) rear end, YES they are the same thing. Most chevy trucks come stock with an open differential.

 

 

Actually, you're the one supplying the misinformation.

 

This is one of my pet peeves. Chevy/GMC trucks do not have posi rears. Period.

 

Pretty much from MY 2000 and up, Chevy/GMC fullsize trucks have either an open diff, or the G80 locker. The only time you'll find a posi on a Chevy/GMC truck is if it's an aftermarket rear.

 

A "posi" rear is a limited-slip differential that has clutch packs, which allows the axles to turn at different speeds (like during sharp turns), but still allows somewhat equal application of power when one wheel loses traction.

 

The G80 locker is NOT a posi. It is an automatic locking differential. When the differences in axle speeds exceeds ~100 RPM, the differntial actually locks the two rear wheels together and they turn as one.

 

This is an important distinction, because a true posi requires a friction-modifier additive to the axle fluid. This allows the clutch packs to slip smoothly and not chatter when you make a sharp turn. The G80 requires no friction modifier. The friction modifier actually reduces the lubricity of the axle fluid - something you don't want to do unless absolutely necessary.

 

I did a lot of research on these different rears. I've been through four rear ends on my '01 Burb - the OEM G80, a posi, then two more G80s. I've experienced clutch chatter first hand.

 

Here's a good video that illustrates the difference:

 

 

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+ 1 on intheburbs' post. I think all the confusion comes because GM uses the same RPO (code G80) for both both types of systems. In both cars and trucks. I also think your as likely to find a G80 axle in a Chevrolet as a GMC. The Posi axles in the '60s used a cone and pocket instead of clutches. They seemed to wear out pretty fast.

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Actually, you're the one supplying the misinformation.

 

This is one of my pet peeves. Chevy/GMC trucks do not have posi rears. Period.

 

Pretty much from MY 2000 and up, Chevy/GMC fullsize trucks have either an open diff, or the G80 locker. The only time you'll find a posi on a Chevy/GMC truck is if it's an aftermarket rear.

 

A "posi" rear is a limited-slip differential that has clutch packs, which allows the axles to turn at different speeds (like during sharp turns), but still allows somewhat equal application of power when one wheel loses traction.

 

The G80 locker is NOT a posi. It is an automatic locking differential. When the differences in axle speeds exceeds ~100 RPM, the differntial actually locks the two rear wheels together and they turn as one.

 

This is an important distinction, because a true posi requires a friction-modifier additive to the axle fluid. This allows the clutch packs to slip smoothly and not chatter when you make a sharp turn. The G80 requires no friction modifier. The friction modifier actually reduces the lubricity of the axle fluid - something you don't want to do unless absolutely necessary.

 

I did a lot of research on these different rears. I've been through four rear ends on my '01 Burb - the OEM G80, a posi, then two more G80s. I've experienced clutch chatter first hand.

 

Here's a good video that illustrates the difference:

 

 

 

:)

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¨ 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.

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  • 3 years later...
On 7/15/2010 at 2:50 PM, Buddy67Lincoln said:

 

to buddy67...

 

read the post from Don (snakedoctor) again... if you use Limited slip gear oil in a G80 Eaton Loacker (i.e., in a chevy/Gm truck) it will affect the locking mechanism and may cause damage...

 

 

No,He'll be fine with the Mobil 1 LS Synthetic Gear Lubricant.....

 

Applications:

 

*Mobil 1 Synthetic Gear Lubricant LS 75W-90 is recommended for use in modern high performance automobiles like SUV's, Vans and Light duty trucks requiring API GL-5 and MT-1 level performance

*Mobil 1 Synthetic gear Lubricant LS 75W-90 is intended for initial fill, topping-off or refilling differentials, final drives, transfer cases and other gear applications where lubricants meeting API Service GL-5 and multi-purpose or mild EP gear lubricants are recommended

*Mobil 1 Synthetic Gear Lubricant LS 75W-90 can also be used in rear axles where API Service GL-4 lubricant is recommended

*Not recommended for automatic, manual or semiautomatic transmissions for which engine oil or automatic transmission fluids are recommended

*Where extended service intervals and warranties are required

 

 

 

Thanks again Dale. Its just for topping off purposes

This is an email I got back from Mobil1 customer service regarding using their Mobil1 75w90 gear oil in my 2013 Silverado with the G80.

 
On Dec 6, 2017 11:08 AM, "Mobil Products" <[email protected]> wrote:
 
Bruce, 
 
The Mobil 1 Synthetic Gear Lube LS does contain the LS friction modifier. Normally the addition of the LS additive does not make a difference in non-LS conditions, but since your application says specifically not to use it, its a safe conclusion that we can't recommend it.  The only other alternative I could think of would be the Delvac 1 Gear Oil, but I would need to know the application before I could confirm. What year and model of vehicle is this G80 in? 
 
 
Thank you for choosing Mobil, 
Jake Ackerman 
Mobil Help Desk Team
If I can help further with this inquiry or with any other questions, please respond to this email or phone us 1-800-ASK-Mobil

If you would like to provide feedback on your experience with us today, please email [email protected]
On Tue, 5 Dec at 8:32 PM , 13z714x4 <[email protected]> wrote: The following inquiry was sent through: ExxonMobil
Brand: ExxonMobil
First Name: Bruce
Last Name:
Position Title:
E-mail:
Company:
Address:
Country: United States
Phone:
Fax:
Current Customer: Yes
Question:
Is your Mobil 1™ Synthetic Gear Lube LS 75w90 recommened for GM truck G80 
locking differentials.  The GM G80 is not Limited Slip and LS additives are 
not to be used.  Does you Mobil 1™ Synthetic Gear Lube LS 75W90 actually 
contain the LS additives or is the lube just compatible with LS differentials?
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Bruce, the differential in your truck is both a locker and limited slip and the factory fluid does contain the limited slip additive (89021677).

 

From the service manual:

 

Locking Differential Description and Operation 

The locking differential consists of the following components:

Differential case -1 or 2 piece
Locking differential spider -2 piece case only
Pinion gear shaft -1 piece case only
Differential pinion gear shaft lock bolt -1 piece case only
Two clutch discs sets
Locking differential side gear
Thrust block
Locking differential clutch disc guides
Differential side gear shim
Locking differential clutch disc thrust washer
Locking differential governor
Latching bracket
Cam plate assembly
Differential pinion gears
Differential pinion gear thrust washers

 

The optional locking differential (RPO G80) enhances the traction capability of the rear axle by combining the characteristics of a limited-slip differential and the ability of the axle shafts to "lock"together when uneven traction surfaces exist. The differential accomplishes this in 2 ways. First by having a series of clutch plates at each side of the differential case to limit the amount of slippage between each wheel. Second, by using a mechanical locking mechanism to stop the rotation of the right differential side gear, in order to transfer the rotating torque of the wheel without traction to the wheel with traction. Each of these functions occur under different conditions.

 

Limited-Slip Function 

 

Under normal conditions, when the differential is not locked, a small amount of limited-slip action occurs. The gear separating force developed in the right-hand clutch pack is primarily responsible for this.

The operation of how the limited-slip function of the unit works can be explained when the vehicle makes a right-hand turn. Since the left wheel travels farther than the right wheel, it must rotate faster than the ring gear and differential case assembly. This results in the left axle and left side gear rotating faster than the differential case. The faster rotation of the left-side gear causes the pinion gears to rotate on the pinion shaft. This causes the right-side gear to rotate slower than the differential case.

Although the side gear spreading force produced by the pinion gears compresses the clutch packs, primarily the right side, the friction between the tires and the road surface is sufficient to overcome the friction of the clutch packs. This prevents the side gears from being held to the differential case.

 

Locking Function 

 

Locking action occurs through the use of some special parts:

A governor mechanism with 2 flyweights
A latching bracket
The left side cam plate and cam side gear
When the wheel-to-wheel speed difference is 100 RPM or more, the flyweights of the governor will fling out and one of them will contact an edge of the latching bracket. This happens because the left cam side gear and cam plate are rotating at a speed different, either slower or faster, than that of the ring gear and differential case assembly. The cam plate has teeth on its outer diameter surface in mesh with teeth on the shaft of the governor.

As the side gear rotates at a speed different than that of the differential case, the shaft of the governor rotates with enough speed to force the flyweights outward against spring tension. One of the flyweights catches its edge on the closest edge of the latching bracket, which is stationary in the differential case. This latching process triggers a chain of events.

When the governor latches, it stops rotating. A small friction clutch inside the governor allows rotation, with resistance, of the governor shaft while one flyweight is held to the differential case through the latching bracket. The purpose of the governor's latching action is to slow the rotation of the cam plate as compared to the cam side gear. This will cause the cam plate to move out of its detent position.

The cam plate normally is held in its detent position by a small wave spring and detent humps resting in matching notches of the cam side gear. At this point, the ramps of the cam plate ride up on the ramps of the cam side gear, and the cam plate compresses the left clutch pack with a self-energizing action.

As the left clutch pack is compressed, it pushes the cam plate and cam side gear slightly toward the right side of the differential case. This movement of the cam side gear pushes the thrust block which compresses the right-hand side gear clutch pack.

At this point, the force of the self-energizing clutches and the side gear separating force combine to hold the side gears to the differential case in the locking stage.

The entire locking process occurs in less than 1 second. The process works with either the left or right wheel spinning, due to the design of the governor and cam mechanism. A torque reversal of any kind will unlatch the governor, causing the cam plate to ride back down to its detent position. Cornering or deceleration during a transmission shift will cause a torque reversal of this type. The differential unit returns to its limited-slip function.

The self-energizing process would not occur if it were not for the action of one of the left clutch discs. This energizing disc provides the holding force of the ramping action to occur. It is the only disc which is splined to the cam plate itself. The other splined discs fit on the cam side gear.

If the rotating speed of the ring gear and differential case assembly is high enough, the latching bracket will pivot due to centrifugal force. This will move the flyweights so that no locking is permitted. During vehicle driving, this happens at approximately 32 km/h (20 mph) and continues at faster speeds.

When comparing the effectiveness of the locking differential, in terms of percent-of-grade capability to open and limited-slip units, the locking differential has nearly 3 times the potential of the limited-slip unit under the same conditions.

 

Locking Differential Torque-Limiting Disc 

 

The locking differential design was modified in mid-1986 to include a load-limiting feature to reduce the chance of breaking an axle shaft under abusive driving conditions. The number of tangs on the energizing disc in the left-hand clutch pack was reduced allowing these tangs to shear in the event of a high-torque engagement of the differential locking mechanism.

At the time of failure of the load-limiting disc, there will be a loud bang in the rear axle and the differential will operate as a standard differential with some limited-slip action of the clutch packs at low torques.

The service procedure, when the disc tangs shear, involves replacing the left-hand clutch plates and the wave spring. It is also necessary to examine the axle shafts for twisting because at high torques it is possible to not only shear the load-limiting disc, but to also twist the axle shafts.
 

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8 minutes ago, swathdiver said:

Bruce, the differential in your truck is both a locker and limited slip and the factory fluid does contain the limited slip additive (89021677).

 


 

Well, I guess if you ask 5 people, you will get 5 different answers.  I emailed Mobil1 regarding their 75W90 and they said they could not recommend their lube for a G80 locker.  I emailed Eaton (makers of the G80) and this is their response:

 

Eaton Performance (Eaton Performance Support)

Mar 12, 12:58 ADT

Hello Bruce,
Both synthetic and mineral based are sufficient. Although the G80 uses clutches, it is a full locker. If you use a friction additive this can reduce torque / load carrying capacity and is not recommended. As far as oil weight, please use the specified oil weight and quantity in your vehicles owner's manual.

For further information / support please call 800 328 3850.

Thanks!
-Eaton Performance Support

 

 

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Well, you're right, there's a ton of information out there and not all of it is right or accurate.  I feel safe sticking with my service manual for accurate information and the factory fluids to keep the truck in top form.  The factory rear differential lube includes the additive and I and many others have had no problems at all.  I'm a Mobil One guy too when it comes to my engine oil, been so since the 1980s.  But for my differentials, I've always stuck with the GM/AC Delco fluids and additives when needed.  Back in the 1980s, we had these little bottles of posi lube to keep the rear from chattering on turns and I think two bottles did the trick.  

 

The aforementioned GM fluid is synthetic if you weren't aware too.  It has a new grape scent too!

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Agreed.  I put Mobile 1 synthetic 75w90 in two years ago at 40k.  It clearly states that it is "compatible with LS applications".  I have had no problems. At this point, I will probably go with it again.  I know a whole lot of GM truck owners using it as well with no reported issues.  

I hate buying anything from the dealers.  I imagine they charge $20 a quart for their purple kool aid.

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They should clear the confusion and state "additional limited slip additive is unnecessary."

The g80 is basically a limited slip that has the ability to put so much "load" on the clutches that they won't slip...this is why an old g80 will no longer function once the clutch linings are worn out or glazed over.

Other than the g80 has more moving parts, this isn't much different than a Dana Power-loc that wedges the clutch packs tighter the more power you apply...

Sent from my SM-G950U using Tapatalk

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