Locking Diff 'clunk'

[ts_hunter]

While spinning the tires and driving in a circular motion (doing donuts in the snow), I am noticing a slight 'clunk' in the rear end. I am not sure if I have the locking differential, but from reading other posts, this may be the source of the clunk when in 2 wheel drive. Ideas?

 

('05 Sierra 4x4 Z71)

[MS3DALE]

Look in your glovebox for "G80",Thats the RPO code for the locking differential.

[NathanL]

If you have the G80 and continue to do donuts in the snow and hear that clunk get ready to buy a new one.

[unit731]

Just a quick review.

 

What does Locking Rear Differential do?

 

Is it only in 2WD or only in 4WD or both?

 

Thanks.

[MS3DALE]

Just a quick review.

 

What does Locking Rear Differential do?

 

Is it only in 2WD or only in 4WD or both?

 

Thanks.

 

It works in 2 or 4-wheel drive,This should help explain how it works....

 

Document ID# 701869

 

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

2 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, or the left differential side gear on the 10.5 inch axle, 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 (left-hand side on 10.5 inch axle) 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.

[unit731]

Thank you!

[asilverblazer]

My GMT900 is much clunkier than my 1999 GMT800 was. With my old truck it was one clunk, engaged. The new one is clunk, clunk, clunk, ...engaged?

[Bostonsfavson]

That's pretty nifty Rich! Do you have one of those, or something similar, for the AWD system in the Sierra Denalis?

[MS3DALE]

That's pretty nifty Rich! Do you have one of those, or something similar, for the AWD system in the Sierra Denalis?

 

Yep,Here ya' go......

 

Document ID# 1708507

2007/2008 GMC Truck Sierra Denali

 

Transfer Case Description and Operation:

The Borg Warner (BW) model 4485, RPO NR3 transfer case is a 1-speed, full time, all wheel drive (AWD), transfer case. The transfer case provides power to both axles, through an external planetary type differential, which has 2 different sets of pinion gears. The planetary differential provides a 40/60 torque split, front/rear, full time. This means both axles are constantly being driven for maximum traction in all conditions.

 

The transfer case external type planetary differential functions the same as a typical rear axle differential. The transfer case differential pinion gears function as the spider gears, and the sun gears function as the side gears.

 

The following actions occur because of the planetary differential:

 

• If the vehicle is on a hoist, the front propeller shaft can be rotated by hand.

 

• The vehicle cannot be driven if 1 propeller shaft is removed.

 

• Operating the vehicle on the hoist can damage the differential pinion gears, by over-spinning.

 

• Operating the vehicle with 1 propeller shaft removed causes over-spinning of the differential pinion gears.

 

The BW 4485 design of the planetary differential allows the use with the Vehicle Stability Enhancement System (VSES) vehicles. The VSES takes use of the planetary differential, by applying braking to a tire that has less traction and dividing the engine torque to the other axle.

 

The BW 4485 case halves are high-pressure, die-cast magnesium. Ball bearings support the input shaft, the front output shaft, and the rear output shaft. The transfer case requires DEXRON®VI Automatic Transmission Fluid (ATF) GM P/N 88861003 (Canadian P/N 88861004), which is red in color.

 

When the BW 4485 is operating in the AWD mode, the power flows from the transmission to the transfer case input shaft. The input shaft delivers the power to the planetary differential. The case of the planetary differential is splined to the input shaft. The planetary differential splits the torque 40 percent through the front differential pinion gears to the front sun gear. The front sun gear is engaged with the drive sprocket, which drives the chain to the front output shaft, and power is delivered to the front propeller shaft. 60 percent of the torque to the rear output shaft goes through the rear differential pinion gears and rear sun gear, which is connected to the rear output shaft and to the rear propeller shaft.

 

Document ID# 1708507

2007/2008 GMC Truck Sierra Denali

[slimjim2525]

That's pretty nifty Rich! Do you have one of those, or something similar, for the AWD system in the Sierra Denalis?

 

Yep,Here ya' go......

 

Document ID# 1708507

2007/2008 GMC Truck Sierra Denali

 

Transfer Case Description and Operation:

The Borg Warner (BW) model 4485, RPO NR3 transfer case is a 1-speed, full time, all wheel drive (AWD), transfer case. The transfer case provides power to both axles, through an external planetary type differential, which has 2 different sets of pinion gears. The planetary differential provides a 40/60 torque split, front/rear, full time. This means both axles are constantly being driven for maximum traction in all conditions.

 

The transfer case external type planetary differential functions the same as a typical rear axle differential. The transfer case differential pinion gears function as the spider gears, and the sun gears function as the side gears.

 

The following actions occur because of the planetary differential:

 

• If the vehicle is on a hoist, the front propeller shaft can be rotated by hand.

 

• The vehicle cannot be driven if 1 propeller shaft is removed.

 

• Operating the vehicle on the hoist can damage the differential pinion gears, by over-spinning.

 

• Operating the vehicle with 1 propeller shaft removed causes over-spinning of the differential pinion gears.

 

The BW 4485 design of the planetary differential allows the use with the Vehicle Stability Enhancement System (VSES) vehicles. The VSES takes use of the planetary differential, by applying braking to a tire that has less traction and dividing the engine torque to the other axle.

 

The BW 4485 case halves are high-pressure, die-cast magnesium. Ball bearings support the input shaft, the front output shaft, and the rear output shaft. The transfer case requires DEXRON®VI Automatic Transmission Fluid (ATF) GM P/N 88861003 (Canadian P/N 88861004), which is red in color.

 

When the BW 4485 is operating in the AWD mode, the power flows from the transmission to the transfer case input shaft. The input shaft delivers the power to the planetary differential. The case of the planetary differential is splined to the input shaft. The planetary differential splits the torque 40 percent through the front differential pinion gears to the front sun gear. The front sun gear is engaged with the drive sprocket, which drives the chain to the front output shaft, and power is delivered to the front propeller shaft. 60 percent of the torque to the rear output shaft goes through the rear differential pinion gears and rear sun gear, which is connected to the rear output shaft and to the rear propeller shaft.

 

Document ID# 1708507

2007/2008 GMC Truck Sierra Denali

 

Rich, man you are an unending wealth of knowledge here, I thank you for all the info you provide. I don't post much, (only 230 or so to date), but I read almost every post in the light duty section. I got to ask, do you type each of those document posts, or is there a way of copying them here? Again, thanks for all your contributions here.

[MS3DALE]

Thanks Jim!!!

Well,most of the time I just "copy & paste" the article or document from GM-DealerWorld.

What helps is that I usually know where to go to find the articles or TSB's/Recalls,then all I have to do is copy & paste them.

[Ranger]

All Z71s get the G80 locker, it just acts differently based on the truck and terrain. My 2500HD was pretty jerky on sand and I could feel it engage at times in the mud.

[michigansilver03]

I'm up in Michigan and in the snow you can feel and hear it kicking it. I works GREAT

[Bostonsfavson]

Thanks Rich! You're definitely one of the most valuable and helpful members on the site. Just a quick question about this part:

 

"The BW 4485 design of the planetary differential allows the use with the Vehicle Stability Enhancement System (VSES) vehicles. The VSES takes use of the planetary differential, by applying braking to a tire that has less traction and dividing the engine torque to the other axle."

 

I assume the VSES mentioned here is StabiliTrak and/or traction control? I had a 2002 Oldsmobile Bravada (AWD) for a couple years. Equipped with the same tires I have on my Denali (Bridgestone Blizzaks) the Bravada seemed much better in the snow. Don't get me wrong, the Denali is good, but I was expecting more. Kind of a bummer.

[MS3DALE]

Yea,I also assume the "VSES" is the Stabilitrak system.