Rear End Fluid?

[Daryl Z71]

I was looking to change the Rear End Fluid in the truck here in a little while. Im going to put Royal Purple in it. I have the G80 and I was wondering what type and where to get it at Positrack fluid I need to add to it too?

[hockeypro1411]

DO NOT ADD ANY KIND OF POSI ADDITIVE!!! Use straight synthetic 75w-90 gear oil.

[Daryl Z71]

DO NOT ADD ANY KIND OF POSI ADDITIVE!!! Use straight synthetic 75w-90 gear oil.

 

 

Oh Ok. I was thinking about the GMT800's I think? I forgot that the GMT900's don't need any. I could have just read the owners manual too, but that wouldn't have told me where to get the additive IF I needed it.

[Crash138]

DO NOT ADD ANY KIND OF POSI ADDITIVE!!! Use straight synthetic 75w-90 gear oil.

 

Im not arguing with you, but why wouldnt you put the posi fluid in there when GM recommends it?

[txdutt]

not sure about the RP stuff but the Mobil-1 gear lube doesn't need the additive--says so on the bottle...

[RyanbabZ71]

DO NOT ADD ANY KIND OF POSI ADDITIVE!!! Use straight synthetic 75w-90 gear oil.

 

Im not arguing with you, but why wouldnt you put the posi fluid in there when GM recommends it?

 

 

With the GMT800's and 900's no additive is needed just 75W-90 synthetic gear oil. GM only recommends it on the pre 99 trucks.

[hockeypro1411]

DO NOT ADD ANY KIND OF POSI ADDITIVE!!! Use straight synthetic 75w-90 gear oil.

 

Im not arguing with you, but why wouldnt you put the posi fluid in there when GM recommends it?

 

 

Because we're thinking of different model trucks here. Adding it to his truck would delay the locking of the differential and cause stuff to break. ...basically.

[dlstewart01]

DO NOT ADD ANY KIND OF POSI ADDITIVE!!! Use straight synthetic 75w-90 gear oil.

 

Im not arguing with you, but why wouldnt you put the posi fluid in there when GM recommends it?

 

 

 

No they don't..... GM has never required an additive in a Truck. They DO in a G80 in a car.

 

The oldest truck factury manual I have is a 1996 and it does not call for an additive either.

 

LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109

SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)

VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS

Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.

This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.

To correct this condition, drain and refill the rear axle with SAE 75W-90 GL5 (P/N 12378261). The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.VEHICLES/COMPONENTS INVOLVED: ----------------------------- Some light duty trucks equipped with locking rear axles, RPO G80.

SERVICE PARTS INFORMATION:

Part Number Description ----------- ------------------12378261 Lubricant, Rear Axle (1 litre)

Parts are currently available through CANSPO.

WARRANTY INFORMATION:

As specified in Light Duty Truck Maintenance Schedules, locking rear axle fluid drain and refill is required owner maintenance at the first engine oil change. Failure to drain and refill the rear axle as specified may contribute to a later axle chatter condition. Refer to the appropriate Light Duty Truck Maintenance Schedule or service manual, section OB, for further details on change intervals.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.

© Copyright General Motors Corporation. All Rights Reserved.

 

How a G80 locker works....

 

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 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

[Hoffxr]

I just did this. I contacted GM, Eaton and Mobil1/Exxon. All stated to use GM synthetic only for a truck with G80. No additive required.

Correct part number for 75w90 is 8890041. It's not cheap but it is what's required, especially for a vehicle under warranty.

I have had lockers stop working in the past and all had aftermarket fluid with many miles on them.

Good luck

[2017SierraSLT]

I recommend to anyone to change their differential fluids. The factory fill in my rear differential not only was half a pint low, it was this nasty black crap and it left a residue on everything. I switched to Amsoil Severe Gear 75W90. It took less than an hour and $32. The gasket is reusable. I would drop the spare tire. Makes everything a lot easier to drain/refill the differential.

[Colossus]

As one who has had a G80 granade on a 1999 GMT-800 with around 70,000 miles, I strongly suggest using the right fluid and changing it every 15,000-25,000 miles. $1600 was a very expensive repair bill for a new G80 back in the day. It is probably even more now.

[Gurrzt]

Im not arguing with you, but why wouldnt you put the posi fluid in there when GM recommends it?

 

The G80 code on these trucks does not indicate an LSD (as in my 2001 Z28 (Sara)).

 

There is no need for the additive . Use a high quality GL5 or GL6 replacement fluid.

[Cowpie]

Since the OP obviously is looking for a boutique 75w90 to put in the diffs, I would recommend the Severe Gear 75w90 from Amsoil. I have used if in a lot of vehicles over the years. It is an exceptional product. If nothing else, just looking up your vehicle on the Amsoil site will yield the information one needs to find what they need to put in the vehicle from engine oil to brake fluid. Amsoil does a pretty good job of staying on top of OEM specs for what it needed and how much the capacity is. Even if they are deciding on another brand of oil or lube, the Amsoil site provides some good info on a vehicle fluid needs.

 

As a side note, I changed out ALL drive train fluids and lubes before the new vehicle turned 3000 miles. I like getting the early metal particulates from the component build out of there. Good thing I did. The rear diff was 2 pints low from the factory, and the transfer case fluid look horrible and the magnetic plug had enough metal shavings on it to make me think someone had stuck a cat's tail in a light socket.

[Jvs]

I've been looking into doing the rear diff fluid on my 2000 1500 with the 5.3

 

I got a couple questions.

 

I want to use 75w90 weight gear oil in my truck correct? Any recommendations on brand?

 

I need a new gasket correct?

 

Anything else to look for? I know I need to loosen the filler cap before draining it (so I be sure I can refill it with new fluid and it's not seized).

[Black02Silverado]

I've been looking into doing the rear diff fluid on my 2000 1500 with the 5.3

 

I got a couple questions.

 

I want to use 75w90 weight gear oil in my truck correct? Any recommendations on brand?

 

I need a new gasket correct?

 

Anything else to look for? I know I need to loosen the filler cap before draining it (so I be sure I can refill it with new fluid and it's not seized).

Wow this is an old thread!

 

Yes, you need a 75w-90 weight fluid for your rear differential. Mobil 1, Royal Purple and AMSOIL Severe gear are the three major recommendations. Redline as well if you can find it locally if not you can get it online. I would get a new gasket. That way you will be covered. I know the newer trucks have a resuable gasket but yours I'm pretty sure isn't.

 

Good idea to make sure to be able to remove the fill plug before starting.

 

If you are interested in AMSOIL let me know and I can get a quote delivered to your door for the Severe Gear 75w-90. Unless you can find a local dealer in your area that carries it. You can do a search on the AMSOIL web site for a local dealer near you.