Slip Diff

markwilks · May 17, 2017

I want to change the front and rear diff fluid. How do I know if I need a additive? I've got a 03 Sierra 2500 HD. Any recommendations on what fluid I should use?

On a side note just had the transfer case fluid changed and no pump rub yet. Going to order the upgrade kit. Local shop said they would put it in for $377. I think that's a good deal. Anyone else done this preemptive swap?

jusbecuz · May 17, 2017

I have an 04 3500 and I run Mobil 1 synthetic 75-90 in both the front and rear with no additional additives.

davester · May 17, 2017

Well, if you were going to get a pump rub fix installed, there was no point to changing the TC fluid, as they SHOULD be putting new fluid in afterwards. And $377 doesn't sound like a crazy amount, as you have to

-drain the fluid

-remove the TC

-take it apart

-hopefully clean out the inside (bottom tends to collect a bunch of rtv from when it was last sealed as well as the mating surfaces

-put in the kit (probably just the pressed metal plate, it looks flimsy & cheap, but it's fine)

-bolt it together

-reinstall it and fill it with fluid

And you want to do it preemptively, as if you wait until there is a hole, all the fluid disappears and parts start failing unless you catch it right away. And then most people wind up buying a new rear half rather than just jb-welding the hole (and the case isn't cheap).

Maybe also ask them to look over the internals (or ask if you can yourself), to check that the bearings and gears look ok. Note, you can't tell if the chain is worn or not. Without the rear half of the case in place, the two shafts aren't held in their correct position and the chain WILL be very loose.

I also used Mobil 1 syn 75w90, no additives, but I also know both axles don't have a limited-slip/locking diff. IIRC, the mobil 1 already has some stuff for LSD already (you should read the bottle to confirm), if you do have a LSD. You can find out if you have one by decoding the RPO codes on the sheet in your glovebox.

Formulabruce · May 17, 2017

The Rear end has a drain plug FYI. hard to see, its in the very middle bottom and its FLUSHED with the case, I think it takes a 3/8 ratchet, could be a 1/2 though, so you do NOT need to take off the back cover. Good luck!! PS, No need for any posi type lube as these do NOT need to 'Slip" around corners, they disengage, the G80 is a locker .

markwilks · May 18, 2017

Thanks guys. So the code I'm looking for if I do have LSD is G80? I probably won't get to the pump upgrade kit for a few paychecks. They said if the fluid is still good they could reuse it. I wanted to get it changed cause I have no idea if it had ever been changed. I looked in my owners manual today and it says 75/90 synthetic for rear and 80/90 for front. Does it really matter if I put synthetic in front and rear? Any other advice or tips? My rear cover is pretty rusty so I think I'll put a new one on.

dewfpo · May 18, 2017

The G80 is a Locker and not an LSD.

DEWFPO

markwilks · May 18, 2017

So I looked at my codes. I do have G80 and also GT5 which I see is a 4.10 ratio. Is the G80 a good thing? I still don't understand what code is limited slip and if I need a additive or not. Thanks for all your help. I'm just a average guy wanting to save some money and do this myself but I don't want to mess anything up to save myself a few bucks.

davester · May 18, 2017

the G80 works, but it also has the reputation as being a 'gov-bomb', in that it's not unusual for them to fail, particularly if they are abused. I believe GM recommends synthetic for the rear diff.

jake111 · May 18, 2017

As someone mentioned above, Mobil 1 synthetic 75-90 is an excellent choice for both the front and rear diffs. It's also what I have used for the last 10 years.

However, the front diff vent cover on 2001-2003 might not be fully compatible with synthetic gear oil. Why? I haven't a clue. The so-called incompatible vent covers were black. The compatible ones are white, so you can easily check if you have one. Or you can just try it. Or you can buy a white cover like from here: http://www.merchant-automotive.com/12479390-White-Vent-Cap.aspx(or if links not allowed, Merchant Automotive white vent cap Part #12479390)

Raptordude · May 19, 2017

How tough is the G80? Will it hurt it to spin around corners in the rain every time it rains?

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Z45 · May 19, 2017

¨ 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.

PS - Adding a limited slip additive to a G80 Locking Differential can cause the mechanism to slip and miss engagement. Probably a reason why some blow up, improper maintenance.

Note - G80 code is used for both a limited slip diff and the locker. The model is what determines usage, e.g. cars get limited slip etc.

jake111 · May 20, 2017

Maintenance free; requires no lube additives.

Maintenance free??? Really??? Yeah, I have a Toyota with "lifetime" ATF, which appears to mean until the transmission destroys itself. I changed the gear oil in my G80 gov-loc at 18k miles and I wished I had done it sooner. The gear oil glistened with metal. That's normal when a diff breaks in. After the initial break-in, every 50k miles is probably sufficient in most cases (more often if you tow a lot; less often if all you do is drive down the highway).

Requires no lube additives??? Actually, the G80 gov-loc has some friction plates that are used to engage the locker and they work much better with the right amount of friction modifier; not too much and not too little. GM's gear lube "grape juice" has the proper additives and "requires no lube additives", which more likely means "don't add more". Mobil 1 synthetic gear oil says "LS" because it has some friction modifiers in it, and it also works very well in the G80.

Z45 · May 20, 2017

Friction modifiers are used to control chatter. That's the sole purpose of them.

Chatter is essentially the build-up and release of energy between the differential clutch packs in a limited slip differential during operation. This phenomenon often occurs when torque is transferred between the clutches or when rotational speeds change. During the stick-slip phase, energy is built up to a point where contact between the clutch plates change between static friction (stick) and dynamic friction (slip), resulting in noticeable vibrations. It is this vibration that causes an audible chatter or ratcheting to be heard within the differential. While certainly an annoying characteristic, there is typically no mechanical damage occurring to the differential. It is most often noticed in parking lots or slow driving while turning where the outside wheel turns faster than the inside wheel.

In an effort to control this chatter, friction modifiers are often used. These additives result in a smoother transition between the two states of friction: static and dynamic. Static friction occurs when the clutches physically lock together, while dynamic friction occurs as the clutches are slipping, ergo Limited Slip Differential.

vucelick · May 20, 2017

the G80 works, but it also has the reputation as being a 'gov-bomb', in that it's not unusual for them to fail, particularly if they are abused. I believe GM recommends synthetic for the rear diff.

That only applies to the 1/2 ton axle gov-locks. The 3/4 and 1-ton 14 bolt gov locks are not prone to failure like the 1/2 tons are.

davester · May 20, 2017

That only applies to the 1/2 ton axle gov-locks. The 3/4 and 1-ton 14 bolt gov locks are not prone to failure like the 1/2 tons are.

I don't think so...I know my truck was shipped with it, but it no longer has the G80 diff. And on another diesel forum, other posters don't think they are overly strong...