Eaton's Recommendations on Locking Rear Diff Service

[DonG]

wxmn6,

The two sets of numbers refer to the thickness or viscosity of the oils. Very similar to motor oil like 15W-40.

The 75W-90 weight gear oil is what comes in your truck differential and is recommended most of the time.

The 75W-140 is listed in your operators manual as an alternative gear oil if your tow heavy loads in hot weather. Heavy loads would be 12,000 lbs. or above. Several people on this site feel that even when towing heavy, the 75W-90 will still be fine.

When towing 14,000 lbs. I use the 75W-140.

I hope this helps.
Don

[jbplock]

Ditto's to Don post.

Also another point regarding the numbers in an oil viscosity spec…

The high number (90 in 75W-90) is measure of the oil’s viscosity when it reaches operating temperature and the low number is a measure of the oil’s cold temperature viscosity.

So, 5W-40 and 15W-40 oil have the same viscosity at operating temp but the 5W=40 will flow better when cold.

Conversely 75W-140 oil will be “thicker” when hot than 75W-90 oil but have similar cold temp performance.

[img]smile.gif[/img]

[wxmn6]

Thanks for excellent information. It's nice to learn about something like that.

I am just curious that if both 75w-90 and 75w-140 have same cold weather performance (which I was concerned because I live in NY where we can see sub zero temperature during the winter), then would it hurt to go with higher viscosity if you don't do alot of heavy towing? I would assume that thicker oil would give more protection? Would it (140) actually do more harm than good (than using 90), or is it just that 140 cost more than 90?

With the information given earlier, I will be going with 75w-90, but just want to learn a bit more about it.

Do the front transfer case also use 75w-90 too? If I am not mistaken, the front transfer case does not have factory sythetic, correct? If I want to go sythetic, is there something else that I need to do (other than draining) to properly clean the case so it would not get contaimated with mixed fluid? Thanks.

[G. Gearloose]

Originally posted by wxmn6:
Do the front transfer case also use 75w-90 too?

Hmmm, you should only have one...

[jbplock]

Originally posted by wxmn6:
... I live in NY where we can see sub zero temperature during the winter), then would it hurt to go with higher viscosity if you don't do alot of heavy towing? ....

Does the front transfer case also use 75w-90 too? If I am not mistaken, the front transfer case does not have factory synthetic, correct? If I want to go synthetic, is there something else that I need to do (other than draining) to properly clean the case so it would not get contaimated with mixed fluid? Thanks. [/QB]

wxmn6,

I’m not an expert on this subject but from what I’ve learned unless you are towing big loads regularly it’s best to stay with the GL-5 synthetic 75W-90. Using the heavier 75W-140 when not needed can lower your fuel efficiency.

And yes last I knew the front diff comes with 80W- 90 mineral oil but you can switch to a GL-5 synthetic 75W-90 (the 4WD transfer case uses DEXRON III). However, GM recommends you have the “white” vent cover on your front Diff for compatibility with the synthetic oil. See Feb03 GM Techlink article for more info.

It’s also not necessary to flush or clean the diffs (front or rear) when doing a routine fluid change to synthetic – the fluids are compatible. Just remove the magnetic plug, drain and refill. Also clean the plug ... the front diff plug on mine had quite a bit of “goo” on it. A small amount of metal filings is normal. (see pics)

Personally I changed both front and rear diffs to Mobil-1 75W-90 at 5kmiles. I also changed the transfer case and Allison to Mobil-1 ATF.

A search of the forum will also turn up a quite a bit of info on this subject.

Hope this helps… [img]smile.gif[/img]

[wxmn6]

Thanks for very helpful information!

[alaskanmax]

According to some folks on the forums it is said that if you lift both rear wheels on the d-max, and rotate one wheel the g-80 limited slip will act just like an open diff and the other wheel will counter rotate. With out opening up my cover on my rear diff or getting stuck how will I know for sure what my used truck has??? I can find no stickers on dash or door areas to say it has a g-80

[DmaxMaverick]

According to some folks on the forums it is said that if you lift both rear wheels on the d-max, and rotate one wheel the g-80 limited slip will act just like an open diff and the other wheel will counter rotate. With out opening up my cover on my rear diff or getting stuck how will I know for sure what my used truck has??? I can find no stickers on dash or door areas to say it has a g-80

Look in the glove box. Your RPO sticker is there, and G80 will be on that list, if you have one.

If you raise both wheels and spin one slowly (tranny in P), the other wheel should turn the opposite direction. If you spin one very quickly, like a good kick, it will lock. You can also test it on a roadside shoulder, one wheel on the pavement, and the other on loose ground (less traction). You should feel it lock when you goose it and one wheel breaks traction.

[SoTxPollock]

ALASKAMAX, where are you located? I may have seen your truck, seems to be the truck of choice where I was last week, sure miss those 50 degree days, came back to 90 degree Texas days.
One word of caution when the locker locks, if you are on slick hard surface be prepared to back off the accelerator or you could find yourself going around in circles. On an open rear end at least one of the wheels will tend to help you keep it straight, but if both start to spin the engine torque and rear differential torque will tend to make the truck back end slide to the right. Enjoy the truck.

[alaskanmax]

I, and truck are both in Anchorage. Today, I recieved my copy of Differentials Identification, Restoration & Repair by Jim Allen and Randy Lyman. I have read most all in the past couple hours, so listen up everyone....buy a copy as this book is really good and will answer any questions on the AAM 11.5/Eaton G-80. ......Thank You to Jim and Randy

[SoTxPollock]

Alaskamax, been looking at my pictures since I got back, sure like your state. I will be back, by the way I have been running Mobil 1 75-90 synthetic for over 100,000 miles now with a G-80 and no problems whatsoever. I run it front and rear.

[jharden1]

Saw a infomercial that claims it locks up at 25 rpm difference

[DmaxMaverick]

Never saw an infomercial about it, but Eaton states the Gov-Loc engages with a 100 RPM differential. It may sound like a lot, but it isn't. At 25 RPMs, you couldn't make a u-turn w/o it locking.

[Turbo Al]

I brought the rear end break in procedure back to the top of forums just to confuse everyone a little more LOL
Al

[TraceF]

My manual seems to indicate that BOTH the front axle and rear axle have synthetic lubricants. No change is required. I'll go to Mobil 1 around 6,000 miles.

[MacDR50]

What rear and front axles do I have or how can I tell? My rear is locking and I have a 4X4. I intend to tow at least 11,000 lbs. I didn't read anything about changing my lubes after a break-in period. The truck hasn't towed anything yet. I intend this to be my last truck so it has to last at least 10-15 yrs and if I look after it and myself maybe longer. I can do all my own fluid changes as I have access to a lift.

[DmaxMaverick]

What rear and front axles do I have or how can I tell? My rear is locking and I have a 4X4. I intend to tow at least 11,000 lbs. I didn't read anything about changing my lubes after a break-in period. The truck hasn't towed anything yet. I intend this to be my last truck so it has to last at least 10-15 yrs and if I look after it and myself maybe longer. I can do all my own fluid changes as I have access to a lift.

All Duramax equipped trucks are the same. The rear is an AAM 11.5" axle. If it has a locker, it's the Eaton Gov-Loc. The front is a GM Corporate 9.25" IFS (I could be wrong on the size). The only gear ratio offered for these trucks is 3.73.

Changing the rear lube at the right time can be important. Once you've seen what comes out, you won't ask why (reminiscent to those with newborn children). And, soon after you begin towing a significant load (11K qualifies). Originally, we recommended a change at 5K with no significant towing. Or, after 500 miles of towing. If you are keeping your rig for all eternity (relatively), it could be different. Lube changes are cheap, and easy. Money very well spent early on. Work hardening and cleaning of the assembly junk is important for long life. In your case, I recommend an initial change (if over 5K), then another change after 500-1,000 miles after a significant towing experience. Also, be careful to not overheat the diff, which is easy when towing and new. Frequent stops (every hour or so) with a cooling period of 15 minutes is good. This ensures thorough work hardening of the gear set. You should have to do this for more than the first 500 miles or so, of towing. DO NOT use dino lube in the rear end, under any circumstance. As we've seen, the brand of synthetic lube hasn't shown any noticeable difference. As long as it's full synthetic (none of that "mix" stuff), it should be fine with any of the popular brands.

[MacDR50]

Thanks Dmax" I have only 1200 miles on the truck and no towing yet. I think I will change just before I start towing in the spring. I should be around 3000 miles by then. (The truck I mean. There are many-many more miles on driver and more than a few that were heavy tows.)

[DmaxMaverick]

Thanks Dmax" I have only 1200 miles on the truck and no towing yet. I think I will change just before I start towing in the spring. I should be around 3000 miles by then. (The truck I mean. There are many-many more miles on driver and more than a few that were heavy tows.)

That sounds OK. You could get away with holding off the first change until you have about 500 miles of heavy towing. It's easy to do, and can be done anywhere, and quickly. The diff has a drain plug, and the rear cover does not need to be removed. You may have to lower the spare to cleanly access the fill hole, though. The factory fill lube is good, it just gets full of junk. The heating/cooling cycles are key, though. Do it right and the diff will last your lifetime.

[MacDR50]

Okay I had my first 500 of heavy towing and went out to my local dealer and asked for the right amount of grape juice to do a rear end fluid change. He sold me three liters and said I needed only about 2.8. I drained the rear end and it looked gray and dirty and the drain plug had a small head of goo the consistency of plumbers putty. I then put in all three liters of the new stuff and I am about 1 inch below the filler hole. Am I OK or should I add more? BTW the owners manual says what kind but not how much. PITA for sure.