Eaton's Recommendations on Locking Rear Diff Service

[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.

[DmaxMaverick]

Your level is OK. The later models had the service lube amount adjusted to reduce incidents of axle seal leakage. It was 3.8L, and is now about 3L, or about 3/4" below the fill hole. Anywhere between 1/2" and 1" is fine.

The goo you found (and hopefully cleaned) on the drain plug and the color is normal for a first/early service. It's all the fine wear-in metal particles.

[MacDR50]

Thanks DM. I will be looking at the front end, most likely before the winter.