More rear end break in questions.

[AOPA Roger]

I am buying a 2003 GMC. As other posts talk about driving with a heavy load for 500 miles and then change the rear end fluid.

I do not pull a trailer for long distances. But this fall, I will use the truck to pull in grain wagons from the field, to the elevator. Miles will probably be around 100 for the whole season. The weight of the wagons apx.31,000.

Should I change rear end fluid at the end of the season? or wait until I accumulate 500 miles of towing?

[Chris N5CWM]

I'm curious about the same thing. Now have 12,000 miles on the Yukon XL. Was going to have dealer change the rear axle lube at 6,000 miles but I balked at the price. I ordered Amsoil Series 2000 SAE 75W90 and have the gasket in the garage. The alxe I have is the older 14 bolt 1 ton full-floating axle not the new AA that is under the 2500HD DMax/496.

http://www.chuckschevytruckpages.com/14bolt.html

[ 08-07-2002: Message edited by: Turbo Al ]</p>

[TxDoc]

Two bits of info from Eaton I received on the locking rear end(G80)were:

1--
"GM introduced the new body style in 1998. At the same time they upgraded
the differential and changed to a synthetic lube. The G80 is specifically
designed to give ultimate performance with this lube, and the lube contains
special additives that help reduce slip stick that occurs between the
clutches. the GM synthetic lube typically runs about 100+ degrees cooler
than mineral base lube, which is better if you do allot of towing or heavy
hauling. I would recommend that you stay with the GM synthetic, not Mobil 1.
If you don't tow or haul you could use the pre 1998 mineral lube without
degrading performance of the differential. this would be a cheaper option
but require more lube changes. "
"Axle tolerances of the 1/2 ton trucks are very tight. The recommended
procedure was to change lube after 500 miles of heavy towing, which would is
considered a break in period. This breaking period generates high heat
conditions in the axle which was found to degrade the synthetic beyond the
point of mineral lube. After the 500 mile lube change, the axle is broke
in and does not generate the high heat. If you use GM synthetic after the
break-in period, it can be considered "lube for life", although some say to
change it at 40,000 - 50,000 mile intervals."

"If you don't tow, don't worry about the oil change as it a waste of money."


"If your truck is pre 1998 or the old style, it comes with mineral lube. IN
that case you need to stay with mineral because the seals are designed to
work with it. If you changed from mineral to synthetic you'll cause your
seals to swell and eventually leak. It's ok to go from synthetic to
mineral because you won't cause swelling. many of your stop leak
chemicals effect seals in this manner, resulting in a short term fix but a
long term problem."

2--
and on service


"From Mr. Ralph Holmquist of Eaton, the maker of the locking differential:
"The maintenance schedule for the rear axle was developed by American Axle &
Manufacturing and GM truck based on multiple tests. The Eaton locker does
not require additional maintenance nor does it add heat to the lube. The
lube will darken due to the carbon wear on the clutch surfaces, much the
same as a disc brake pad & rotor. This does not damage axle components such
as seals or bearings. However, a new axle can produce excessive
temperatures (plus 350 degrees F) due to the ring & pinion breaking in that
will break the lube additives down. Avoid high loads, trailer towing and
high speed extended driving during the initial break in of the vehicle.
After the break in period axle temps will level at a much lower figure.
Lube changes are a good idea because the additives are replenished and
contaminates such as casting sand are eliminated. The axle is filled at the
factory with a synthetic 75w90 GL5 rating made by Texaco under part # 2276.
The GM service # is 12378261. This is the only lube we have done extensive
testing with to insure locker compatibility. The only negative to using one
of these other lubes is an increased potential for clutch chatter. This
really doesn't hurt anything and can be corrected by changing the lube. I
noticed in the latest GM owners manual the term "or equivalent" when
referring to the lube specification. Look for a GL5 rating on the bottle to
make sure the ring & pinion, seals & bearings are protected. Limited slip
additive is not needed."
The following information outlines the type of
rear axle lubrication utilized in GM Truck axles.
I. Fluid Type
II. The 1500, 2500 and 3500 GMT 800 Trucks utilize SAE 75W-90 Synthetic Axle Lubricant. The GM part number is 12378261 and the specification is 9986115.

[BadDog]

Ok, now I am confused. My new 2500HD has just under 3k on it now. Several have told me to change the diff lube "NOW". However, that was mostly highway 65-75 mph running mostly empty. No towing yet. However, soon I'll be towing roughly 8k for about 800 miles round trip.

So, should I change the fluid now? Sounds like I should definitely use the synthetic ("75w90 GL5 rating made by Texaco under part # 2276.
The GM service # is 12378261."). Doesn't sound like I need the heavy stuff.

Now I'm just not sure if I should change before the trip or after...

[JimWilson]

AOPA Roger;

I don't think you need to worry about the diff fluid for what you're going to be towing, because if you try and pull 31,000# you'll most likely blow it to pieces so the fluid will no longer be necessary! :D :D That was a typo, wasn't it?

[JimWilson]

BadDog;

I have an 02 2500HD CC SB with the Eaton locker and I changed the rear diff fluid with Amsoil 75/90 when the truck had 3K. I hadn't really towed at that point either. I would think that by then most of the casting crap has broken free, and the clutches have shed most of what they're going to, so it was a good time to drain all that out and start fresh. But I'm compulsive; I'll be changing it again at 50K, so that should tell you something about me! :D

[IndigoDually]

I drove a couple thousand miles then checked the fluid and it was clear and light. I hooked up a trailer 4500-7500 lbs and within a hundred miles or so and the oil was cloudy and dark (full of friction material which is normal). I feel that the load is what really gets the clutches working and not just driving unloaded.
I am using the Amsoil and have not had and problems or chatter.

John

[Colorado Kid]

Bad Dog, If I were in your situation I'd make the trip and then change the lube. Towing will change the contact pattern of the gears due to the extended relatively high torque. The new contact pattern, until well established (i.e. 500 miles of towing) will generate a lot of heat, similar to the original break-in, so you'll want to change the "cooked" lube out after the 500 (or in you case 800) miles of towing. Your lube is probably already lightly cooked, but your break in has been pretty gentle so far.

I've accomplished about 500 miles of towing with my rig, and went to get the "grape juice" at the stealer, but they were out and have been back orderred for a month! (Don't tell me to use other lube...I write specs and I intend to use what's specified for the once in a lifetime change.)

[BadDog]

Thanks for the replies. I'll check it now, if it looks ok, I'll make the trip first, then change.

Now, about the "GM grape juice vs. any ol' synthetic" debate. Is there really any need to pay the GM price? I know there is the "less chance for warranty quibble" argument. But that also applies to letting the dealer do the work, which I'm not inclined to do. I just got off the phone with my nearest dealer (moderate truck volume) service manager and he said flat out that he would just put Mobile synthetic in there if it was his and there would be "absolutely no problem" with warranty. Now, I know this is just words, and reality may vary if I go in there with a receipt for Mobil Synthetic from Napa with a blown gov-lock, but is there any *real* benefit to GMs stuff? Is there any documented case where the dealer refused coverage due to

[FightinTXag]

I went with Mobil synthetic. I doubt any service department would look very closely at the lube if you were to have a problem. Somebody had their dealer change the diff fluid and they used Mobil synthetic so most dealers don't even know that the grape juice is the only approved oil. I doubt there's anything magical about the GM lube. I doubt I will have any type of problem with the rearend in the first 36,000 miles. If I were going to leave the gear oil in for a long time I might consider the GM lube, but I plan on changing the rear diff fluid every 25,000 along with the tranny fluid.

[Colorado Kid]

They wouldn't have to "look very close", you can smell the grape stuff a block away. It isn't the warranty I'm worried about, I figure the chances of it failing within 36,000 miles is nil. The folks at Eaton wrote a spec, only one product has gone to the trouble to prove it meets it, That's what I'm going to use. It'll cost less than $100. I paid three times that much just to add the locker, if it cost $100 to take care of it the way the manufacturer says, then that's what it costs.

[vince]

When I took my 01 2500HD for it's first service @ 3 kmiles the dealer informed they were changing the rear diff to synthetic. The front diff was changed but with mineral. Who knows?

[FightinTXag]

I don't think the average service tech would notice if the gear oil didn't come out smelling like the last one he drained weeks ago. Even if he did, I don't think he would know that the foul smelling stuff was the only warranty-approved fluid.

I agree there is very little chance there will be any problems within the warranty period.

Mr. Holmquist from Eaton:
"The only negative to using one of these other lubes is an increased potential for clutch chatter. This really doesn't hurt anything and can be corrected by changing the lube."

$100 to change with the grape juice or $25 to go with Mobil 1 and risk hearing some chatter until switching back to the grapejuice. I took a gamble. I don't have any clutch chatter so all must be well. I'll take the money I save and change it more often. I think more frequent changes with Mobil 1 synthetic will be better overall than longer change intervals with the grape juice.

[AOPA Roger]

Jim, 2 wagons of corn, 250 Bushells in each * 56# per bushell +1500 or so pounds for each wagon empty. This is not unusual for a farm truck to pull 2 loaded wagons to the elevator. I have 350,000 on my D**** and I did not even repalce the clutch until almost 300,000 miles. I think that this truck should hold up at least as good as my current one.

Roger

[Chris N5CWM]

Well I just finished changing the grape juice in
my 14 bolt 10.5" full floater axle in the Yukon
XL 2500.
It has 19,000 miles; I posted earlier 16,000. The
stuff in it definitely smelled like grape juice!
Not too dirty but the magnet at the bottom of the
carrier had the usual accumulation of fine metal
grit and some small flakes. Surely break-in metal.
I used Amsoil Series 2000 75W90 and it took 2.5
quarts to fill it up.
I know, most of you on here have the new 11.5"
American Axle; I'm posting my info for the sake
of comparison. ;)
I did learn something. There IS a DRAINPLUG at
the bottom of the 10.5" axle. Had I known that
I would not have taken off the cover. Spent more
time scraping off old gasket material!

-Chris