Hi to all,Our TV is a GMT400 1997 GMC K2500 7.4L Burb with 149k miles. It is 8-lug, 3.73, GVWR 8,600, with GAWR RR of 6,000 lbs for the full floater rear axle. We believe the rear end also has 149k miles and has never been rebuilt (14-bolt 10.5" full floater with G80 limited slip).When we got this TV last year we changed all fluids including diff fluids, so now the rear diff has standard GM fluid with the standard GM additive for limited slip. We purchased a heavier camper last month and we pulled it back home at about 7k lbs, through the Tn mountains/hills for about a 6 hour pull.The 7.4L & 4L80E handled the hills and load great and never once had to downshift or even have the torque converter unlock. But after that haul I detected a little hum in the rear-end for a day or so (when driving with power applied). It may have been my imagination but I don't think so, and the hum went away after a day or two and a few miles of not towing. This made me think of doing all I can to protect that rear diff for future towing long distances from home.All opinions for action(s) greatly appreciated. My main questions are whether to just put synthetic fluid in the rear diff with possibly some protective additive(s), or if folks might think this diff should be rebuilt (or internally inspected) at 149k miles before doing a lot of heavy towing. Thanks in advance to all, Marty

I would service it and check for wear on the teeth and stuff in the bottom of the housing. And yes, you should be running synthetic.

X2. For that much weight you need full synthetic.

Thanks guys. I'll set up an appointment to have it checked.Any recommendations on particular synthetics, and/or additives greatly appreciated too.

Mine is the same, except 1996, 4.1's and over 165K miles of extremelyhard driving/towing/offroadingGM's G80 is NOT a limited slip, but a LOCKER with clutchesIt does NOT need, nor does GM recommend any LS modifierLS Modifier reduces the clutch pack from sticking...exactly what the G80 wants todo when the centrifugal switch turns on the locerRecommend you flush and refill with synthetic diff fluid WITHOUT any frictionmodifier for LS diff'sI've used Mobil synthetic since new. A trick, as this era does NOT have a drain plug.Remove the diff cover and flush.Scrap off any old gasket on the diff housing and cover.Degrease it wellThen glue (Permatex) the new gasket on the diff cover side. Lightly coat the diff housing flange with diff fluid. Ditto the gasketside that will mateThen reassemble and tighten the diff bolts 'lightly'DO NOT FILLLeave it this way for a couple hours...best over nightThen tighten the bolts to sped and refillNext flush: just loosen all of the diff cover bolts, but leave themonPlace a catch pan underneath the diff pumpkinThen whack the diff cover. Normally dead center, but on GMT400's thegas tank is very close and in the wayThe diff cover will just pop off and a gush of fluidThen take the bolts off.Wipe the diff cover and gasketLet it drain and then repeat the wipe of fresh fluid on the matingsurfaces.No new gasket, as the old one will be in one piece.Tighten and then refill...doneFlush mine every 20K miles or two years...which ever comes first

I normally add molyb to all my lube...except for anything that has a clutch (diff &automatic)Too slippery and will ruin it.Just synthetic diff fluid straight up

GM 14-bolt FF Rear End with 149k miles | Good Sam Community

37 Replies

wintersun
Explorer II
Dec 12, 2014
If the lubricant is SAE grade 75W90 it makes no difference whether or not it is synthetic or not. What is important is the load that the film of fluid can take before there is metal to metal contact.

Seal materials have changed over the years and you want to be sure that the new fluid is compatible with the old seals. Usually GM specifies a new fluid that is backward compatible with older vehicles, but not always. Currently for the rear end that would be the synthetic axle lubricant equivalent to GM PN 89021677 in the USA). With a limited slip rear differential I would stick to the GM fluid and as it is not going to be changed unless something breaks the cost should not be a factor.
CampingN_C_
Explorer
Dec 12, 2014
I used to have the same rig as you and BenK. I replaced the fluid with full synthetic when I bought it used as it had some noises also. I also had not one sign of a leak after many miles of towing somewhat heavy and in the mountains, and I used plain old Permatex sealer.
However I don't think you'll have a problem with what they did, especially with the weight you're towing.
Also, it's not a bad job to do yourself.
Dave_H_M
Explorer II
Dec 12, 2014
Marty, there is a lot of houp a la gong on here.

As far as a stealer not putting the correct diffy fluid in there, well it is your choice.

My ole F250 came with full syn in the diff and no gasket. As i understand so did yours. I serviced the thing twice and reinstalled the pan with permatex red. Nary a sign of seepage.

If I were you i would do it right or have it done right and move on to bigger and better things.

good luck.
Thunderbolt
Explorer
Dec 11, 2014
JIMNLIN wrote:
I had a new '91 2500 chevy with the GM 10.5" 14 bolt corporate rear axle with the G80 option. The owners manual and GMs service manual says no LS additive. Well the first 2000 miles the rear axle banged and clunked going around corners. Back to the dealer to see what was happening. I showed the driveway tech both manuals and he confirmed no additive needed per GM.
We went to their axle mechanic and he says to have the fluid service folks put the LS additive in it. Doing so stopped the banging and clunking issues. I ran the truck up to 215k miles and no more issues as long as I put a additive package with each fluid change.

This truck was in commercial service for the first 150k miles of the time I had it so it had fresh non synthetic lube every 25-30k miles. Never tried a synthetic fluid in any of those older axles.

My 03 did this as well, out of warranty. I called Eaton and was told how to fix it. Put one wheel in dirt the other on asphalt and accelerate enough to get the diff to lock. Do this 15-20 times and then put the other wheel in the dirt and repeat. After doing that they said change the fluid. They said it is to deglaze the clutches. After doing this my problem was solved. They stressed not to add friction modifier.
MartyW
Explorer
Dec 02, 2014
Thanks BurbMan. I'll mention your synthetic p/n to the local chevy dealer folks and see what they say.

I was really surprised when my friend said, that by the book, he can't run synthetic in his 2005-2006 either, and he is positive that my diff and his diff are the same. Since he pulls a heavy 11k lb camper he would like to run the synthetic too but he sticks with the gm stated fluid for his truck. He does run Mobil1 syn oil in his 6.0L engine. He says to just change the standard diff fluid every 20k miles.

I'll keep asking questions to this dealership to see what answers I get. At least for now I got fresh diff fluid in it with no additive, so something good has been accomplished. I'll also try to find some good independent axle shops to see what answers I get there.

Many thanks again to all,
Marty
BurbMan
Explorer II
Dec 02, 2014
MartyW wrote:
They were happy to do a differential service but they would not add synthetic fluid. They said the synthetic fluid would cause the sealer to leak as the stock setup has no gasket.

No gasket, as in the cover is sealed with a bead of RTV, that must be removed and replaced at every fluid change. It is true that synthetic fluid will attack some types of RTV sealer and cause a leak. BUT there are types of RTV or sealer that are NOT affected by synthetic fluid. There have to be because synthetic fluid was installed from the factory.

From my Owner's Manual:

Rear Axle Lubricant: SAE 75W-90 Synthetic Axle Lubricant (GM Part No. 12378261) or equivalent meeting GM Specification 9986115.

The only thing the Lube-Lock gasket does is save you from the mess of having to scrape the old RTV off and re-RTV at every fluid change.

Not sure if things were different for your year truck, but the axle is the same, and there's no reason not to use synthetic.

Of course, I also run Mobil 1 oil in the big block...

I wouldn't sweat it too much though. Since you have fresh fluid in there, run it for a season and see how it does. You can always add synthetic at the next fluid change. You might want to ask the dealer about the P/N referenced in my owner's manual and see what they say.

On edit: the 1997 owners manual says:
Differential, Front and Rear Axle: Axle Lubricant (GM Part # 1052271) or SAE 80W90 GL-5 Gear Lubricant.

Bear in mind that the neither manual distinguishes between the 1500 and 2500 series trucks, so that means the half tons got the same lube, either regular in 1997 or synthetic in 2001.
MartyW
Explorer
Dec 02, 2014
JimNLin wrote "We went to their axle mechanic and he says to have the fluid service folks put the LS additive in it. Doing so stopped the banging and clunking issues" and this is exactly what happened with our 2000 Astro with G80 (but it is half ton and not 14-bolt). The LS additive helped and has almost eliminated the banging and clunking in that one.

I'm told that the Chevy folks at my dealership are following their stated procedures for that model year on the '97 K2500 14-bolt. The LS additive is not added to the 14-bolt, and it is now out.
Thunderbolt
Explorer
Dec 02, 2014
THis is my understanding as well. I believe all trucks with the G-80 are locking differentials

MARKW8 wrote:
I think the G-80 was only what GM used as the option code. Depending on year or model it could have been locking or a limited slip. Both have clutches but the locking diff gets no additive.

Mark
JIMNLIN
Explorer III
Dec 02, 2014
I had a new '91 2500 chevy with the GM 10.5" 14 bolt corporate rear axle with the G80 option. The owners manual and GMs service manual says no LS additive. Well the first 2000 miles the rear axle banged and clunked going around corners. Back to the dealer to see what was happening. I showed the driveway tech both manuals and he confirmed no additive needed per GM.
We went to their axle mechanic and he says to have the fluid service folks put the LS additive in it. Doing so stopped the banging and clunking issues. I ran the truck up to 215k miles and no more issues as long as I put a additive package with each fluid change.

This truck was in commercial service for the first 150k miles of the time I had it so it had fresh non synthetic lube every 25-30k miles. Never tried a synthetic fluid in any of those older axles.
Thunderbolt
Explorer
Dec 02, 2014
This is incorrect. The G-80 is a locker and it does have clutces. I have spoke directly with Eaton and was told not to use any friction modifier. The use of friction modifier will shorten the life and or cause the differential to malfunction.

ScottG wrote:
It's either a locker or it has clutches. It's isn't both. And it will do no harm (or good) to add friction modifier to a locker dif.
If it actually has clutches then a friction modifier is generally recommended. Failure to use a FM where it is required causes the clutches to chatter and engage roughly which causes wear and breakage.