First Chevy Ive owned, just purchased a '96 C2500, 5.7, 14 bolt, 8 lug, semi-float G80. The guy I bought it from said it had a brake grabbing but I dont think it is the brakes. It has a thumping sound especially at higher speeds. I jacked it up and ran it up to 60 mph but no thumping sound. I checked the mid-shaft bearing, looks good, u joints look good best I can tell. One thing I did notice is the drive shaft seems not to be perfectly in line between transmission and rear end, a slight degree of angle (from left to right, not up and down). I dont know if this is normal, or? I would think that if this situation was a factor which is causing the thumping sound, it would sound off while on the jack stands. So, before I send her to the mechanic, I decided to open up the differential and see what it looks like, and change the gear oil....trying to figure out which oil to use was so confusing...didnt know whether to scratch my teeth or brush my ass, but have finally decided on Valvoline High Performance 80w/90 or MasterPro 80w/90...both have no LS additive...probably go with the Valvoline. I have doubts that this will remedy the thumping sound, but at least I will have fresh gear oil.
Thump thump, G80 and confusing gear oil
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someotherguy
Truly Awesome
Correct, no "posi" or "limited slip" additive. The RPO code G80 means different things depending on application, but in this generation of trucks it means you have the Eaton Gov-Loc locking differential which does not require an additive and in fact can have issues if you add any.
So while there's a lot of debate about the correct oil at this point (the official GM stuff appears to have been discontinued; there's a current thread here about it) - the one thing you want to be 100% sure to NOT do is add a bottle of any type of friction modifier additive. That's a mistake people can make because they think their truck has a posi unit, when it does not.
Richard
So while there's a lot of debate about the correct oil at this point (the official GM stuff appears to have been discontinued; there's a current thread here about it) - the one thing you want to be 100% sure to NOT do is add a bottle of any type of friction modifier additive. That's a mistake people can make because they think their truck has a posi unit, when it does not.
Richard
www.gmt400.com/threads/is-acdelco-10-4051-gear-oil-discontinued.62501/
is ACDelco 10-4051 gear oil discontinued?
cant find it anywhere...what's the best alternative for '96 G80?....confusing
www.gmt400.com
Best I can tell, the current recommended lube for truck Gov-Lock differentials is ACDelco 10-4016 "Grape Juice".
I have the Valvoline no-friction-modifier, "top-off only" on "Positraction" axles 80W-90 in mine.
There is a built-in angle in the shaft; u-joints demand a small degree of deflection to keep the grease spread around and prevent dry galling and rust. All clutch posi diffs are famous for thumping, hitching or grabbing when the oil is old. are your rear tires different diameters, one worn and one new, etc? that will cause the clutches to grab-slip if the oil is worn, and would disappear when on jackstands.
thanks for that info
hopefully im making progress...im ready to pull diff cover off, noticed where some gear oil had leaked. cracked cover apart and I would estimate it only had around half a quart fluid. I was surprised that everything looked very good according to all the info ive seen, no broken parts, little flipper things work. There was hardly any metal dust around magnet, but something very strange...when I dipped my finger in magnet to start cleaning, this slimy amber colored stuff stuck to my glove and stretched around 8 inches or so...i imagined whale snot. The gear oil was very clean, amber color, what little there was. There was no gasket, someone had only used the black rtv...there was much rtv that had pushed into the diff where the oil would contact it. Im wondering, considering the slime, if the rtv reacted to the oil, or someone used limited slip oil? I didnt get a pic of the strings of slime when I first pulled it, but you can see it on the end of my glove in pic.
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No idea on the slime.
Concerned for your wheel bearings and grease seals, in that they've probably not been getting much lube if there was only a pint of grease in that axle. Spec should be 5.5 pints, 2 3/4 quarts.
Anything look overheated/blued?
Pernatex makes an RTV Silicone especially for use with axle lube, and some of their other RTV formulations should also be acceptable. I've used RTV, I've used gaskets.
Before you put the cover back on...make sure you can get the fill-plug screwed-out. If it's jammed in place, you'll be filling the axle through the vent hole, and that will be NO fun at all.
Concerned for your wheel bearings and grease seals, in that they've probably not been getting much lube if there was only a pint of grease in that axle. Spec should be 5.5 pints, 2 3/4 quarts.
Anything look overheated/blued?
Pernatex makes an RTV Silicone especially for use with axle lube, and some of their other RTV formulations should also be acceptable. I've used RTV, I've used gaskets.
Before you put the cover back on...make sure you can get the fill-plug screwed-out. If it's jammed in place, you'll be filling the axle through the vent hole, and that will be NO fun at all.
thanks...first thing i did was remove the fill plug, thanks to you guys on this forum. I dont see anything blued, i have a gasket and some ultra black permatex. I hope bearings/seals are ok, but at least this is a start. I just have to wonder if the diff was filled with an oil with LS additive, maybe a synthetic, which leaked out and then heated up, creating that slimy substance... it was so stretchy. I think I will use some brake cleaner or isoprophl alcohol to clean the botton of the carrier, at least around the magnet, wipe it out good, then go with it
I've seen some stringy slime before, but I also have no idea what caused it. Your teeth look good, I'd fill it up and give it a half a hundred miles for the new lube to find it's way into the clutches. Hopefully it resolves the grabbing and doesn't leak.
Out of curiosity, what do have in the way of endplay on the half-shafts as measured at the wheels?
Lack of gasket implies someone's been in there before you - recently? While you are there, check the cross-shaft (the one retained by the bolt) doesn't protrude overly. There are (at least) two available and they are slightly different lengths. When I changed mine (along with the C clips) I got the longer one and it fouled the diff housing (before I shortened it).
Lack of gasket implies someone's been in there before you - recently? While you are there, check the cross-shaft (the one retained by the bolt) doesn't protrude overly. There are (at least) two available and they are slightly different lengths. When I changed mine (along with the C clips) I got the longer one and it fouled the diff housing (before I shortened it).
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