black oil/coating in gm 8.5 diff

[kc8adu]

i just bought a diff as a replacement for a whiner in an 88 chevy g20
it came from a 93
i opened it and found pitch black oil and a black film on non contact surfaces.it washed off easily with carb cleaner.
my neighbor thinks it is a moly additive.
i think it is the original oil with umpteen hundred thousands of miles on it.
it had the typical strong odor of diff oil.
it was much thinner than the new 80w90.
its not a posi.
does diff oil break down to a black mess?

 

[ssmokn]

If the ring, pinion and spider gears look good don't worry about it.

I would install all new bearings (carrier and axle) setup the new rear end, install your favorite oil and fahgettaboutit

I would also go back and do another change after 1 or 2K miles just to drain out any potential crap or metal. Plus you can look at the rear again and make sure nothing weird is going on.

A copy car that I rebuilt had some of the nastiest fluid I ever saw come out of a rear end. It was black as tar and nasty looking (and smelling). I dissassembled the whole rear and and everything was in top shape (almost 90K at this time).

I replaced all bearings (since I had it apart) and installed an Eaton posi - 40K miles later it is running strong.

Again, I wouldn't worry about it as long as everything looks good and you don't see anything unusual in the oil (like excessive metal).

 

[kc8adu]

i should have done that.
i traded severe howl/whine for rumble.
i suspect an axle bearing.gets worse when turning right and feels like running over rumble strips when coming to a stop.doesnt change with load as in accel/decel so i doubt is gear related.it is strictly road speed related.probably plastigage the carrier as it is easy to get to without teardown.

 

[kc8adu]

btw it will get a hard drive magnet apon reasembly.

 

[kc8adu]

well i did the axe bearing swap.
i was mistaken though about the carriers.
they are a roller bearing rather than a shell as i thought.
fixed the noise.
the new oil i put in before the install was very dark but not as bad as the unrefined crude i drained the first time.probably try to get some autorx in there and run it awhile just to kill any remaining sludge.
guess i got lucky as this diff makes no gear noise.
which is rare esp with the vans aggravating the noise

 

[Ken2]

Differential gear oil does oxidize and turn black, and sooner if hotter. It can thin if it was a lower quality 75W-90 with cheaper viscosity index improvers.


Ken