Dark Oil In Rear Differential?

[strafingmoose]

Hey guys

 

So I made a GM dealership inspect the truck I bought and one of the things he found is that the oil in the rear differential was very dark and its level was low.

 

I asked the seller to have the oil there changed and also, to see if it had any damage to it. Showed me the bill (of a GM dealership) on which it said oil was changed and "everything OK".

 

I know black oil in transmission means that it's almost dead, but what about the rear differential ?

 

Anything more specific I should have asked the seller to look at ?

 

thanks.

 

edit: the truck was bought with 60k miles on. the seller told me the previous oil in it was syntehtic and needed to be changed only at each 60k miles.

[BurbanZ71]

What year is the truck?

 

On my '05, an many others I have heard, the rear diff fluid is stinky, dark nasty smelling stuff. I changed mine at 30,000 just because I thought it would be a good idea - it was dark and quite smelly.

 

No issues with my diff.

 

GM Spec is Synthetic gear oil - I believe 75-90W.

[Zembonez]

It's very much like a dealership to point out maintenance items that they want to sell and make them seem somehow worse than they are. Can't really blame them... they are there to sell service. Unless the rear axle is leaking, I'd just change the fluid and enjoy the truck.

[Westlotorn]

Dark Diff Oil with no metal in it might have a small leak allowing the level to run too low. This can cause the oil to overheat burning the oil.

 

The 1/2 ton diff does not fill to the fill hole any longer there is a measured distance below that hold in the newer rear diffs.

 

Maybe it was full and all is OK. Is it quiet? Any damage in a rear differential will talk to you at freeway speeds.

 

These were not the best read differentials ever built but you should get 110 to 175,000 miles before a rebuild is necessary.

 

Rear differentials can't leak without you seeing a trail, If you see oil splash around the pinion ( driveshaft yoke ) the seal is out and should be changed. Note: can't change this seal without changing the crush sleeve.

 

Sometimes the rear cover gasket leaks. You would see oil at the bottom of the rear cover.

The only other possible leaks would be at the axle shafts and the rear wheels. If all looks dry and clean you don't have a leak.

As far as the oil change on a rear differential many go 150,000 miles or more and never get an oil change. These GM designs are not the best so I would change the oil every 50K and hope to never have failure.

[strafingmoose]

@BurbanZ71

 

It's a 2003

 

@Zembonez

 

It was leaking, the bill that the seller showed me included fixing a leak too.

 

Thanks for your recomforting answers guys

[Zembonez]

Thanks for your recomforting answers guys

Sounds to me like you got a great truck.

[2500ak]

I changed my diff oil because my G80 was acting up. Worst smelling oil I've ever had to deal with.

[d3athsong]

Worst smelling oil I've ever had to deal with.

 

i hope you put something that smelled equally as bad back in.

[jaredaggie]

This is a dumb question but where are you finding the oil they specify for the rear diff on a vehicle? Is that under the fluids section of the manual?

[d3athsong]

This is a dumb question but where are you finding the oil they specify for the rear diff on a vehicle? Is that under the fluids section of the manual?

 

I dont have a manual in front of me, but im would look at under fluids or capacities.

[pm26]

Rear end oils are hypoid gear oils with special additives, hence the odd smell. Regular hypoid gear oil used in almost all light duty vehicles will be 80W90 weight, synthetic oil will be 75W90 weight.

 

In hot climates, or for heavy towing they sometimes recommend using 80W140 viscosity which will remain thicker under high temperature conditions.

 

For your vehicle, I recommend Mobil 1 75W90 gear oil (about $ 8/quart at Wally World). If you prefer the genuine GM stuff, your favorite GM dealer will be happy to sell their synthetic 75W90 gear oil for as low as $ 24/quart. You need to buy 3 quarts to fill your differential, but double check the capacity in your owner's manual, as it is a bit less than 3 quarts.

[d3athsong]

You need to buy 3 quarts to fill your differential, but double check the capacity in your owner's manual, as it is a bit less than 3 quarts.

 

You'll know its too full if its running out of the

hole you are filling.

[swipter]

When changing the rear diff fluid do you just pull the plug and drain or is it better to remove the cover and put a new gasket? Also, are you going to get the same performance and length of wear out of the Mobile 1 fluid?

 

THanks

[bobf1234]

When changing the rear diff fluid do you just pull the plug and drain or is it better to remove the cover and put a new gasket? Also, are you going to get the same performance and length of wear out of the Mobile 1 fluid?

 

THanks

 

On my '04 Silverado you have no choice. You have to remove the cover and change the gasket, there is no drain plug. I would think it would be better to do that anyway, isn't there a magnet that needs to be cleaned in the housing?

 

And just 1 quick question for someone who knows... I'm getting ready to change my fluid, does the G80 affect the amount of fluid I need to use or is it still .6 - 1.6 inches below the fill plug for the 10-bolt differential?

[Westlotorn]

Shops that rebuild rear differentials will tell you to avoid using silicone for a gasket material if you plan on running synthetic oil in your rear end.

 

They say the synthetic will cause leaks with the silicone gasket.

 

The factory used silicone for years, maybe they are buying a better grade than we can in the local parts store?

 

If you have a Posi traction or locking rear differential make sure you buy the correct oil. These need special additives to lubricate the Posi unit.

Parts stores all sell both but you need to read carefully to make sure you get the correct one.

 

Read your owners manual on the fill technique, GM 10 bolt light duty differentials actually give you a measurement down from the fill hole as a fill level. I think it is something like 15/16 of an inch below the fill hole, at least in the 2002 10 bolt 1/2 ton rear end. All the older models said to fill till it spills out.