Need Definitive Diff Lube Qty Answer

[RRoller123]

While servicing the bad seals on my rear axle, I want to also take the opportunity to change the Diff Lube (I use 80W/90 Castrol Hypoid).

The spec (Glove Box Owner's Manual) says that the capacity of the rear axle differential is 1-2/3 quarts.

But in order to get this much into the differential, you have to raise the front end somewhat or it fills deeper than the fill hole and runs back out. Maybe 1-1/3 quarts or a little less will actually fill the differential. Last time I filled it, we had the rear diff plate off, cleaned everything, and I am sure all the fluid was out, including what fills the axle housings on either side. I raised the front end a little to get the full 1-2/3 quarts in.

Now I am wondering if the extra fill may have contributed to early seal failure? ALTHOUGH I did have the wheel bearing replaced on the side that now leaks, and I suspect that the mechanic who pressed the bearing on the axle damaged the seals. This is more likely.

SO. Do we follow the spec or just fill on level ground until it reached the level of the fill hole?

Anyone have a definitive answer to this? I can't find an answer through the search function.

Thanks!

Pete

[baltobernie]

Two possibilities I can think of, Pete.

First is a poor quality seal. I had a very reputable shop and experienced mechanic finally get it right after the third try. Good now for 20k miles. He also sticks his finger in the filler hole, and if the oil level is anywhere close to the top, he pronounces it fine. So I'm voting that a little under is better than a little over.

[RRoller123]

I tend to agree with you, Bernie, and will probably just fill it up to the hole, unless there is a definitive answer out there. It certainly can't hurt to have it filled just to the hole, but maybe that extra ~1/3 quart could cause some pressure problem? I really don't know. But there is plenty of time since we are about a month away from drive season still..... More opinions are always welcome.

[blazingspider]

I've always been under the impression that you fill the unit until the gear oil begins to seep out from the fill hole with the car level. Plus or minus an once or 2 probably doesn't matter much as its just splash lubrication and remember, that the differential is vented, at least the older ones are, so there's no build up of pressure, assuming of course the vent isn't clogged.

[klweimer]

I'd go to the hole and call it good. Some diffs are vented which would relieve any pressure build up, I don't know if these are or not.

[RRoller123]

Another thing to consider in this discussion: If you fill only to the hole, ~1-1/3 quarts, there doesn't seem to be enough fluid depth to get the fluid to run to the outsides of the axle, i.e. pull the axle with the car on level ground and no fluid runs out. Fill the Diff to the spec 1-2/3 quarts and pull the axle and fluid runs out. But since the axle floats on air in there I guess it doesn't matter. And the bearing on the later cars runs dry (sealed lube) out at the end anyway. Interesting question, no definitive answer yet.

[131]

I've never measured a volume for filling the diff on any car I've owned, car level, fill to bottom of hole, definitive answer. Oil isn't required at the axle ends, the bearings are sealed, axle seals stop oil leaking out to damage the brakes.

[RRoller123]

Bingo, I think you have hit upon the correct answer. The spec from the factory manual and owner's manual is therefore wrong. Wouldn't be the first time.

Pete

[Nanonevol]

Pete, where would the oil come out if overfilled and axle pulled? Just trying to educate myself - "if" (lost a bit of oil when the driveshaft came off) something goes wrong.

[RRoller123]

On my car and I think all later cars, if the car is level and you pull the axle, with the specified amount of Diff fluid, 1-2/3 Qt, "excess" oil will flow out past the seal, which is out at the end of the axle versus inside near the Diff. It is a little disconcerting when you first pull the axle and Diff oil flows out. But if you run with the oil filled only to the plug, the seal at the end of the axle runs dry, no oil runs out when you pull the axle on level ground. I have verified both of these conditions fooling around with this in my garage. So the seal out at the end of the axle runs in a dry mode, unless there is some flow from an extended turn, or time spent with the car on an angle. I am still somewhat curious about what they intended with the spec. ~1-1/3 versus spec 1-2/3 Qt, and if they intended the seal to run in dry mode. Maybe that is why the extra fluid was spec'd out? To allow the seal to stay wet?

[131]

The seal doesn't run dry, it will always have some oil getting to it. If you pull an axle and oil pours out, you've got way too much oil in the housing.

[RRoller123]

Only filled with the amount the spec calls for! But I agree it is too much. This time around just filled it to the oil fill hole. The entire outer area has been completely dry when I fill it just to the hole and then later remove the axle. Axle dry, seal dry. It takes a long time on a tilt to get the oil to flow out there.

Pete