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Differential fluid empty?

After a loud helicopter sound was coming from our car on the highway, we had it checked out, and were told that our front differential was completely out of fluid. We asked how that could happen and were told that it was due to either poor maintenance or from being revved like when you get stuck. We haven’t revved the car at all, and according to our most recent oil change receipt (800 miles ago), the differential fluid was fine. Our car has 102K miles and has been treated pretty well all of those miles. Is there another possible cause?

Leak.

some oil change places say they check the fluids, but if the tech doesn’t do it you’ll never know. That is until you have a problem.

Your “front differential” is part of your transmission… You can’t tell is it’s “completely out of fluid” unless you try to drain it by removing the drain plug, which oil change places FREQUENTLY do with Subaru’s, thinking they are draining the motor oil…

So if they did drain it, would it have been drivable for the last 800 miles and two and a half months?

Perhaps…The gearbox might tolerate short trips at low speeds where nothing got very hot…That gearbox has very little friction, all the bearings are ball or roller and can survive with little lubrication. But there will be a critical area, a bevel gear, that will start complaining…

With the trans axle now full and no leaks discovered, the only way out would be if someone drained it out. The transmission drain plug is not far from the crankcase drain plug. Is your engine oil over-full by chance?

Was the oil change at a quick-lube place?

Yes. They offer a quick registration renewal, so we cheaped out and went there.

Hmmmmm. I have not checked the engine oil, and the car is currently at a repair place. Most of the 800 miles were around town, so hopefully the repairs won’t be terribly expensive…?

If this came on suddenly it’s usually due to someone removing the wrong drain plug while performing an oil change.

The final drive drain plug is close to the engine oil drain and if someone is in a hurry, not paying attention, etc. it’s easy to remove the wrong plug.
What then happens is either the engine oil is overfilled or the differential is not refilled and the differential gives up.

Another cause is an internal seal leak in which the diff. oil transfers to the automatic transmission part of the assembly. This usually occurs over a longer period of time and causes the ATF to be overfull. Results are the same, blown transaxle.

This is not a rare thing to happen with a Subaru. Examine the differential drain plug for fresh marks where someone may have removed it by accident. If it has fresh marks then you need to revisit whoever changed the oil last.

The front differential on an Outback is only integrated with the transmission if it’s a manual. If you have an automatic, the transmission and front differential aren’t integrated, use separate fluids, and have separate drains.