Interesting Find 2017 Subaru Outback Differential Service

[BISCUT]

I used my friend's lift to drain/fill front and rear differentials on 17 Outback 3.6. FF was drained on both front and rear at 25k and filled with Amsoil 75/90. This drain/fill was at 127k and Valvoline synthetic 75/90 went in. Rear fluid looked almost new and was sooo simple to do with a lift. Plug magnate was pretty clean. The plastic bags are a dream to use over the old hard plastic containers.

The front diff looked worse for the wear (yes, subjective for sure) and the plug magnate had double the amount of metallic sludge. Nothing I'd say to be concerned about but wondering if there's anything going on in a Subaru front diff that is harder on it than the rear diff. Or simply wearing harder up front. I didn't take any pics as I wanted to free up his lift asap as he was doing me a favor in letting me use it.

A little laugh at my expense. Drained the front and was searching for a way to access the fill hole.....my friend saw me searching and gave me the hand to get out from under the vehicle. He lowered it without a word and handed me a long nosed trans funnel and shined a light on where the front fill hole was....point taken! SO much easier to fill from the top! 😀

 

[IndyIan]

All but the WRX are fwd until the fronts spin. I do find our gets the rears involved so fast that it feels like its fulltime AWD or 4wd, but the rear diff isn't doing much 99% of the time.

 

[Bottom_Feeder]

I assume that Outback has a CVT and not a manual?

 

[MRC01]

All but the WRX are fwd until the fronts spin. ...

The stick shifts used to be open diffs front & rear with a viscous coupled center.

 

[IndyIan]

The stick shifts used to be open diffs front & rear with a viscous coupled center.

Yes I had to do some searching but the manual cars do still have a true center diff with the coupling. Most drivetrain layout diagrams don't show this, so I was thinking subaru was using a viscous coupling in the rear drive shaft with no center differential, like the CVT versions, which use a clutch pack now.
So I guess the manual trans cars do a 50:50 split most of the time.

 

[BISCUT]

I assume that Outback has a CVT and not a manual?

17 3.6 is a high-torque CVT.

 

[MRC01]

I still think the rear does nothing in most driving with a viscous center coupling until the fronts spin significantly faster? ...

Yep, that's the basic physics behind viscous couplers. Essentially it is 2 parallel discs surrounded by fluid. The engine drives the front wheels, rear wheels drag along so both plates spin at the same speed and there is no relative motion between the plates or the fluid. Yet all the power is delivered to the front wheels. Whenever the front and rear wheels aren't spinning at the same speed, then one plate moves faster than the other, which drags the fluid, which drags the other plate, transferring some power to it.

 

[tcp71]

Not every Subaru all-wheel-drive system is created equal

Though they all carry the same badge and moniker, there are actually quite a few different versions of Subaru's AWD system in use today.

www.cnet.com

Anything CVT is front biased 60/40 torque split

 

[BISCUT]

All but the WRX are fwd until the fronts spin. I do find our gets the rears involved so fast that it feels like its fulltime AWD or 4wd, but the rear diff isn't doing much 99% of the time.

Interesting. It does feel similar to the 08 328XDrive I had for a few years. If that is the case, kudos to Subaru as the number of times I've been "off course" in snow the engagement was seemingly instantaneous.

 

[BISCUT]

Yep, that's the basic physics behind viscous couplers. Essentially it is 2 parallel discs surrounded by fluid. The engine drives the front wheels, rear wheels drag along so both plates spin at the same speed and there is no relative motion between the plates or the fluid. Yet all the power is delivered to the front wheels. Whenever the front and rear wheels aren't spinning at the same speed, then one plate moves faster than the other, which drags the fluid, which drags the other plate, transferring some power to it.

Thanks for the explanation. Makes sense now.

 

[MRC01]

Which makes me wonder: in the Subaru models having a viscous coupler, what fluid does it use? Does it share the front transmission/differential fluid? Or does the coupler require separate fluid?

 

[nthach]

Which makes me wonder: in the Subaru models having a viscous coupler, what fluid does it use? Does it share the front transmission/differential fluid? Or does the coupler require separate fluid?

It uses the transmission fluid, IIRC.

 

[BISCUT]

Which makes me wonder: in the Subaru models having a viscous coupler, what fluid does it use? Does it share the front transmission/differential fluid? Or does the coupler require separate fluid?

Shared CVT fluid. Front diff is Indy.

 

[gathermewool]

So far as I know, the viscous coupling in Subarus is a sealed unit. It doesn’t share fluid with anything.

Everyone forgets about the poor STI. It has LSDs front and rear, and a driver-controlled center diff.

 

[MRC01]

So far as I know, the viscous coupling in Subarus is a sealed unit. It doesn’t share fluid with anything.
...

That makes sense, as the viscous coupling is similar to an auto tranny torque converter, and tranny/diff oil (GL4 or GL5) isn't like auto tranny fluid. Yet it's a bummer, since it means the coupler would have to be replaced rather than serviced.

 

[gathermewool]

That makes sense, as the viscous coupling is similar to an auto tranny torque converter, and tranny/diff oil (GL4 or GL5) isn't like auto tranny fluid. Yet it's a bummer, since it means the coupler would have to be replaced rather than serviced.

I frequent the Subaru forums and I don’t recall the last viscous coupling failure I’ve read about. It’s a pretty simple device.

 

[BISCUT]

Now I'm really confused Not the first time and sure won't be the last time

 

[SubieRubyRoo]

All but the WRX are fwd until the fronts spin. I do find our gets the rears involved so fast that it feels like its fulltime AWD or 4wd, but the rear diff isn't doing much 99% of the time.

Eh, this is somewhat true, but certainly not anywhere near “99%” (max is 90% FWD unless the fuse is pulled to disable the center diff) until the car is at freeway speeds. The center differential is electronically controlled, and at low speeds is directly linked to maintain a near 50/50 distribution especially when there is a speed difference between the front and rear diff speeds. As vehicle speed increases, the center diff progressively “slips” and will eventually get to 90/10 front/rear on all but the models with the Sport center diffs. The WRX STi is adjustable and does not use the same progressive scheme.

To OP, yes, I think the front diff experiences more use/wear because when the steering is actuated, the front wheels spin at significantly different speeds due to the Ackerman angle. I’ve seen the same thing you did where front diffs are much darker and more metal “fluff” on the magnet. I’d either do UOAs to check fluid, or change the front diff at half the OCI of the rear.

The other thing that most don’t know is that nearly all Subaru rear diffs other than WRX are open; the Legacy/Outback had “sealed” limited slips but I don’t know the latest gen. The one on my ‘05 Outback cannot change the LS diff fluid itself; you can only change the fluid that contacts the gearset- the diff itself is completely sealed and unserviceable.

 

[SubieRubyRoo]

Eh, this is somewhat true, but certainly not anywhere near “99%” (max is 90% FWD unless the fuse is pulled to disable the center diff) until the car is at freeway speeds. The center differential is electronically controlled, and at low speeds is directly linked to maintain a near 50/50 distribution especially when there is a speed difference between the front and rear diff speeds. As vehicle speed increases, the center diff progressively “slips” and will eventually get to 90/10 front/rear on all but the models with the Sport center diffs. The WRX STi is adjustable and does not use the same progressive scheme.

To OP, yes, I think the front diff experiences more use/wear because when the steering is actuated, the front wheels spin at significantly different speeds due to the Ackerman angle. I’ve seen the same thing you did where front diffs are much darker and more metal “fluff” on the magnet. I’d either do UOAs to check fluid, or change the front diff at half the OCI of the rear.

The other thing that most don’t know is that nearly all Subaru rear diffs other than WRX are open; the Legacy/Outback had “sealed” limited slips but I don’t know the latest gen. The one on my ‘05 Outback cannot change the LS diff fluid itself; you can only change the fluid that contacts the gearset- the diff itself is completely sealed and unserviceable.

Also, the manual non-WRX Subarus do not have an electronic center diff and go with a straight clutch. The manuals are always 50/50 (or 25% x 4, if you will), and never vary unless one wheel is completely tractionless. IMO, the pinnacle of Subaru AWD were the Outback/Legacy with LSDs and manual transmissions.

The current iteration is fairly capable especially at lower speeds (X-mode essentially locks in 50/50 distribution below 25mph) but in on-road, high speeds it is not a “full” AWD in the sense of a 4wd truck. Jason Fenske of Engineering Explained has a great video that describes the 4 iterations of Subaru AWD.

 

[gathermewool]

Eh, this is somewhat true, but certainly not anywhere near “99%” (max is 90% FWD unless the fuse is pulled to disable the center diff) until the car is at freeway speeds. The center differential is electronically controlled, and at low speeds is directly linked to maintain a near 50/50 distribution especially when there is a speed difference between the front and rear diff speeds. As vehicle speed increases, the center diff progressively “slips” and will eventually get to 90/10 front/rear on all but the models with the Sport center diffs. The WRX STi is adjustable and does not use the same progressive scheme.

To OP, yes, I think the front diff experiences more use/wear because when the steering is actuated, the front wheels spin at significantly different speeds due to the Ackerman angle. I’ve seen the same thing you did where front diffs are much darker and more metal “fluff” on the magnet. I’d either do UOAs to check fluid, or change the front diff at half the OCI of the rear.

The other thing that most don’t know is that nearly all Subaru rear diffs other than WRX are open; the Legacy/Outback had “sealed” limited slips but I don’t know the latest gen. The one on my ‘05 Outback cannot change the LS diff fluid itself; you can only change the fluid that contacts the gearset- the diff itself is completely sealed and unserviceable.

Does that fuse still exist? My 97 Legacy had one for emergencies only - pull the fuse and some indicator comes on (“FWD” or something). I never even thought to check my current Subies

I thought that all recent WRX’s have an open front and rear diff. The only Subarus with an LSD are the STI and BRZ.

I also believe that the real difference between Subaru AWD and other makes, besides the symmetrical thing, is that the center diff in a Subaru mechanically connects the front to the rear, has a front bias, and uses a clutch pack to transfer more power to the rear. Many other basic AWD systems are only connected when the clutch pack engages, otherwise it’s an open connection. The consequence is that the rear wheels need to slip before ANY power is sent rear.

With that said, some modern non-Subaru (and even Subarus with X-Mode) have a button or dial to energize the clutch preemptively for slippery conditions. One of my Subarus has X-Mode, and it disengages at around 20 something MPH, I believe to prevent excessive wear or damage to the clutch pack.