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09 Legacy GT, 08 Legacy 2.5i, 09 Outback 2.5i
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Discussion Starter · #1 ·
2009 Outback 2.5i 4EAT here. What type of center diff is in these transmissions? Working on my son's car today it has had a ripped right rear outer CV boot for a few months now. Getting under it today we find the driveline spins easily and the outer CV joint is free spinning. I am surprised the car would even move under it's own power with one rear wheel basically unconnected. Is the center diff some type of limited slip that would transfer all power to the two front wheels in this scenario?
 

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2005 OBXT Limited, VF37, STI intake, 5MT
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In the 4EAT (automatic) cars, they do not have a "coupled" differential like the manual transmission cars.

You have a "transfer clutch" that activates when commanded by channeling fluid pressure to a set of friction clutches in the tail housing. This allows for a variable transfer of 10% up to 50% rear power transmission on demand.

During normal driving, the split is 90/10 (front/rear). The transmission drives the front pinion internally, so you have a FWD car most of the time. Spinning the driveshaft by hand (when the car is off) is normal.
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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it's a wet clutch pack in the tail section of the trans. When slippage is detected at one end of the car, a solenoid's duty cycle is altered to hydraulically engage/disengage the clutchpack to redirect torque.


doh - beat while typing.
 

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4EAT uses a multi-disk transfer clutch to connect the output of the transmission to the rear propeller shaf, however the rear drive train isn't needed to propel the car. The output of the transmission is also mechanically-connected through a pair of 1:1 gears and a pinion drive shaft to the front differential, so the front is always 100% connected to the transmission output. The transfer clutch, which is electronically-controlled by the TCU, and varied in torque transfer by hydraulic pressure, can be totally open and the car will still drive. With the engine off, the transfer clutch is not engaged, so the rear drive spins easily.

doh - me too
 

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09 Legacy GT, 08 Legacy 2.5i, 09 Outback 2.5i
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Discussion Starter · #5 ·
The front drive worked 100% as you described while this was broken. Car is fixed with a used axleshaft now and my son is super happy that the issue is solved. A bit of a PITA to drop the exhaust and diff to get the shaft out but I've experienced worse...
 

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09 Legacy GT, 08 Legacy 2.5i, 09 Outback 2.5i
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Discussion Starter · #7 ·
Did you have a look at the "failed" CV joint to see what happened to it?
Have not ripped the joint itself apart. My son is going to bring it to auto tech school to dig into it more. What I can tell it the ball carrier is still in there and it looks a bit dry. The splines on the outer end of the axle shaft where it mates the inner part of the cv joint are sheared off. It spun freely by hand before taking apart.
 

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I believe there should be a "C" clip that holds the splined axle shaft in the ball carrier (inner race) in the BJ joint. If the clip is missing, or breaks away (albeit unlikely), the shaft can pull out of the carrier (the boot will stretch with it, to a degree). The axle shaft can pull out if the inner DOJ CV joint is stiff, and doesn't extend as it should. This could happen if the car is raised, or when making a fast turn and the car leans enough.
In any event. once axle splines clear the splines in the carrier, the axle will spin freely, and as the car settles back, the axle will grind the splines as it tries to reenter the carrier. That spinning would also have torn the boot completely. If the splines in the BJ CV joint ball carrier are also worn down, that would fit the scenario.
 

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2005 OBXT Limited, VF37, STI intake, 5MT
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It looks like someone had pressed the inner race (the 3-socket, splined hub) unto the shaft without lining up the teeth. I've never seen one flatten the splines naturally. This probably occurred while an inexperienced tech was doing a re-boot on the axle.

And I'm glad it worked out in the end. Worn u-joints can cause all sort of "unrelated" vibration issues.
 

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09 Legacy GT, 08 Legacy 2.5i, 09 Outback 2.5i
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Discussion Starter · #14 ·
MPT vs VTD?

Old thread sorry but I am looking to learn more about the types of center diffs in these cars. Based on previous responses I think my son's 09 Outback 2.5i/4EAT has the MPT style where the trans output is connected solid to the front driveline and there is a hydraulic clutch pack that can lock the rear output to deliver 50/50 split front/rear. In other words no real "center diff" here. I was reading this post about MPT to VTD conversion trying to learn more. ('06-'08) MPT to VTD conversion - Subaru Forester Owners Forum In that post the attached 4EAT PDF states that all vehicles with Vehicle Dynamic Control now have the VTD or true "center diff". One thing I don't understand is that with the VTD what is the purpose of the clutch pack mechanism? I think my son who is in auto tech school said the center diff on my 09 GT is a Torsen type differential.

I am curious because we have three of these vehicles. From what I've read so far I think the type of center diff in each one is as follows.
1) 2008 Legacy 2.5i/4EAT without Vehicle Dynamic Control - MPT
2) 2009 Outback 2.5i SE/4EAT with Vehicle Dynamic Control - MPT or VTD?
3) 2009 Legacy 2.5 GT/5EAT with Vehicle Dynamic Control - VTD (I think)

Or...am I way off the mark so far?
 

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Not far off at all. For the record, ALL VDC systems (4EAT and 5EAT) have a VTD-type "center diff". The VTD, combined with some smart ABS controller logic, is what makes the VDC system work. The ACT-4 (or MPT-type transfer clutch) is still variable, but doesn't have the sort of the variability, mechanical connection and "natural split" of the VTD system. This is also why the VTD-equipped cars cannot use a compact spare tire (and does not use the "FWD" fuse). No way to disconnect or disable the AWD system.

The VTD still uses a clutch pack, but as a way to "boost" rear output and control the front/rear split. If it didn't have that control, you'd get 50/50 split, but with no way to allow or control for slip at the front or rear.

The 5MT also has a sort of "clutch pack" center diff, but use viscous silicone fluid that sealed into the differential rather than pressurized trans fluid. This is a natural 50/50 split, but slips at very low speed to allow for parking and tight turns without binding the system.

Early Subaru 5MT and 3AT transmissions actually used a 100% mechanical, push-button center diff-lock. Great for loose surface and snow traction, but can't handle turning corners (no slipping allowed).

So, with your vehicles....

1. Correct
2. VTD
3. Correct
 
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