[bshusted]

Recently (October), my wife and I purchased an '01 VDC for use as her daily driver. I am not unfamiliar with AWD as my daily is an A4 Quattro.

Today while adjusting the parking brakes on the Subaru, I noticed that turning one of the rear wheels did not turn the opposite side in the opposite direction; rather it spun the rear drive shaft instead. The Quattro system that I am familiar with uses a Torsen differential as the center differential. The torque split is fixed and therefore, no clutch is required. The behavior exhibited by the Subaru would represent catastrophic failure in the Audi.

My question is, is there something wrong with the MPT clutch? I have done some research and confirmed that the MPT is controlled by the fluid pressure in the transmission. Pressure is bled off to bias front torque, with no "bleed" representing 50:50 torque distribution. Am I correct in understanding that in the non-running (no pressure) state that the MPT is not connected to the output shaft and the behavior I noticed is okay?

 

[rasterman]

if I understand it correctly, you are seeing normal behavior. Pressure in the clutch pack builds up with a difference in F/R speeds. Turning it by hand, you aren't able to get enough slip or pressure to notice anything, but under power it's a whole different story.

 

[jp1203]

The VDC models all have a very slight RWD bias. It's very close to being 50:50, but not quite.

Spinning one rear wheel on my L.L. Bean will spin the other in the same direction because it is limited slip, but I'm not sure if it spins the rear drive shaft. Can't remember for sure if the VDC models have a rear LSD or not, but I'm pretty sure they don't since they accomplish the side-to-side distribution of power by braking individual wheels.

 

[bshusted]

So does anyone have a suggested method to test the AWD as we are short on snow and ice currently. I'd rather know now if I have an issue rather than when it's really needed.

Anyone know what would/should happen with the engine running in park or neutral? Would the hydraulic pressure on the clutch keep it from spinning the drive shaft then and have "normal" open differential properties?

I am certain that it is not a limited slip. If I spun one of them fast enough, hard enough, the inertia if the driveline would make the other spin opposite. If it was limited slip it isn't anymore.

 

[rasterman]

I am certain that it is not a limited slip. If I spun one of them fast enough, hard enough, the inertia if the driveline would make the other spin opposite. If it was limited slip it isn't anymore.

Subaru (as a brand) has had durability issues with their LSDs. Most people never notice when they are worn out, because they do not lose any other property. So the issue doesn't get much attention. It is possible that you have one and that it has worn out. Search this board, I think there is a thread about it somewhere.

You can demonstrate the AWD on wet grass, or soft sand if that is any more accessible to you. It might be best to ask someone else to drive a few launches while you observe from outside.

 

[cardoc]

The VDC uses VTD (variable torque distribution) and has a torque split of 45/55 front to rear. It is AWD all the time, similar to the Audi/VW and is unlike the other Subaru models of the era that had 90/10. The rear differential is a LSD because when the VDC becomes active, it needs to control the wheel independently to maintain vehicle stability. The programming in the early models would detect an issue faster than you would and correct it, sometimes without you even noticing it. It is normal for one wheel to spin and the other remain stationary. When enough rotational force is applied to the differential, the opposing wheel, right rear, is active and carries the same torque as the left.

If your near a beach, hit the sand.

You can also insert a 10 amp fuse in the FWD slot in the underhood fuse box by the washer reservoir to turn off the VDC programming. A light to the left of the speedo will light up "VDC Off". This allows full torque transfer to all the wheels without the limitation of the VDC working in conjunction with the ABS, transmission controls and engine controls to keep the wheels from spinning, retaining the AWD. It also helps in drag launches.

This is mine in the dirt. We were bored and I lost a short race with a very hopped up Cobalt SS hitting 17-18 psi boost on a special built engine from GM Performance. His is not off road worthy.

The car was in a class by itself with torque transfer. The reduction gear of the VDC is totally different from all other Subarus.

 

[bshusted]

I appreciate your response. So you're saying that what I experienced is normal?

What type of LSD is used in the rear differential? If it is a clutch type, it would seem that if I can rotate the rear wheels independently that the clutches are toast.

 

[cardoc]

I appreciate your response. So you're saying that what I experienced is normal?

What type of LSD is used in the rear differential? If it is a clutch type, it would seem that if I can rotate the rear wheels independently that the clutches are toast.

Maybe this will help. They load in .pdf. First page is the Auto trans, second is the differentials. Scroll through.

Directory /Auto/Japan/Subaru/Legacy Outback/2001/Service Manual/2001 AUTOMATIC TRANSMISSION/AUTOMATIC TRANSMISSION/

http://jdmfsm.info/Auto/Japan/Subar...XLE SECTION/Differentials/MSA5T0105A28288.pdf

Main page: Directory /Auto/Japan/Subaru/Legacy Outback/

 

[jp1203]

The VDC uses VTD (variable torque distribution) and has a torque split of 45/55 front to rear. It is AWD all the time, similar to the Audi/VW and is unlike the other Subaru models of the era that had 90/10.

There was never, to my knowledge, a Subaru with a 90/10 torque split. According to Subaru themselves, the models with the 4EAT are "usually" 60/40:

SUBARU | Subaru Technology | Symmetrical AWD

 

[cardoc]

http://jdmfsm.info/Auto/Japan/Subar...ON/AUTOMATIC TRANSMISSION/MSA5T0109E29293.pdf

In the above reference, the only difference is the USDM does not have a "hold" feature for specific gears. In other words, you can't put the lever in 3 and hold 3, it will shift up to 3 and no further. You can hold 2.

Back to the year of discussion. I am aware of the "arguments" concerning the torque split. Symmetrical AWD systems are the updated systems based on early (2001 up) VDC/VTD and torque split base has changed throughout all models. European and Japan markets got updates earlier than the US starting in 2004 builds.

The MPT clutch alters torque transfer to the front/rear in bias based on driving conditions. At minimal it is a 90/10 split and increases incrementally according to needs based on data from the sensors on the vehicle. On the VDC/VTD models, the split is 45/55 and the LSD allows for independent wheel control via braking based on ABS/VDC activation or torque transfer one side to the other based on wheel spin.

Have you ever seen a modified WRX on a dragstrip? Front wheel burnouts. Unless the owner changed the reduction gear setup in the tailshaft or altered the MPT solenoid function, lights em up. Until the MPT engages the rear enough to push the car.

 

[jp1203]

http://jdmfsm.info/Auto/Japan/Subar...ON/AUTOMATIC TRANSMISSION/MSA5T0109E29293.pdf

In the above reference, the only difference is the USDM does not have a "hold" feature for specific gears. In other words, you can't put the lever in 3 and hold 3, it will shift up to 3 and no further. You can hold 2.

Back to the year of discussion. I am aware of the "arguments" concerning the torque split. Symmetrical AWD systems are the updated systems based on early (2001 up) VDC/VTD and torque split base has changed throughout all models. European and Japan markets got updates earlier than the US starting in 2004 builds.

The MPT clutch alters torque transfer to the front/rear in bias based on driving conditions. At minimal it is a 90/10 split and increases incrementally according to needs based on data from the sensors on the vehicle. On the VDC/VTD models, the split is 45/55 and the LSD allows for independent wheel control via braking based on ABS/VDC activation or torque transfer one side to the other based on wheel spin.

Have you ever seen a modified WRX on a dragstrip? Front wheel burnouts. Unless the owner changed the reduction gear setup in the tailshaft or altered the MPT solenoid function, lights em up. Until the MPT engages the rear enough to push the car.

http://www.subaruoutback.org/forums/367598-post38.html

That's on a 4EAT 4 cyl Outback and according to the Duty C loads, the split is never really less than 80/20. In 1st, it's basically 50/50 until you start picking up speed, then it slowly backs off the amount of power it's sending to the rear.

I can tell you that under no conditions can I get my H6 Outback to spin just the front wheels. If wheels are spinning, either the others are keeping it moving or both axles are spinning and I'm not moving.

Maybe cruising at a high speed it gets somewhat close to 90/10, but otherwise it's nowhere near that. It's a "minimum" torque split, and one that seems pretty tough to hit.

 

[plain OM]

When "cruising", what does it matter?

If you've ever driven a manual transmission car, you might have found you can "cruise" with the clutch pedal partially depressed without the engine revving up. That's because clutch friction, even when the clutch pedal isn't fully released and there's not full pressure on the clutch plate, is enough to handle the torque from the engine required to keep the car rolling.

Granted, if the accelerator pedal is pressed further, the additional torque developed by the engine could exceed the gripping force of the clutch, and the engine will rev up. But with the Subaru AWD, the control system responds to the accelerator position and engine torque. When it detects the change, it increases the clutch pressure, (same as lifting the clutch pedal on a MT), thereby preventing the slip from happening.

In fact, even in 1st gear, the level is not set at any predetermined point, as is demonstrated in the scatter diagrams here The capacity of the transfer clutch is constantly varied depending on the driving conditions, and particularly the accelerator, gear position, and torque delivery through the torque converter.

Another observation coming out of the data logging in the linked thread is that the clutch control (duty cycle) is always at a minimum of 30% when the engine is on and the transmission is in a driving gear, and while the relationship isn't exactly linear, that reflects an approximation of the same as far a clutch torque transfer capability. In other words, the clutch is always at a point where it can resist slipping up to approximately 30% of its maximum torque transfer capacity. And as is evident in the MT comparison, even with this seemingly low percentage, it's enough to ensure that the rear wheels are pushing equally with the fronts.

This is a very dynamic system that doesn't have fixed levels such as 90/10, 80/20 or 60/40. It's constantly varying in response to conditions.

 

[rasterman]

This is a very dynamic system that doesn't have fixed levels such as 90/10, 80/20 or 60/40. It's constantly varying in response to conditions.

Well said. This system is analog- like water finding its own level, it just does its thing. It gets manipulated by the computer accounting for things on a digital scale, but the real world result is mechanical and direct. As long as the ATF pressure is up to minimum, the system will always distribute torque.

 

[cardoc]

http://www.subaruoutback.org/forums/367598-post38.html

That's on a 4EAT 4 cyl Outback and according to the Duty C loads, the split is never really less than 80/20. In 1st, it's basically 50/50 until you start picking up speed, then it slowly backs off the amount of power it's sending to the rear.

I can tell you that under no conditions can I get my H6 Outback to spin just the front wheels. If wheels are spinning, either the others are keeping it moving or both axles are spinning and I'm not moving.

Maybe cruising at a high speed it gets somewhat close to 90/10, but otherwise it's nowhere near that. It's a "minimum" torque split, and one that seems pretty tough to hit.

Maybe there's a misunderstanding. I was referring to the models other than the VDC in stating the 90/10 torque difference. All other models besides the VDC for Gen 2 have a minimal 90/10 ratio and increases incrementally according to demand. While on the other hand, the VDC only has a 5% ratio change. Its either 45/55 or up to 50/50. That's what makes it different.

 

[jp1203]

Maybe there's a misunderstanding. I was referring to the models other than the VDC in stating the 90/10 torque difference. All other models besides the VDC for Gen 2 have a minimal 90/10 ratio and increases incrementally according to demand. While on the other hand, the VDC only has a 5% ratio change. Its either 45/55 or up to 50/50. That's what makes it different.

I'd tend to agree more with that, though I don't think the non-VDC cars are ever as low as 90/10 (see the linked thread), and I wouldn't say it "increases from 90/10" because it seems to start out at a nearly equal split and takes power away from the rear wheels as the car picks up speed.

 

[susiemk]

OEM Rear LSD on 91+ R160 Subarus is Viscous (V-LSD) so the goo between the mini "clutch plates" has to heat up before you get "strong" lock-up - ? which year VDC's came with rear LSD's.

Older cars (97 & older) usually have a strange looking differential (internally) that is easy to ID as an LSD while newer cars (2000+) have a rear LSD that actually looks like an open diff internally (most folks will think it is an open diff on visual examination).

I've seen some V-LSDs that will spin both rear wheels in the same direction & others that do not or barely turn the opposite wheel - I tend to pick the ones off of 4eat's (easy life for the rear diff) that turn both rear wheels in the same direction when I'm junkyard hunting BUT these can actually be units that have technically "failed" (in "locked" ) as they are only supposed to "lock" when the goo is heated by a spinning wheel - that said they work fine.

I've read that 4eat's on "normal" models default to 50/50 OR front biased torque split depending on the gear they are in on normal driving - as/if wheels slip power is re-routed up to 50/50.
BUT
4eat WRX's from 2002 have the 45/55 torque split so do 2003-4 LGTs (2005 LGT's got the 5eat) - these also come with V-LSDs.

5 speed Subarus have a 50/50 viscous center diff - it takes a "second" to "balance" out the wheel speeds front to rear on a hard launch but they'll balance out fairly quickly & the rears are pushing forward while the fronts are spinning (launch = weight off of the front tires (less traction) & onto the rears (more traction) which probably is a factor in the wheelspin too).
If the center diff is broken it usually fails "LOCKED" so turning is difficult (similar to strong 4eat torque bind) & driving is noisy (wheelbearing type noise).