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4WD & AWD systems explained

1460 messages,  Last post on Sep 21, 2012 at 9:29 AM

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What is this discussion about? Chevrolet Suburban, Jeep Grand Cherokee, Subaru Outback, Ford Explorer, Nissan Pathfinder, GMC Envoy, Performance Mods, Tires, Suspension, Transmission, Truck, SUV

Messages Page 145 of 147

#1434 of 1460 Re: 2005 Outback AWD Failure [larryv] by wwest

Aug 13, 2012 (10:28 am)

Replying to: larryv (Aug 11, 2012 8:39 pm)
Subaru is not shy regarding marketing "AWD" systems that have VERY serious shortcomings. The current WRX, for instance, uses a completely non-functioning VC, Viscous Clutch, in order to tout "AWD" capability.
 
Sounds as if your's is one of the many F/awd systems out there in the market today that are really "ONE-WHEEL" drive systems. Totally open center differential, "AWD" mode only if all four tires have roughly EQUAL traction.
 
Most of those have a TC implemented "AWD" system, braking of "slipping" wheels to simulate traction accompanied INSTANTLY by full engine dethrottling...not very helpful in the situation you describe.
 
Turn off TC = ONE-WHEEL drive.

#1435 of 1460 Re: 2005 Outback AWD Failure [larryv] by rsholland

Aug 13, 2012 (12:59 pm)

Replying to: larryv (Aug 11, 2012 8:39 pm)
Did you turn off the traction control? I think that would have helped. Subaru says to turn it off in deep snow and mud. It certainly would have helped your clutch.
 
Bob

#1436 of 1460 Re: 2005 Outback AWD Failure [rsholland] by wwest

Aug 13, 2012 (8:04 pm)

Replying to: rsholland (Aug 13, 2012 12:59 pm)
Since turning off TC results in the car reverting to a simple ONE-WHEEL drive he got exactly the results one would expect.
 
With TC on the engine dethrottling would have been so severe the engine torque wasn't enough to pull that wheel over the "hump".
 
TC off functionality is pretty much restricted to "feathering" the throttling right up to, but not beyond, the point of loss of traction. Wheelspin, rocking the car back and forth, to get unstuck, can also be used constructively.

#1437 of 1460 Re: 2005 Outback AWD Failure [rsholland] by dcm61

Aug 14, 2012 (7:00 am)

Replying to: rsholland (Aug 13, 2012 12:59 pm)
Did you turn off the traction control?
 
Wake up before you post, Bob ... 2005 Outback 2.5L does not have traction control.

#1438 of 1460 Re: 2005 Outback AWD Failure [dcm61] by rsholland

Aug 14, 2012 (8:01 am)

Replying to: dcm61 (Aug 14, 2012 7:00 am)
What's the little yellow light that shows a car skidding on the instrument cluster? I can turn that off in my car, and you're saying that can't be turned off in the Legacy 2.5?
 
Bob

#1439 of 1460 Re: 2005 Outback AWD Failure [rsholland] by xwesx

Aug 14, 2012 (8:53 am)

Replying to: rsholland (Aug 14, 2012 8:01 am)
VDC wasn't available on the 2.5L models until 2008 as an option, then standard in 2009.
 
Even so, the car should have had traction in the front axle unless the center differential had gone out. It's unusual that he would have power to the rear axle under that circumstance, though, as typically it is the rear axle that would stop working as opposed to the front axle. In 2007, 08, and 09, the rear axles on the Outback were limited slip, but they were open prior to (and after) that, so under limited traction conditions they are generally 2wd (one on each axle).
 
I managed to get my '96 Outback stuck more than a few times in deep snow by taking too much weight off the wheels, and it would always default to spinning the left front and right rear tires.

#1440 of 1460 Re: 2005 Outback AWD Failure [dcm61] by wwest

Aug 14, 2012 (8:56 am)

Replying to: dcm61 (Aug 14, 2012 7:00 am)
"..does not have..."
 
That would be strange indeed. Even as far back as 2001 it ws well understood industry wide that TC could prevent a LOT of loss of control accidents rising from the use of to much engine torque applied to the drive wheels.
 
Prior to that period TC was found mostly only on "pure" FWD vehicles, non-AWD versions.
 
That was one of the aspects facilitated by DBW, the engine could be dethrottled without compromising the catalytic converter.

#1441 of 1460 Re: 2005 Outback AWD Failure [xwesx] by wwest

Aug 14, 2012 (9:06 am)

Replying to: xwesx (Aug 14, 2012 8:53 am)
VDC probably refers to stability control INCLUSIVE of TC, the 2008 timing would be about right for that happenstance.
 
Not knowing anything at all about the specifics of this particular "AWD" system in question the description, only the wheel with traction spinning freely, indicates a fully open center diff'l with the TC turned off.
 
"..were limited slip..."
 
But more likely than otherwise "simulated" limited slip using TC braking.
 
Sounds as if your '96 had some method to automatically "lock" the center diff'l or PTO, VC's were commonly used for that back then. Once TC was implemented those methods because useless, non-functional.

#1442 of 1460 Re: 2005 Outback AWD Failure [wwest] by xwesx

Aug 14, 2012 (10:02 am)

Replying to: wwest (Aug 14, 2012 9:06 am)
But more likely than otherwise "simulated" limited slip using TC braking.
 
No, they weren't, they were viscous LSD. After 2009, however, they did go to open front/rear differentials using the VDC system to compensate. It is a poor compensation, at best.
 
The WRX retains a rear LSD, while the STi has both front and rear LSD.

#1443 of 1460 Re: 2005 Outback AWD Failure [xwesx] by dcm61

Aug 14, 2012 (1:45 pm)

Replying to: xwesx (Aug 14, 2012 8:53 am)
In 2007, 08, and 09, the rear axles on the Outback were limited slip, but they were open prior to (and after) that,
 
2000 (w/AWP) and 2001 to 2006 2.5L also had rear LSD. Not sure which years/models of H6 had rear LSD.

Messages Page 145 of 147

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