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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 147 of 147

#1452 of 1460 Re: 2005 Outback AWD Failure [larryv] by larryv

Aug 23, 2012 (1:45 pm)

Thanks xwesx. Yours was the most detailed response and best suggestions. Unfortunately, I don't have any means to put the car up on blocks to run the suggested resistance tests. But I could take it to the dealer/shop. Is the age and condition of the fluid in the center VC supposed to be checked with regular servicings? And, first apologizing for being such a car mechanic peasant, exactly how is the VC supposed to send more power to the wheel(s) with the greater traction? If the VC fluid is shot, how does that prevent sending more power to the wheels with greater traction? I don't believe my car has traction control. Nothing in my paper work mentions it and I don't see any button on the dash or console marked as such. I do a lot of fishing and drive on non-paved roads a bit and so want to make sure all the AWD features of this car are working properly and what to do if I ever fall into a similar situation again. Thanks. LRV

#1453 of 1460 Re: 2005 Outback AWD Failure [larryv] by xwesx

Aug 23, 2012 (3:08 pm)

Replying to: larryv (Aug 23, 2012 1:45 pm)
This is a good explanation on the operation of a viscous coupling LSD: Viscous Coupling on How Stuff Works
 
Now, this says it is in a sealed housing; I am not sure whether that is true or not for the Subaru center differential on the manual transmission. The car calls for GL5 gear oil in both the transmission and the rear differential, and the transmission is housed inside the transaxle, which includes the transmission, center differential, and front differential. I know the front diff and transmission use the same oil and are open to each other in terms of fluid transfer, and I think the center is as well. The VC in the rear axle is open to the fluid within the main housing.
 
Basically, if the friction qualities of the oil are too low, you'll get excess slippage. When it is on the edge of "good enough," one might notice the slipping through a "chattering" feeling/noise within the car due to it engaging/releasing in quick succession. If this is due to the oil qualities rather than age, one can often simply add friction modifiers to the oil to get it to stop slipping. I think the service interval calls for 50,000 miles or more between fluid changes, and often people let it go much longer than that. The older the fluid, the more thermal breakdown the oil has experienced and the less it is able to deal with high-shear-force situations. In my opinion, even 50,000 miles is a questionably long period of time for the stock fluid, especially if you drive it in conditions that are likely to engage the VC frequently.
 
For instance, the oil in my Forester, which is a synthetic that calls for 75,000 miles or five year intervals, was obviously well-used in the transmission/center/front diff when I changed it last month. I have 45K on the car now, and installed the oil at 2.5K. I didn't feel bad about changing it, that's for sure! I live in Fairbanks, Alaska, and slippage situations are frequent during the winter months (about half the year, Oct to April). The fluid in the rear differential, in contrast, looked like I had just installed it even though it was the exact same age. On my '10 Forester, the rear is an open differential.
 
Now, if the VC isn't engaging, it doesn't necessarily mean that the VC is bad, it simply means it isn't working with the fluid it has available. Fresh fluid, with, possibly, friction modifiers added, and it is good to go again. If you've never had the oil changed in the car, you might try that first just to see if it helps. It is a terribly easy chore to replace the oil, and takes about 1.5 gallons total (check your owner's manual for transmission and rear diff capacities). You will need a Torx T60 or T70 socket in order to pull the transmission plug (it is a T70, but the T60 works too and is often much easier to come by), and a long-necked funnel is helpful for the transmission fill, while a hand pump for your gear oil bottle is helpful for the rear differential.
 
If you want an easy way to test whether the system is working well or not, take your car and a friend (in a different vehicle) to some place with sand, and drive out on it. If your AWD system is not working correctly, you're going to get stuck in short order. If you do get stuck, just strap up to the other vehicle and get pulled out.
 
Finally, your car does not have the "VDC" system in it. You don't have any buttons that toggle traction control. The traction control system in that car is purely mechanical: By default, the power is split 50/50 to front and rear axles. Any change in power split, both between axles and within the rear axle, is reactionary based on the speed at which the drive shafts connected to the VC units are spinning. VDC was not added to the manual transmission cars until 2009. Prior to 2008, it was only available in the "3.0" (H6) Legacy/Outback.

#1454 of 1460 Re: 2005 Outback AWD Failure [xwesx] by wwest

Aug 24, 2012 (7:49 am)

Replying to: xwesx (Aug 23, 2012 3:08 pm)
Viscous Clutches/couplings are generally quite well sealed, heretically sealed since at time they operate with a great deal of internal pressure. Only a professional shop would be able to change out the fluid.
 
Post about 2002 VC's were so severely derated absent TC you had no real "locking". From what I can find out at Subaru your '05 has a standard open differential with a VC between the 2 "output" shafts. If there is a sustained difference in the speed of those 2 shafts, front/rear, then the VC fluid should stiffen dramatically thereby "locking" the 2 shafts together.
 
On a 4 wheel dyno the VC in my '00 F/awd RX300 would take about 15 seconds to re-apportion engine torque ~70/30 F/R. The '01 came with TC and VSC thereby making the VC useless and as such it was dropped entirely from the RX330 model.

#1455 of 1460 "It's about confidence" by steve_ HOST

Sep 19, 2012 (7:31 am)

"It's not even a debate," says Jim Vurpillat, global marketing director for General Motors Co.'s Cadillac brand. Luxury-car buyers expect all-wheel drive to be available on any serious luxury sedan, he says, which is why Cadillac is offering the technology on its new compact ATS and large XTS sedans, as well as its midsize CTS model.
 
What is debatable is whether all-wheel drive delivers enough safety and performance benefits for the $2,000 to $3,000 extra charge car makers typically add to the price tag, as well as the mileage penalty that the extra weight of all-wheel-drive hardware still exacts in most cars.
 
Consumer Reports compared the slick weather performance of a Toyota Corolla to a Land Rover and concluded that the Corolla—with its thin tires and light body—stopped much more quickly in snow than the burly, all-wheel-drive, luxury SUV."
 
All-Wheel Drive Goes From Novelty to Necessity (Wall St. Journal)

#1456 of 1460 Re: "It's about confidence" [steve_] by ateixeira

Sep 19, 2012 (8:26 am)

Replying to: steve_ (Sep 19, 2012 7:31 am)
OK, but how well would an AWD Corolla perform compared to a FWD one? That's the real question.
 
Articles like that get headlines, though.
 
Plus AWD is to supposed to help you get through. If you had to get your wife-in-labor to the hospital to deliver your first born in the middle of a snow storm, would you take the Corolla or the Rover?
 
Answer: your Subaru. It's lighter and brakes better, but still offers AWD.

#1457 of 1460 Re: "It's about confidence" [ateixeira] by steve_ HOST

Sep 19, 2012 (8:52 am)

Replying to: ateixeira (Sep 19, 2012 8:26 am)
K, there's a project for you. Put the fuse in on your Forester and see if you get stuck this winter.

#1458 of 1460 Re: "It's about confidence" [steve_] by ateixeira

Sep 19, 2012 (9:10 am)

Replying to: steve_ (Sep 19, 2012 8:52 am)
That just feels wrong. An idiot light pops up that says "FWD", as if FWD is a defect, so you know how Subaru feels about that.

#1459 of 1460 Re: "It's about confidence" [ateixeira] by wwest

Sep 20, 2012 (12:47 pm)

Replying to: ateixeira (Sep 19, 2012 8:26 am)
"..how well would an AWD Corolla perform.."
 
Considering it would of necessaty be a F/awd in which the clear majority of the time it would be FWD only......

#1460 of 1460 Re: "It's about confidence" [wwest] by ateixeira

Sep 21, 2012 (9:29 am)

Replying to: wwest (Sep 20, 2012 12:47 pm)
Yup, front axle would already be slipping by the time any power was sent aft.

Messages Page 147 of 147

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