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4WD and AWD systems explained

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Comments

  • mfhyattmfhyatt Member Posts: 7
    Someone told the guy that looked at the car that it may be the mpt clutch. It is located it the back of the transmission. What is your opinion on that and do you know what changing that entails?
  • ateixeiraateixeira Member Posts: 72,587
    Could very well be, though to be honest failures are so rare I don't recall a single one among Subaru Crew members. Try a Google search, maybe?

    -juice
  • paisanpaisan Member Posts: 21,181
    The center diffy clutch could have gone. Fairly uncommon, but not out of the question. It may need a new/used trans swapped it. That will likely be your cheapest way out of it.

    -mike
  • arnoldaarnolda Member Posts: 3
    My 1998 subaru legacy automatic spins the tires when climbing up a steep wet asphalt driveway. It seems first the front right spins then the a back wheel spins. Other front-wheel drive sedans and 2wd trucks can drive up this driveway with no problem. Does anyone know what is wrong with my subaru?
  • ateixeiraateixeira Member Posts: 72,587
    It's probably the tires. The best AWD system in the world coupled to the best traction control in the world still can't accomplish anything without traction from the tires.

    AWD is working because it's sending power to the rear axle.

    -juice
  • arnoldaarnolda Member Posts: 3
    I put new tires on my subaru legacy outback and first thought that it solved the tire spinning problem. The first time it successfully ascending the hill, but the second time it just spinned the wheels again. Could there be some sort of electronic traction control device that is working intermittently?
  • ateixeiraateixeira Member Posts: 72,587
    It doesn't have traction control. It could be the AWD overheats at times and is beginning to fail, but you don't always feel it. Just a guess.

    FWIW, AWD failures are very rare. We see differentials, clutches, wheel bearings, gaskets, but very rarely do we see issues with the AWD system itself.

    -juice
  • paisanpaisan Member Posts: 21,181
    You guys are missing the most obvious thing I've seen here. You are probably hitting the gas too hard. Also what kind of tires are you using? If you have 1 spinning in the front and one in the rear your AWD system is working properly.

    -mike
  • arnoldaarnolda Member Posts: 3
    I put new tires with more aggressive tread and even tried lowering the tire pressure to about 25psi. I don't think I am hitting the gas to hard, but I tried it hard and 'slow and steady'. There may be one other thing you should know about the car. I ran over a pillow in the road (it was dark and raining) and it got sucked up into the the shrouding that covers the driveline just infront of the rear differential. There is a small 3" dia. hole at the bottom of the shrouding. That entire polyester pillow went through that hole and got compressed around the driveline and u-joint, to about the size of a grapefruit. The car came to a sudden stop and would not move at all. With the help of a tow truck driver and a pair of vise-grips we pulled that pillow out of there, and the car seemed fine. Do you think this could have damaged something in the AWD system?
    Thanks for you help!
  • airdix81airdix81 Member Posts: 2
    i have a hyundai santa fe with 3.5 v6. i have the tod electronic 4wd system. it also had a tcs system. can anyonr tell me if this is a good system for snow? does it have a LSD

    thanks
  • ccaywoodccaywood Member Posts: 1
    Nice!
  • paisanpaisan Member Posts: 21,181
    My guess is that you definitely damaged the 4wd system when you wrapped that pillow up in it. Seems to be the only logical explaination of it.

    -mike
  • russlarussla Member Posts: 74
    Hi Mike, are you still maintaining your Isuzu page?

    Thanks

    RLR
  • paisanpaisan Member Posts: 21,181
    It's just down right now, should be up shortly.

    -mike
  • motionneonmotionneon Member Posts: 3
    Hello -

    I have a 2005 tribute with electronic 4wd. The 4wd light came on and my seat heaters stopped working after my last windshield wiper control repair, leading me to believe its just a fuse. Right now the 4wd indicator light blinks 3x/minute, which means that it needs service, but does anyone know if the electronic 4wd will still function if its simply an electrical/fuse problem?
  • ateixeiraateixeira Member Posts: 72,587
    I'm not sure, but the fuse theory seems feasible since it is controlled electronically (vs. mechanically).

    -juice
  • mamb99mamb99 Member Posts: 1
    The 4x4 lights on the instrument panel blink 3 times every few minutes. Does anyone know what this might mean. Also I was wondering if anyone knows how to find the original code for the keyless pad. I heard it was on the computer module but not having any luck. Thanks.
  • tejayntejayn Member Posts: 1
    I have a 2002 ford explorer. its an automatic with the 4.0 and automatic four wheel drive. the 4x4 high light on the dash flashes 7 times every few minuets and the four wheel drive is intermittently engaged. I have hooked it up to a scanner but i cant get into the four wheel drive system. any ideas or advise would be appreciated.
  • ryan07ryan07 Member Posts: 1
    My 2000 Nissan Pathfinder 4WD system has recently been causing problems. It is slow to engage and the only way to disengage it is to shift the 4wd lever into 2wd and then shift the transmission into reverse and 'goose' the gas. Any suggestions as to what the issue may be?

    Thks
  • cthornburgcthornburg Member Posts: 1
    reading your situation with your 2004 was like i wrote it. i just bought a 2004 yukon with stabilitrack, the dealer was confused as i was. have you fiqured out how to use? the manual was very confusing. thanx
  • alexstorealexstore Member Posts: 264
    can someone explain Highlander's Hybrid 4WD system compared to regular 4wd Highlander?
  • ateixeiraateixeira Member Posts: 72,587
    Yes, but surprisingly, it's based on the FWD Highlander.

    Basically, only the front axle gets power from the engine. There is no driveshaft going to the rear.

    The rear axle is powered by electric power only. The electric assist goes to both axles, funny enough.

    So the gas engine is FWD and the electric boost is what makes it AWD.
  • alexstorealexstore Member Posts: 264
    so basically 4wd is limited till what speed?
  • ateixeiraateixeira Member Posts: 72,587
    I'm not sure if it is speed restricted. Presumably you'd get an electric boost at any speed if you floor it (passing on a highway, for instance).
  • steverstever Guest Posts: 52,454
    I think this video just about sums AWD up:

    AWD Technology
  • ateixeiraateixeira Member Posts: 72,587
    Love the use of duct tape...
  • knobbyrobbyknobbyrobby Member Posts: 2
    I have a 1998 Ford Explorer and it makes a weird winding noise when turning. At first I was thinking it was the power steering but my mechanic thinks its the AWD unit. It seems like my gas mileage has drastically dropped so I am also thinking its the same. It almost seems as though the AWD is constantly locked in because when you turn it also gets very rigid. Have you ever heard of this happening and is this possibly a recall item I dont know of. I'm sure it will be astronomical to fix this if it is the AWD...and the noise is driving me nuts. Not to mention I dont know if it dangerous to keep driving it like this or if it is doing more damge.
  • xwesxxwesx Member Posts: 16,794
    One easy way to distinguish between the steering or the AWD "whining noise" is to turn your wheels side to side while stopped. If you get the same whining, it is the power steering pump. Aerostar vans are notorious for whining power steering pumps. The pumps work fine, but they sure to whine about it!

    As for AWD, I expect that the Exploder probably has a part-time, electronically controlled system. The Ranger has a switch-controlled electronic 4WD system that has all sorts of problems with the electronics. If the Explorer shares components in its system, it could be that the AWD is engaged all the time due to some system failure. Is there a light on the dash that will typically let you know when the system is engaged? As the system is part-time, it is likely hard on it to be running on dry pavement, etc., due to the lack of slippage available. If you turn hard at slow speeds, you should be able to feel the truck "binding" a bit and likely hear wheels slipping as the system relieves pressure. Try it on a gravel surface so the wheels will let loose easier.
    2018 Subaru Crosstrek, 2014 Audi Q7 TDI, 2013 Subaru Forester, 1969 Chevrolet C20, 1969 Ford Econoline 100, 1976 Ford F250
  • knobbyrobbyknobbyrobby Member Posts: 2
    What really happens is when you're driving it feels like the feeling you get when engauged in 4 wheel drive. The steering gets rigid and jumpy. Especially when on the highway and you hit a bump doing like 55-65 MPH. The whole front end gets a massive vibration and pulling. It also feels like the steering wheel is drifting side to side. I firmly believe its the AWD locked in constantly....although it started gradually happening and stopping but now doing it all the time. It feels very dangerous and my gas mileage is terrible. I'm sure this is a ridiculous price tag to fix if not a reacll item.
  • ateixeiraateixeira Member Posts: 72,587
    I know they used to have a viscous coupling, but can anyone tell me what Toyota is using for the current models?
  • chris301chris301 Member Posts: 2
    I have a 96 Cherokee. Only recently have I had the need for 4WD. I have heard a faint whining sound when engaged in 4WD and driving, that stops when the car stops. Now, the car seems to be in 4WD, even when I switch it back to 2WD. After a bit, I'll hear a "thud" and can feel it disengage, and then I am definitely back in 2WD. What's happening here? It's only happened twice, both today. Its cold and snowy where I am. I don't want to ruin anything. Any thoughts?
  • xwesxxwesx Member Posts: 16,794
    It is probably worth your while to check the fluid levels in both the transfer case and the front differential. If you have never replaced it, consider doing so now. It sounds to me like something is binding a bit - probably in the transfer case - and preventing the system from disengaging. Also, are you engaging and disengaging it properly? Some of the "older" shift-on-the-fly systems (not sure if Jeep falls under this classification) required a short jaunt in reverse to allow the transfer case to disengage, even though you can shift into 4WD while traveling forward.

    Any problems with it engaging at the time you request it?
    2018 Subaru Crosstrek, 2014 Audi Q7 TDI, 2013 Subaru Forester, 1969 Chevrolet C20, 1969 Ford Econoline 100, 1976 Ford F250
  • chris301chris301 Member Posts: 2
    Thanks much for the reply. I've had no problem engaging whatsoever. I'll replace the fluid tomorrow. I'll try reversing first to disengage as well and will post results.
  • kurtamaxxxguykurtamaxxxguy Member Posts: 1,798
    Glad to have found this forum as I have some AWD queries:

    1. Is it possible to change the Subaru Outback auto-trans AWD bias ? I understand it is RWD-biased and perhaps that is why the car has a reputation for spinning out (per CU and some other tests I have read) ? Would be nice to be able to readjust it to prevent that.

    2. How does the RAV4's AWD compare to the Outbacks? It seems the RAV4 is reactive, save for a user setting that offers full AWD for speeds below 20 mph.

    I'm also curious if VW has abandoned the torsen based Quattro and is now relying on Haldex, and how the Haldex system compares to that used in the Subaru.
    :shades:
  • ateixeiraateixeira Member Posts: 72,587
    I'll take a shot at trying to answer each question.

    1. Not really, though you can insert a FWD fuse to send all power to the front axle. Note that this is for temporary use only, such as when you get a flat tire.

    2. I was bummed that Toyota took away the full-time system that existed in prior RAV4 models. The new one is part-time, on-demand. At an event Toyota hosted themselves, the AWD system failed to climb a hill they had picked for their press demo:

    http://www.drive.com.au/Editorial/ArticleDetail.aspx?ArticleID=11004&vf=23

    VW: Haldex makes what is probably the quickest reacting AWD system on the market, but it's still reactive. Volvo and Ford also use the same supplier for their systems.
  • xwesxxwesx Member Posts: 16,794
    The AWD on the Subaru automatic actually defaults to a FWD bias, though it can shift to rear depending on the traction situation, as much as 80-20 either way. But no, it is not driver controllable. The STi has a driver-adjustable system, at least on the new model.
    2018 Subaru Crosstrek, 2014 Audi Q7 TDI, 2013 Subaru Forester, 1969 Chevrolet C20, 1969 Ford Econoline 100, 1976 Ford F250
  • paisanpaisan Member Posts: 21,181
    Not anymore Wes. The new 5EATs are 45/55 torque split. The 4EATs are 80/20.

    -mike
  • xwesxxwesx Member Posts: 16,794
    Ah, so while the 4EAT sticks with the FWD bias, the 5s (in... XT and H6?) are slightly RWD bias. What is the variability allowed in the center differential on the 5EAT to shift power based on traction?
    2018 Subaru Crosstrek, 2014 Audi Q7 TDI, 2013 Subaru Forester, 1969 Chevrolet C20, 1969 Ford Econoline 100, 1976 Ford F250
  • kurtamaxxxguykurtamaxxxguy Member Posts: 1,798
    ---and now Saab uses Haldex in a very complex new system. Unfortunately (your mileage may differ !!) that vehicle is tuned for the track, not all around use.

    Interesting article about the RAV4. Does not inspire confidence. Did Toyota change anything for '07 or are they using the same system?
  • ateixeiraateixeira Member Posts: 72,587
    No change as far as I know.
  • ateixeiraateixeira Member Posts: 72,587
    Subaru's VTD/VDC (Variable Torque Distribution with Vehicle Dynamics Control) is very impressive because it really meets every criteria you could possibly want from an AWD system:

    * it's full-time
    * it's pro-active
    * it can send 100% of torque to either axle
    * the front axle is managed by the VDC
    * the rear axle is managed by the VDC

    The only thing missing is a low range, or the ability to increase the bias side-to-side proactively like Acura's VTM-4.

    VTM-4 can't do the 100% part, so it's not perfect either.

    IMHO these two are about as close as you get to ideal AWD.

    If you think about it, the RAV4 can only meet 2 of those 5 criteria. Same with the Highlander.

    4WD have different criteria, of course. paisan could probably tell you more about that.
  • kurtamaxxxguykurtamaxxxguy Member Posts: 1,798
    agreed that Subaru's AWD system is very nice, but as you said, apparently the side-side bias is fixed to whatever the limited slip diffs decide to do.

    I do not know if the limited slip diffs in Subarus are torsen or (I assume) clutched bevel/ring design.
  • ateixeiraateixeira Member Posts: 72,587
    The only model that gets a Torsen diff is the Legacy spec.B, on the rear axle.

    The others that have a rear limited-slip use a viscous coupling. Simple but effective.
  • kurtamaxxxguykurtamaxxxguy Member Posts: 1,798
    Is the Subie's _front_ differential limited slip as well?
    Via viscous coupling?
  • naatz1naatz1 Member Posts: 188
    The Subaru has a good AWD system however their vehicles (Forester, Legacy, and now new 08 Tribeca) are just too small for a 6ft3 driver with 6ft+ backseat passengers. I have seriously considered each in the last 7 years of new car buying and they always lose.

    The bad press and a few other features caused us to drop the RAV4 and Highlander for country MN winter driving considerations and boat towing.

    I think the new roomy Saturn Outlook(Acadia/Enclave) has a fairly "tough" AWD system w/4500lbs of towing capacity but there's no way to lock it in < 25 mph. Even the Hyundai Veracruz had that, however like most imports only 3500 lbs towing max. So you have a Grand Cherokee with solid (but heavy) 4WD/AWD and a true low range, 6500 lbs towing with the "small" 4.7L torquey V8 and semi-crummy MPG. A go through anything AWD, just pay for it in cost of ownership.

    One of the above may fit everyones requirements, it depends ....
  • paisanpaisan Member Posts: 21,181
    No, all the subarus have an open front diffy, except the STi which has a viscous front diffy.

    On VDC models you get an ABLS on the front and rear where it will lock a faster moving wheel to create a limited slip. I have this on my Armada and it works well in all but full-out track conditions where you can overheat the brake fluid (saw this on an E500 AMG a few years back)

    -mike
  • kurtamaxxxguykurtamaxxxguy Member Posts: 1,798
    Sounds like the archilles heel of Subies (save STI) would be if both rears and one front wheel had no traction, but other front wheel did. Not likely but in winter, anything is possible :surprise:

    That is, unless the Subie has VDC, which would lock that spinning wheel and you'd be good to go.
  • ateixeiraateixeira Member Posts: 72,587
    The Outlook is big, I'd call it full-sized.

    We test drove one, but ended up buying a Sienna minivan. Just to give you an idea of the size class it's in.

    The Forester is a good 3 sizes smaller. Sportier and more fun to drive, but a lot smaller, naturally. I'd say a RAV4 is 2 sizes smaller, and the new Highlander is still one size class down from the Outlook.

    It all depends on what you want.
  • paisanpaisan Member Posts: 21,181
    Sounds like the archilles heel of Subies (save STI) would be if both rears and one front wheel had no traction, but other front wheel did. Not likely but in winter, anything is possible

    That is, unless the Subie has VDC, which would lock that spinning wheel and you'd be good to go.

    The system is still better than all the other AWD systems out there though No other AWD system out there has an LSD front axle w/o traction control.

    The advantage of the Subaru system is that because it has power going to both axles at all times you are less likely to get into a situation where you are stuck/spun/etc. than if you are driving one of the systems with a reactionary AWD system.

    -mike
  • ejonavinejonavin Member Posts: 36
    I think you mean Acura's SuperHandling-AWD. VTM-4 can't bias side to side like SH-AWD. With VSA,VTM-4 could brake one wheel slipping to give the other more traction and acting as a LSD.

    I'm waiting for the Super Terrific Awesome Handling All Wheel Drive that can send all torque to one wheel. :D
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