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4WD and AWD systems explained

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  • drew_drew_ Posts: 3,382
    Yes, the GM's AutoTrac system can be used in "auto" mode at all times, regardless of surface (in fact, I'd recommend that one leaves it in this mode even on dry/wet surfaces, rather than the "2WD" mode). In "auto" mode, whenever rear wheel slippage is detected, power is automatically transfered to the front wheels to assist. When rear wheel slippage stops, the system reverts back to RWD mode. The 4WD High mode should not be used on anything but slippery surfaces though, since it locks the centre differential.

    Hope this helps!

    Drew
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  • gpm5gpm5 Posts: 785
    In refernece to the GM auto with 4WD lo, hi, and auto, does it or doesn't it have a center diff. You said it doesn't but now you said in 4WD it locks it. Thanks.
  • gpm5gpm5 Posts: 785
    Is permenant vs full-time referring to being able to turn AWD on or off. My trooper has 4WD torque-on-demand (TOD) that is used in any condition. Normally, 15% front and up to 50:50 depending on slippage. 15% may not fall into the classification of AWD though. The 4WD TOD can be turned of to rear wheel drive. It also has 4WD Lo. Also, this is a vehicle with a form of AWD that falls into the less than $30k price that someone referred to above.
  • drew_drew_ Posts: 3,382
    Oops, my mistake. I was thinking of something else at the time. There is NO centre differential.

    The GM Autotrac equipped vehicles are basically rear wheel drive rigs. When the rear wheels slip, the AutoTrac activates an electronic control module, which uses a wet clutch pack, activated by an electric stepper motor, to send torque to the front wheels that still have traction. It is a "4WD on demand" system (or auto 4WD system), activating 4WD when needed. It has part-time 4WD and 2WD options, as well as a low range in the T-case.

    The Isuzu Trooper's TOD 4WD system is considered full-time 4WD. It is similar to the Acura MDX's VTM-4 full-time AWD system, except that it is RWD biased, instead of the FWD bias of the MDX's system. Heck, the Isuzu TOD system is even made by the same company as the MDX's VTM-4 system...Borg Warner. It is considered 4WD and not AWD because it has low range gearing; AWD does not.


    Drew
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  • hung0820hung0820 Posts: 426
    I have a Santa Fe LX 4WD! What is my type of 4WD categories fit into? Isn't Full-Time 4 Wheels Drive or Full-Time AWD, or Is just of 4WD? Please respond. Thank you.
  • drew_drew_ Posts: 3,382
    The Sante Fe has a full-time AWD system with a viscous coupling centre differential.
  • I have a question to which would be prefered and why.

    AWD with the skid control/traction or AWD with the limited slip differential? Can't have it both ways.

    Also, on the AWD version does the TC only control the front tires or all? Obviously the 2WD version only controls the front. Thanks.
  • drew_drew_ Posts: 3,382
    Read my messages here:
    drew_ Feb 11, 2001 7:06pm

    drew_ Feb 11, 2001 8:00pm

    VSC incorporates 4 wheel traction control, which is a component of stability control. Watch the video clip in my 2nd message (#35) linked above. Since 4 wheel TRACS simulates a locking differentials for the front and rear axles, a mechanical rear LSD would be redundant.


    Drew
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  • loboyloboy Posts: 75
    Very informative with the descriptions of different types of 4WD systems. I agree that many people (myself for one) are misusing the 4WD/AWD misnomer. Now my question: Are the Pathfinder/QX4 systems classified as AUTO 4WD, similar to the Trooper above, and not AWD, since they have low range gearing?
    Thanks
  • drew_drew_ Posts: 3,382
    Absolutely! You got it. The QX4/Pathfinder LE's system is auto 4WD. Nissan calls it "All-mode 4WD".
  • woodyr1woodyr1 Posts: 142
    Drew:

    I own a Nissan Pathfinder with a part-time 4 wheel drive system. A direct mechanical link exists to engage the transfer case and operate the front drive shaft.

    I am considering the purchase of a replacement vehicle and am looking at a Toyota 4Runner. The 2000 model (SR5) has a floor lever, like my Nissan, which locks in the front drive shaft almost instantly upon engagement, a part time system. The 2001 models have 'one touch 4 wd', which works through a centre differential and apparently allows one to drive in any road condition in 4 wd, without damage to tires or mechanical drive components.

    I live in central British Columbia amongst the mountains and valleys, and engage 4 wd, as conditions warrant (snow, ice, etc). I drove a 2001 4Runner, and noticed a delay from when the one-touch 4 wd button was pressed until the 4 wd icon illuminated on the dash. Do you know what mechanical processes exist to account for this delay? My present vehicle actuates 4 wd almost instantly as engaging the floor lever. I like the advanced features of the 2001 Runner, such as traction and skid control, but am concerned that the 'lag' in engaging 4 wd may present a delay in obtaining traction when needed. With a centre differential, leaving the 4 wd engaged would not cause any wear problems, but the increased fuel consumption and loss of power may be noticeable. I have an opportunity to buy a 2000 4Runner SR5, which may be more suitable to my environment.

    Thanks
  • ateixeiraateixeira Posts: 72,587
    Why not take a look at the Highlander? It has more room and a lower price, plus permanent AWD. No worries, no buttons, 225hp, and better gas mileage.

    If you don't need the low range or ladder frame, it looked pretty nice at the auto show I went to.

    -juice
  • drew_drew_ Posts: 3,382
    Nice to hear from another BC'er. AFAIK, the one touch 4WD engages the front prop shaft in exactly the same way as your Pathfinder, except that there are more electronics involved to actually engage the mechanics. Do you recall if the wheels were turned, or how quickly you were going at the time? I know that it does not necessarily engage until certain parameters are met (to prevent damage/excessive wear and tear to the system).

    Personally, I would recommend that you leave the system in 4WD mode at all times. If there is any fuel economy difference it will be negligable, and you will have twice the amount of usable traction than in 2WD mode at all times. As such, in any emergency situation, VSC may not have to intervene as early or as forcefully. FWIW, Consumer Reports ran their Jeep Grand Cherokee Laredo with SelecTrac 4WD (similar to the Toyota's system) in both 2WD and 4WD full-time modes and found no difference in fuel economy. They recommended leaving the system in the latter mode at all times since it helped to stabilise handling. As for loss of power, this shouldn't be the case at all since 4WD simply redistributes the power from the 2 rear drive wheels to all four wheels. As a result, you also get more even tire wear.

    To see a technical view of why full-time/permanent AWD/4WD is better than 2WD, read the following links:
    http://www.4x4abc.com/4WD101/tractionturn2.html

    http://www.4x4abc.com/4WD101/tractionturn4.html

    Hope this helps! Any questions are welcome, of course.
  • drew_drew_ Posts: 3,382
    Good suggestion, but I have a feeling Wes will want to take the vehicle off-road now and then :-) The 4Runner would definitely seem more at home in central BC than the Highlander (though it's also a good choice). In snow though, the 4Runner would definitely have the advantage.


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  • ateixeiraateixeira Posts: 72,587
    You mean the Highlander just for snow, right?

    -juice
  • drew_drew_ Posts: 3,382
    Huh? No I mean the 4Runner for snow. With its greater ground clearance, and permanent 4WD system + stability control, it should be better than the H.L.


    Drew
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  • woodyr1woodyr1 Posts: 142
    Hi Drew:

    Thanks for the information. To answer your question, I was going straight at about 30 kmh, when I pressed the one-touch 4 wd button. The icon on the dash flashed a few times, then became solidly illuminated. It was fully in 4 wd at this time.

    I appreciate your input, as in winter conditions, leaving it in 4wd will be of benefit. With all the electronics and speed sensors etc. do your forsee expenses with servicing, or reliability? I understand that the system is covered by the 5 year/80,000 km power train warranty.

    Thanks

    Wes
  • drew_drew_ Posts: 3,382
    What you describes actually seems normal to me. I recall the same behaviour when I test drove the '99 4Runner Limited about 3 years ago (back then only the Limited version had the full-time 4WD system).

    With the 4Runner's excellent track record in mind, I do not forsee any problems with leaving the vehicle in 4WD mode all year around. Actually, a friend of mine has a '99 4Runner Limited (bought it brand new in late 1998 ) that is in 4WD mode all year around, and I have yet to hear him mentioning anything regarding reliability problems, or the lack of power/loss of fuel economy with his truck.

    I hope this helps in your decision. Please keep us updated. Happy hunting!

    Drew
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  • ateixeiraateixeira Posts: 72,587
    Guess the word "just" threw me off. As in the 4Runner would be better if you did more than just that.

    My wife's aunt has a Limited model and is happy with it. The only catch is the step in height is very high.

    -juice
  • drew_drew_ Posts: 3,382
    Yes, the 4Runner's step-in height is a higher than say, the M-class or the Highlander. I suppose it's partly because of its fairly significant ground clearance.


    Drew
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