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SUVs Toyota Sequoia Toyota 4WD systems explained
2400 messages, Last post on Nov 22, 2009 at 10:30 AM
You are in the Toyota Sequoia Forum. Your Hosts are steve_ & tidester
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I frequently get asked questions about various 4 wheel drive system on Toyota products. In the past, I replied on an individual basis. Finally, I wrote a fairly comprehensive piece on it but felt it was too long to post here. I have broken this up into four parts. The first is a list of terms and definitions. Next is an explanation of the Active-Track system on the Sequoia, Land Cruiser and 4Runner. Next is conventional 4WD systems like the Tundra and Tacoma have. Finally is a discussion of the AWD of the Highlander and RAV4. I hope this is helpful. |
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Transfer Case: A device behind the transmission that sends power to the rear only or to the front and rear in an exact, 50-50 split Center Differential: A devise that allows the power to be split between the front and rear in different amounts. Front or Rear Differential: A devise that splits power to the right or left side of the vehicle. It looks like a ball when viewed from the rear. Open Differential: A differential that is very rugged and reliable that splits power 50-50 but if one wheel begins to slip, power is routed only to that wheel. Limited Slip Differential (LSD): A differential that splits power 50-50 but if one wheel begins to slip, power is routed to the other wheel. This is accomplished in one of two ways: Either by a series of clutches or multiple plates encased in a heavy oil. The clutch type needs rebuilt somewhere between 60K and 100K miles. When the clutches fail, you will have no indication of it unless you become stuck. Rear Locking Differential: A push button mechanism that sends a worm gear into the rear differential to physically lock the axle together. NEVER use this on dry pavement. Center Differential lock: On certain vehicles, you can lock the center differential, which creates the effect of a transfer case. NEVER use this on dry pavement. Traction control (TRACS): A system that uses your ABS sensors to detect wheel slip. TRACS operates in one of three different ways. It can either apply brakes to the wheel that is slipping, it can reduce the engine RPMs to slow down the wheels or it can do both. In any case, by reducing wheel spin, forward movement is accomplished, albeit slow and controlled. Vehicle Skid Control (VSC): A lateral skid sensor that detects understeer or oversteer. If you begin to skid sideways, one, two or three corners of the vehicle have brakes applied and throttle is adjusted to get you moving in the direction the steering wheel is turned. Two Speed Transfer Case: Gives one the ability to gear the vehicle down for maximum pulling power. Works best with a transfer case or locked center differential. Part Time 4WD: This generally refers to a system that allows the use of the vehicle in 2 wheel drive. Prior to the Active Trac system, a part time system meant a transfer case type system. AWD, All-Trac, Full time 4WD: These terms refer to a 4WD system that is always engaged. It implies the use of a center differential rather than a transfer case. The A-Trac system defies this as a simple definition now. |
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Beginning with the 2000 model year Land Cruiser, Toyota began using a new 4WD system called ActiveTrac. This same system was incorporated into all 2001 4Runners and the new 2001 Sequoias. The same basic system has also been used in the Mercedes M Class as well as post ‘99 Humvees. These systems operate in essentially the same way with a few exceptions. When engaged, you have three open differentials working for you (front, rear and center). Open differentials are extremely reliable and require very little maintenance. If you have equal traction at all 4 wheels, power is evenly divided between them all. If one wheel begins to slip, the open differentials begin to send all available power to that one wheel. Normally, this would be very bad. This is when a traction control system (TRACS) takes over. TRACS, applies brakes selectively to a slipping wheel. This braking action literally fools the differentials into sending power everywhere except the slipping wheel. When you are in 2WD (in the Sequoia and Runner), you still have traction control working for you. Obviously, this only will send power left to right but this is better than nothing. There is one thing to be careful of in this condition. When you are in 2-wheel drive, there is a second part of the TRACS that can be hazardous if you are not paying attention. This is the engine speed limiter. This combines the braking action of TRACS with a rev limiter. Your engine speed will be cut back to 1500 to 2400 RPM. This allows for controlled forward movement but it will be slow. The danger with this is if it engages when you are trying to pull into fast moving traffic. This rev limiter only operates in the 2WD mode, so if you know you have any reduced traction, make sure you are in 4WD. On all three vehicles, you have the option of locking the center differential. It is rare that anybody would ever need to do this. On the Land Cruiser and 4Runner, this is accomplished by pushing a button on your dash. On the Sequoia, you shift into 4 wheel low and shift the transmission into “L”. This turns off the TRACS computer and the VSC system. The vehicle is now in a conventional 4WD mode. All 4x4 Toyota trucks have operated in this condition. You should not ever use this mode on dry pavement as you will damage the drive system and tires. The other part of this system is the VSC or vehicle skid control. VSC will selectively apply brakes and throttle to prevent understeer or oversteer. It works in both 2 and 4 wheel drive. This is a rather amazing system and does an incredible job of giving the driver control of the vehicle. Understeer is responsible for a large number of SUV rollovers and oversteer is very common on icy surfaces. The Sequoia will allow you to turn off the VSC but only when you are in 4WD. The only reason to turn this off is if you are off road and want to be able to slide sideways. On the Runner and Land Cruiser, the VSC and TRACS are disabled when you lock the center differential. The Land Cruiser is always in the 4WD mode. The 4Runner and Sequoia can be used in either 4WD or 2WD. It is safe to leave either in the 4WD mode at all times. You will loose a bit of fuel economy, but will handle better. Unexpected loose gravel and slippery surfaces will not be a problem. To engage the 4WD system on the Runner and Sequoia, press the button. The green and amber lights will flash on you dash. While it is flashing, the system has not fully engaged and you should avoid sharp corners at this time. If you are accelerating up a hill, these lights will continue to flash. If this happens, take your foot off the gas for a moment and tap the brake. This gives the differential a chance to engage fully into the 4WD mode. The same procedure applies to disengaging the system. To get into 4WD low, you must first be in 4WD. Stop the truck and place the transmission in neutral. Now, move the floor shifter forward to the low range. This takes a firm hand. This mode is only to be used to remove yourself from a very difficult situation. Once you are unstuck, shift back into the high gear range.
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These are typical part time systems. Under good road conditions, you are in 2WD with the rear axle getting all the power. Power is again split between the right and left wheels. An open differential will route all power to one wheel if it can turn faster than the other. If this happens, engage the 4WD system. This sends exactly half the power to the front axle where another open differential splits power. Between the front and rear axle, you will normally be able to gain forward traction but because of the open differentials, there is a possibility that you wont. Open differentials are vastly more reliable and longer lasting than limited slip differentials, which is why Toyota has stuck with them. With this part time system, you can engage it up to 62 MPH (50 MPH if you don’t have a push button system) but it really isn’t appropriate to drive it at this speed. Because the front and rear axles are turning at exactly the same speed, you can damage the system on dry pavement. This system is only appropriate for more severe conditions. The advantages to this type of 4WD are simplicity and speed of engagement. You are not relying on brake sensors for your 4WD system and it should be more rugged. Also, unlike the Sequoia and 4Runner, the system engages the moment you shift into 4WD. The other models take several seconds and feet to engage.
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These utilize a limited slip center differential and open front and rear differentials. It is a viscous coupling center differential. If one of the front wheels begins to spin faster than the rear, the heavy liquid in the center begins to firm up which routes more power to the rear. Once torque is equalized, the 50-50 power split is resumed. This system is always engaged and requires no driver input. It is possible to become stuck with this system. This is because of the open front and rear differentials. If both right tires were on ice, all power would be routed to these wheels. This is a fairly unlikely occurrence on a light duty vehicle like these. On the Highlander, you can get VSC, which includes traction control. If the right wheel begins to slip, brakes are applied to this wheel and power is sent to the left. On the 4WD model, there is no rev limiter associated with the traction control.
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| On the 4Runner can you use 4Lo without locking the center differential? If you can, you still have traction control I assume. | |
| That is correct. On the Runner, you can run with the full TRACS/VSC system in 4 Low or lock up the center and have a conventional 4WD system. | |
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on 4Runners. Can you use the 4 wheel drive in hi mode when driving on snowy highways? I know you're not suppose to on Dry roads. How much benefit do you get from the locking diffy when off-roading? Don't see many 4Runners with this option. Looking at possibly getting a used 4Runner. Leo |
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Off-roading in a TRACS-equipped '01 4Runnner: tbird45 "Toyota 4Runner" Jun 4, 2001 8:03pm Drew Host Vans, SUVs, and Aftermarket & Accessories message boards |
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I thought Toyota got rid of the locking rear diffy on the 2001's? I thought I also read somewhere that this option can't be had with an automatic transmission? Leo |
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