3.60 - How does my 4x4 really work ? (All)
Description
This article is from the Toyota Tundra FAQ, by Steve Yee with numerous contributions by others.
3.60 - How does my 4x4 really work ? (All)
How Toyota 4WD systems work (By Steve Clifford, Springfield Toyota, edited for
clarity):
Sequoia, 4Runner and Land Cruiser
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 \u201899 AM General 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
operates by applying 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 4Runner), 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 \u201cL\u201d. 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 4Runner 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 lose 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 4Runner and Sequoia, press the button. The
green and amber lights will flash on your 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.
Tundra and Tacoma
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 won't. 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\u2019t
have a push button system) but it really isn\u2019t 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.
RAV4 and Highlander
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.
(Thanks to Steve Clifford of Springfield Toyota for providing this writeup)
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