All-Wheel Drive and 4-Wheel Drive - Vehicles we focus on

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of the vehicle begin to slip, torque is
routed to the opposite end, where traction
is still present.
Although operation of all-wheel drive
vehicles is mostly transparent, they can
make more noise <strong>and</strong> have more unusual
characteristics than 2-wheel drive systems.
There are more parts rotating, <strong>and</strong>
more gears meshing.
When an on-dem<strong>and</strong> system engages
the second axle, the occupants may hear
<strong>and</strong> feel the components working. The
driver may also be aware of sensations
<strong>and</strong> noises when making turns, even on
high-traction surfaces where the second
axle is not called on to operate.
4-<strong>Wheel</strong> <strong>Drive</strong>, Part Time
In part time 4-wheel drive systems,
torque is normally sent only to the rear
axle. When the driver chooses, the front
axle can also be engaged. Further, the
driver can select bet<strong>we</strong>en high <strong>and</strong> low
ranges. Some part-time systems also
offer an Auto function that selects
bet<strong>we</strong>en 2-wheel <strong>and</strong> 4-wheel drive
modes according to need, without the
driver having to do anything. Even in
these systems, ho<strong>we</strong>ver, the driver must
choose bet<strong>we</strong>en high <strong>and</strong> low range.
In a part-time system, the occupants
may feel or hear “something” when the
front axle engages. The transfer case
makes more noise in 4-high mode than in
2-high mode. And the extra gear reduc-
Typical part-time 4WD transfer case
tion of 4-low mode is prone to making
even more noise.
Part time systems have no differential
or other device to allow front <strong>and</strong> rear
propshafts to turn at different rates. So, if
the vehicle is operated in 4-wheel mode
- Thanks to Tracy Timmerman
on hard pavement, driveline binding can
cause the tires to “crow-hop,” which can
be heard <strong>and</strong> felt. This results because all
four of the tires do not rotate exactly the
same number of rpm, either side to side
or front to rear. Eventually, this difference
causes the driveline to begin to bind, or
”wind up,” <strong>and</strong> one tire or other must
temporarily break traction with the pavement
to relieve the pent-up energy. On
unpaved surfaces, tires can slip as necessary
to overcome binding, without causing
crow-hop or noise.
Running the engine with the transfer
case in Neutral may result in noise, <strong>and</strong>
shifting into or out of Neutral can result
in some normal gear clash.
4-<strong>Wheel</strong> <strong>Drive</strong>, Full Time
The automatic 4-wheel drive system
provides torque to all four wheels all the
time, similar to the all-wheel drive system.
But like other 4-wheel drive systems,
it also has a low range. The driver
can select (depending on the model) 4
HI, 4HI Lock, 4 LO Lock, <strong>and</strong> neutral.
Typical full-time 4WD transfer case
4-<strong>Wheel</strong> <strong>Drive</strong>, Active
This system provides the driver with
five choices: 2 HI, Auto 4WD, 4 HI, 4 LO,
<strong>and</strong> neutral.
These vehicle can be driven continuously
in Auto, although there is a fuel
economy <strong>and</strong> noise penalty.
When driven in 4-HI or 4-LO, these
vehicles can experience crow hopping as
<strong>we</strong>ll. Ho<strong>we</strong>ver, when driven in Auto, the
operation of the clutch pack compensates
for different driveshaft speeds.
<strong>All</strong> Systems
Some noises in these vehicles aren’t
new -- they’ve been present in
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