five-position shift pattern of a conventional 4WD gearbox is as follows.
Two-wheel-drive range. In this high-range mode, the gear ratio is at 1 to 1.
Both of the output shafts revolve at the same rate as the transmission output
Four-wheel-drive range with transfer case differential locked up (part-time,
high-traction mode). A straight 1 to 1 gear ratio.
Four-wheel-drive high range with transfer case differential not locked up (full
time, all-surface mode). A straight 1 to 1 gear ratio.
front and rear axles are disengaged from the power train and the vehicle may be
towed without uncoupling the propeller shafts. However, torque is not
transferred to the front axle assembly.
Four-wheel-drive low range with transfer case differential locked up. This mode
of operation has a gear ratio generally at 2:1 (although it may go as high as
26:1), which cuts the road speed in half. Thus, the vehicle can be operated in
4WD low range over difficult obstacles and through mud, snow, and sand with the
engine running up in its power band but with relatively slow vehicle speed.
The transfer case
shift pattern is usually marked on the shifter knob. Manufacturers use different
patterns and designations for the various gear positions. On some vehicles, an
indicator light is located in the dash panel to show the driver which mode of
operation the vehicle is in. when the indicator lamp is not lit, it usually
indicated the vehicle is 2WD or neutral.
One system on the
market contains a transfer case that does not have a low-range position. The
transmission includes a low-geared sixth-speed position, which can be engaged
when the transfer case is in the four-wheel-drive mode. In fact, there are
single-speed transfer cases available that permit the operator to employ either
2WD or 4WD through a simple gear train. In one position, the gear train permits
the power flow to the rear driveline only. An integral main drive gear in the
transfer case (driven by the transmission output shaft) provides power to the
rear driveline at all times. In addition, it is in mesh with an idler gear on a
short auxiliary shaft. The idler gear, in turn, meshes with a front-axle drive
gear to rotate the front driveline when in the 4WD mode. Or, the front-axle
drive gear is moved out of mesh with idler gear when the 2WD mode is desired.
On some models,
particularly more recent ones, the shift to 4WD can be accomplished while the
vehicle is moving (shifting-on-the-fly). Most vehicles must be stopped before
the change can be made. Also, the shift into a four-wheel-drive low mode
normally requires that the vehicle is stopped first.
some of the power
on-the-fly systems feature a magnetic clutch on the transfer case that speeds up
the front drive shaft, differential, and axles to the same speed as the
transmission. At the instant the speed is synchronized, an electric motor on the
transfer case completes the shaft, smoothly and quietly. The operator cannot
damage the system because it does not shift until the speeds are synchronized.
The newest of all 4WD
systems is the automatic design. While there are several different systems on
the market, one of the more commonly used systems has a microprocessor to
control the interaxle differential, thus providing varying amounts of torque to
the front and rear driving wheels. That is, in a fraction of a second, a
computer device senses the difference in wheel speeds and engages a clutch-type
device to begin driving the rear wheels. When the wheel speeds are equal again,
the system reverts to front drive.