GMC Sierra 2018 Owner's Manual page 266

The settings are:
N (Neutral) : Use only when the
vehicle needs to be towed.
See Recreational Vehicle Towing
0 416 or Towing the Vehicle 0 416.
2 (Two-Wheel Drive High) : Use
for driving on most streets and
highways. The front axle is not
engaged. This setting provides the
best fuel economy.
4 (Four-Wheel Drive Low) : This
setting engages the front axle and
delivers extra torque. Choose 4 if
driving off-road in deep sand, deep
mud, or deep snow, and while
climbing or descending steep hills.
When engaged, keep vehicle speed
below 72 km/h (45 mph).
Shifting into 4 will turn Traction
Control and StabiliTrak off. See
Traction Control/Electronic Stability
Control 0 272.
4 (Four-Wheel Drive High) : Use
when extra traction is needed. The
front axle engages and helps when
driving on snowy or icy roads, when
off-roading, or when plowing snow.
The vehicle can be shifted from 2
to 4 while the vehicle is moving.
Shifting Into 4
Turn the knob to 4 at any speed
up to 121 km/h (75 mph), except
from 4 . The indicator light will flash
while shifting and will remain on the
selected setting.
Shifting Into 2
Turn the knob to 2 at any speed,
except when shifting from 4 .
Shifting Into 4
When 4 is engaged, vehicle speed
should be kept below 72 km/h
(45 mph).
1. The ignition must be on and
the vehicle must be stopped or
moving less than 5 km/h
(3 mph) with the transmission
in N (Neutral). It is best for the
vehicle to be moving
1.6 to 3.2 km/h (1 to 2 mph).
Driving and Operating
2. Turn the knob to 4 . Wait for
the 4 indicator light to stop
flashing before shifting the
transmission into gear.
If the transmission is in gear and/or
moving more than 5 km/h (3 mph),
the 4 indicator light will flash for
30 seconds and not complete the
shift. After 30 seconds the transfer
case will shift to 4 . Turn the knob
to 4 to see the indicator. With the
vehicle moving less than 5 km/h
(3 mph), and the transmission in
N (Neutral), attempt the shift again.
Shifting Out of 4
1. To shift out of 4 the vehicle
must be stopped or moving
less than 5 km/h (3 mph) with
the transmission in N (Neutral)
and the ignition in on. It is best
for the vehicle to be moving
1.6 to 3.2 km/h (1 to 2 mph).
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