GMC 2011 Sierra Owner's Manual page 355

Turn the knob to the Four-Wheel
Drive Low position. You must
wait for the Four-Wheel Drive
Low indicator light to stop flashing
and remain on before shifting the
transmission in gear.
It is typical for the vehicle to exhibit
significant engagement noise
and bump when shifting between
Four-Wheel Drive Low and
Four-Wheel Drive High ranges
or from transfer case N (Neutral)
with the engine running.
If the knob is turned to the
Four-Wheel Drive Low position
when the vehicle is in gear and/or
moving, the Four-Wheel Drive
Low indicator light will flash for
30 seconds and not complete the
shift unless the vehicle is moving
less than 5 km/h (3 mph) and the
transmission is in N (Neutral).
After 30 seconds the transfer
case will shift to Four-Wheel
Drive High mode.
To shift from Four-Wheel Drive
Low to Four-Wheel Drive High,
or Two-Wheel Drive High, 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/RUN.
The preferred method for shifting
out of Four-Wheel Drive Low
is to have the vehicle moving
1.6 to 3.2 km/h (1 to 2 mph).
Turn the knob to the Four-Wheel
Drive High or Two-Wheel Drive
High position. You must wait
for the Four-Wheel Drive High or
Two-Wheel Drive High indicator
light to stop flashing and remain
on before shifting the transmission
into gear.
It is typical for the vehicle to exhibit
significant engagement noise
and bump when shifting between
Four-Wheel Drive Low and
Four-Wheel Drive High ranges
or from transfer case N (Neutral)
with the engine running.
If the knob is turned to the
Four-Wheel Drive High,
or Two-Wheel Drive High switch
position when the vehicle is in gear
and/or moving, the Four-Wheel
Drive High, or Two-Wheel Drive
High indicator light will flash for
30 seconds but will not complete the
shift unless your vehicle is moving
less than 3 mph (5 km/h) and the
transmission is in N (Neutral).