GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 146

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 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 3 mph (5 km/h) and the transmission
is in N (Neutral). After 30 seconds the transfer case
will shift to Four-Wheel-Drive High mode.
Shifting Out of Four-Wheel-Drive Low
To shift from Four-Wheel-Drive Low to Four-Wheel-Drive
High, AUTO or Two-Wheel-Drive High, the vehicle
must be stopped or moving less than 3 mph (5 km/h)
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 to 2 mph (1.6 to 3.2 km/h). Turn the knob to the
Four-Wheel-Drive High, AUTO or Two-Wheel-Drive High
position. You must wait for the Four-Wheel-Drive
High, AUTO or Two-Wheel-Drive High indicator light
to stop flashing and remain on before shifting the
transmission into gear.
2-48
Notice: Shifting the transmission into gear before
the requested mode indicator light has stopped
flashing could damage the transfer case. To help
avoid damaging the vehicle, always wait for the mode
indicator lights to stop flashing 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 N (Neutral) with the engine running.
If the knob is turned to the Four-Wheel-Drive High,
AUTO, or Two-Wheel-Drive High switch position
when the vehicle is in gear and/or moving, the
Four-Wheel-Drive High, AUTO or Two-Wheel-Drive
High indicator light will flash for 30 seconds but will not
complete the shift unless the vehicle is moving less
than 3 mph (5 km/h) and the transmission is in
N (Neutral).