Chevrolet 2008 Avalanche Owner's Manual page 124

If the knob is turned to the Four-Wheel Drive Low
position when your 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 your
vehicle is moving less than 3 mph (5 km/h) and the
transmission is in NEUTRAL (N). 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, your
vehicle must be stopped or moving less than 3 mph
(5 km/h) with the transmission in NEUTRAL (N) and the
ignition in ON/RUN. The preferred method for shifting out
of Four-Wheel Drive Low is to have your 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 your transmission
into gear.
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Notice: Shifting the transmission into gear before
the Four-Wheel Drive Low indicator light has stopped
flashing could damage the transfer case. To help
avoid damaging your vehicle, always wait for the
Four-Wheel Drive Low indicator light to stop flashing
before shifting the transmission into gear.
The vehicle may have significant engagement noise and
bump when shifting between Four-Wheel Drive Low
and Four-Wheel Drive High ranges or from NEUTRAL
while the engine is running.
If the knob is turned to the Four-Wheel Drive High,
AUTO, or Two-Wheel Drive High switch position when
your 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 your vehicle is moving less than 3 mph
(5 km/h) and the transmission is in NEUTRAL (N).