2010 GMC Sierra 1500 Crew Cab Owner's Manual - Page 189

Shifting Into Four-Wheel-Drive Low When Four-Wheel-Drive Low is engaged, vehicle speed should be kept below 45 mph. Extended high-speed operation in Four-Wheel-Drive Low may damage or shorten the life of the drivetrain. To shift to the Four-Wheel-Drive Low position, the ignition must be in ON/RUN and the vehicle must be stopped or moving less than 3 mph (5 km/h) with the transmission in N (Neutral). The preferred method for shifting into 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 Low position. You must wait for the Four-Wheel-Drive Low indicator light to stop flashing and remain on before shifting the transmission into gear. 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 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. 3-51