GMC SIERRA CK2 1999 Owner's Manual page 102

Electronic Transfer Case (If Equipped)
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If your four wheel drive vehicle has the electronic
transfer case, the transfer case switches are below and to
the left of the climate control system.
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Use these switches to shift into and out of four
drive. You can choose among three driving settings:
2HI: This setting is for driving in most street and
highway situations. Your front axle is not engaged in
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two wheel drive.
4HI: This setting engages your front axle to help drive
your vehicle. Use 4HI when you need extra traction, such
as on snowy or icy roads, or in most off
4LO: This setting also engages your front axle to give
you extra traction. You may never need 4LO. It sends
the maximum power to all four wheels. You might
choose 4LO if you were driving off
or deep snow and climbing or descending steep hills.
Indicator lights in the switches show you which setting
you are in. The indicator lights will come on briefly when
you turn on the ignition and the last chosen setting will
stay on. If the lights do not come on, you should take
your vehicle in for service. An indicator light will flash
while shifting. It will remain illuminated when the shift is
completed. If for some reason the transfer cannot make a
requested shift, it will return to the last chosen setting.
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wheel
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road situations.
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road in sand, mud