GMC 1999 Suburban, Owner'S Manual, Page: 92 / 424

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Automatic Transfer Case (If Equipped)
The transfer case switches are below and to the left of
the climate controls. Use these switches to shift into and
out of four
-
wheel drive. You can choose among four
driving settings:
2HI: This setting is used for driving in most situations.
Your front axle is not engaged in two
-
wheel drive. This
setting also provides the best fuel economy.
AUTO 4WD: This setting is ideal for use when road
conditions are variable. When driving your vehicle in
AUTO 4WD, the front axle is engaged, but the vehicle’s
power is sent only to the rear wheels. When the vehicle
senses a loss of traction, the system will automatically
engage four
-
wheel drive. Driving in this mode results in
slightly lower fuel economy than 2HI.
4HI: Use 4HI when you need extra traction, such as
on snowy or icy roads or in most off
-
road situations.
This setting also engages your front axle to help drive
your vehicle.
4LO: This setting also engages your front axle and
delivers extra torque. You may never need 4LO. It sends
maximum power to all four wheels. You might choose
4LO if you are driving off
-
road in deep sand, deep mud,
deep snow and climbing or descending steep hills.