GMC 2002 Sierra - Manual (Page 127)

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Manual Transfer Case (If Equipped)
The transfer case shift lever is on the floor to the right of
the driver. Use this lever to shift into and out of
four
- wheel drive.
The 4WD indicator will
light up when you shift into
four
- wheel drive and the
front axle engages.
Some delay between shifting and the indicator’s lighting
is normal.