GMC 2001 Sierra - Manual (Page 108)

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CAUTION: (Continued)
If you have four - wheel drive, your vehicle
will be free to roll -- even if your shift lever
is in PARK (P) -- if your transfer case is in
NEUTRAL. So, be sure the transfer case is in a
drive gear, two
- wheel high (2H) or four - wheel
high (4H) or four - wheel low (4L) -- not in
NEUTRAL. See “Shifting Into PARK (P)” in the
Index. If you’re pulling a trailer, see “Towing a
Trailer” in the Index.
REVERSE (R): Use this gear to back up. If your vehicle
is equipped with the Allison Transmission, the “R” may
blink on the PRNDL if the shift inhibitor is active.
NOTICE:
Shifting to REVERSE (R) while your vehicle is
moving forward could damage your transmission.
Shift to REVERSE (R) only after your vehicle
is stopped. If your vehicle is equipped with
the Allison Transmission, do not idle in
REVERSE (R) for more than five minutes.
Extended idling in REVERSE (R) may cause
transmission overheating and damage. Always
select PARK (P) whenever idle time exceeds
five minutes.
To rock your vehicle back and forth to get out of snow,
ice or sand without damaging your transmission, see
“Stuck: In Sand, Mud, Ice or Snow” in the Index.