GMC 2007 Sierra - Manual (Page 172)

If torque lock does occur, you may need to have
another vehicle push yours a little uphill to
take some of the pressure from the parking pawl
in the transmission, so you can pull the shift
lever out of PARK (P).
Shifting Out of Park (P)
(Automatic Transmission)
Your vehicle has an automatic transmission shift
lock control system which locks the shift lever
in PARK (P) when the ignition is in the LOCK
position. You have to fully apply your regular
brakes before you can shift from PARK (P) when
the ignition is in RUN. See Automatic Transmission
Operation on page 140 and “Torque Lock”
under Shifting Out of Park (P) (Automatic
Transmission) on page 172.
If your vehicle has an Allison Transmission ® , the
transmission has a park pawl designed to hold the
higher weight of commercial medium duty
trucks. Therefore, the Allison Transmission
® uses
larger parts than other automatic transmissions.
These larger parts may sometimes make a
clunking noise when the selector lever is moved
from DRIVE (D) or REVERSE (R) to PARK (P).
This is normal and there is no damage occurring
during these shifts.
If you cannot shift out of PARK (P), ease pressure
on the shift lever and push the shift lever all the
way up into PARK (P) as you maintain brake
application. Then, move the shift lever into
the gear you want.
If you ever hold the brake pedal down but still can
not shift out of PARK (P), try this:
1. Turn the key to ACCESSORY. There is no
shift interlock in this key position.
2. Apply and hold the brake until the end of
Step 4.
3. Shift the transmission to NEUTRAL (N).
4. Start the vehicle and then shift to the gear
you want.
5. Have the system fixed as soon as you can.
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