GMC 2006 Sierra - Manual (Page 149)

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 2-26 and “Torque Lock”
under Shifting Out of Park (P) (Automatic Transmission)
on page 2-55.
If your vehicle has an Allison Transmission ® , the
transmission is equipped with 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 clunk 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’t 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.
2-55